Indian Patents. 229248:"A PROCESS FOR LOWERING THE MARTENSITIC TRANSFORMATION TEMPERATURE OF SHAPE MEMORY ALLOY HAVING A LOW MARTENSITIC TRANSFORMATION TEMPERATURE"

Title of Invention	"A PROCESS FOR LOWERING THE MARTENSITIC TRANSFORMATION TEMPERATURE OF SHAPE MEMORY ALLOY HAVING A LOW MARTENSITIC TRANSFORMATION TEMPERATURE"
Abstract	The present invention an improved process to lower the martensitic transformation temperature, by a low temperature re-betatising treatment from 110° C. to 30° C. i.e. a lowering of 80° C. wherein previously high temperature betatised material has been subjected to re-betatising at lower temperature in order to utilize the material suitably.

Full Text	FIELD OF INVENTION The present invention relates to a process for lowering the Martensitic Transformation Temperature(As) of shape memory alloy having a low martensitic transformation temperature, said alloy comprising Copper and Zinc in the range of 62-86% of Copper and 10 -28% of Zinc along with 6% to 10% of Aluminum, by a re-betatising treatment of previously high temperature betatised material. BACKGROUND AND PRIOR ART OF THE INVENTION Cu--Zn—Al Shape Memory Effect (SME) alloys are promising smart and intelligent engineering materials. (Wayman C. M., Journal of Metals, 32 (June 1980), p. 129 137 and Michael A. D & Hart W. B Metal Material Technology, 12(1980), p. 434 440. These have attracted much attention because of their low cost and ease of fabrication relative to nitinol (White S. M., Cook J. M. & Stobbs W. M, Journal De Physique, C4 (ICOMAT-82), P-779 783. But nitinol has superior properties, long fatigue life andis biocompatible. There are about twenty elements in the central part of the periodic table Golestaneh A. A., Physics Today, (April 1984), p-62 70 whose alloys exhibit shape memory like Ag— Cd, Au~Cd, Cu-Al-Ni, Cu~Al~Mn, Cu~Au~Zn, Cu-Sn, Cu-Au~Sn,Cu-Zn, Cu-Zn-Al, Cu~Zn-Sn, Cu-Zn-Ga, Cu-Zn-Si, In-Ti, Ni-Al, Ni-Ti, Fe-Pt, Fe-Pd, etc. (Wayman C. M., Journal of Metals, 32 (June 1980), p-129 137 and Michael A. D & Hart W. B Metal Material Technol., 12(1980), p. 434 440. Shape memory alloys (SMA) have a unique property, i.e., these materials remember their past shapes/ configurations. The Important characteristics of these alloys are their ability to exist in two distinct shapes or configurations above or belowa certain critical transformation temperature. It undergoes diffusionless martensitic transformation Golestaneh A. A., Physics Today; (April 1984), p. 62 70, which is also thermo elastic in nature, i.e., below the critical temperature a martensiticstructure forms and grows as the temperature is lowered, whereas, on heating the martensite shrinks and ultimately vanishes. The martensite in shape memory alloys is soft in contrast to martensite of steels. Deformation of these alloys is not by slip, twinning or grain boundary sliding but by growth or shrinkage of self-accommodating, multi-oriented martensiticplates/variant Saburi T., Wayman C. M., Takala K & Nenno S., Acta Metallurgica (January 1980) P-15. On heating, the strained martensite reverts back to its parent phase, thereby, the original undeformed shape is recovered. The change in structure can be linked with change in shape and dimensions and the alloy exhibits a memory of high and lowtemperature Hence, the present invention is directed towards increasing or decreasing of martensitic transformation temperature. In Cu~Zn-4% Al alloy Adnyana D. N., Wire Journal International, (1984), pp. 52 61, lowering of martensitic transformation temperature has been comparatively low, i.e., around 20° C. 25° C. There is always evaporation of volatile and low melting elements like zinc, aluminum, tin, lead etc during the melting of copper base and other alloys, especially in the air melting furnaces. These losses cannot be avoided but can be minimizedby taking all the care during melting, adding precisely weighed quantities of each element, compensating for the elemental losses and rigidly following precautions during melting. Vacuum furnaces precisely control these losses but their installations are costly and are thus unaffordable to the small and medium scale melting/foundry units. Cu—Zn~Al shape memory alloys (SMAs) are no exceptions to these. The martensitictransformation temperature (As) is an important parameter in shape memory alloys and is extremely sensitive to the composition. A slight variation of either zinc or aluminum (±0.5%), as a result of melting losses, shifts the martensitictransformation temperature by ±50° C. The material thus cast and processed reduces to a scrap and has to be remelted thereby resulting in wastage of efforts, manpower and machinery. Experimental studies show that it is possible to raise As temperature by 15° C. 20° C. by the use of either a compensating bias spring or by selective etching/leaching out of zinc by thermal treatments. But lowering of Astemperature, once obtained poses problems. U.S. Pat. No. 4,634,477 recites about shape memory alloys. However, this patent does not mention about the reduction in martensitic temperature. OBJECTS OF THE PRESENT INVENTION The main object of the present invention is to provide a shape memory alloy having a composition of Cu—Zn—Al (6%) with lower martensitic temperature. Another object of the present invention relates to provide a shape memory alloy having good memory response. Yet another object of the present invention is to provide a shape memory alloy having good recovery and fatigue life properties. Still another object of the present invention is to provide a shape memory alloy that can prevent quench cracks. Another object of the present invention is to provide a process for lowering the martensitic transformation temperature (As) of a shape memory alloy having a composition of Cu—Zn—Al (6%) Yet another object of the present invention is to provide for an improved process, in order to lower the transformation temperature, by a low temperature re-betatising treatment from 110° C. to 30° C. i.e. a lowering of 80° C. SUMMARY OF THE PRESENT INVENTION Accordingly the present invention provides a process for lowering the Martensitic Transformation Temperature(As) of shape memory alloy having a low martensitic transformation temperature, said alloy comprising Copper and Zinc in the range of 62-86% of Copper and 10 -28% of Zinc along with 6% to 10% of Aluminum, by a re-betatising treatment of previously high temperature betatised material, said process comprising the following steps of: (i) selecting an alloy composition comprising Copper and Zinc in the range of 62-86% of Copper and 10-28% of Zinc along with 6% of Aluminum; (ii) melting the alloy composition in an induction furnace operating in air under charcoal cover followed by casting intodesired shapes to form a shaped material; (iii) homogenizing the shaped material at 800° C. for a period of about two hours followed by cooling; (iv) surface machining the shaped material for removing oxide scale formation; (v) re-heating theshaped material at about 575° C. for about three minutes (vi) quenching said shaped material with cold water for obtaining a fully martensitic structure; (vii) recording the temperature and structure of the material. In an embodiment of the present invention said alloy have a martensitic transformation temperature lowered by about 80° C. In another embodiment of the present invention, wherein said alloy displays good shape memory at a re-betatising temperature of about 575° C. In yet embodiment of the present invention, wherein said alloy having good fatigue properties thereby preventing quench cracking. In still another embodiment of the present invention, wherein said alloy once processed can be utilized for some other temperature device or application. In further another embodiment of the present invention, said alloy have good shape memory response properties. In another embodiment of the present invention, the loss of Zinc or Aluminum raises the martensitic transformation temperature whereas increase of these elements lowers the transformation temperature. In yet another embodiment of the present invention the material once cast and processed can be utilized for some other temperature device or application. In still another embodiment of the present invention,the shape memory response properties are not affected. In yet another embodiment of the present invention, the two-step betatising and resultant lowering of transformation temperature is valid for higher Aluminum content of 6-10% shape memory alloys. BRIEF DESCRIPTION OF THE ACCOMPANYINGPROCESS FLOW SHEETS, METALLOGRAPHS, BAR CHARTS AND CURVES FIG. 1 Shows experimental flow sheet of the process of production of Shape memory alloy in the sheet form and its betatising (memorizing) heat treatment. It also depicts its structure, SME response and martensitic transformation temperature. FIG. 2 Depicts microstructures of material betatised at 750° C./3 min/CWQ. FIG. 3 Depicts microstructures on heating the betatised material at various temperatures like 200° C, 300° C, 400° C, 500° C, 600° C. and 700° C. FIG. 4 Shows microstructures of seven more betatised samples, reheated (rebetatised) at 550° C, 575° C, 600° C, 625° C, 650° C, 675° C, 700° C. (increments of 25° C.) for tenminutes and cold water (room temperature) quenched. FIG. 5 Flow diagram explains in details the condition of material, its microstructure and shape memory response on heating the previously high temperature betatised material at 200°C, 300°C, 400°C, 500°C, 600°C & 700°C. Fig 6 Flow diagram explains in details the condition of material, its SME response, martensitic transformation temperature (As) and its microstructure on low temperature rebetatising of the previously high temperature betatised material at 550°C, 575°C, 600°C, 625°C, 650°C, 675°C & 700°C. Fig 7 shows bar chart explaining re-betatising temperature Versus martensitic transformation temperature (As). Fig 8 shows the curve explaining re-betatising temperature versus martensitic transformation temperature. It also depicts optimum lowering of martensitic transformation temperature (As 80°C) on re-betatising at 575° C. EXAMPLES The following examples are provided to illustrate further the invention. However, the same shall not be considered to limit the scope of the invention. EXAMPLE 1 The charge consisting of commercially pure Copper, Zinc & Aluminum was melted in an induction furnace under a charcoal cover and cast into sand moulds in plates of sizes 150 x 100 x 12.5 mm. These were then homogenized at 800 °C for two hours and cooled. These were then surface machined to remove oxidized layer. These homogenized plates were analyzed for chemical composition. The plates (12mm thick) were reheated at 750°C for one hour and hot rolled down to one-mm thick flat sheets with number of reheating inbetween the reduction passes. These sheets were held in fixtures (1.0-mm thick sheets) and were betatised at 750°C for 3 minutes and then cold (ordinary) water quenched. These were trimmed to desired dimensions, approximately, 20-25 pieces of size 100mm X 10-12 mm X 1mm. From one flat sheet (betatised strip) a small rectangular piece (10 x 10 mm) was cut and mounted in a acrylic compound, polished on grades of silicon carbide papers then on diamond paste impregnated microcloth rotating wheel, etched in potassium dichromate etchant and its micro structure was seen under the optical microscope. The structure was fully martensitic. On the remaining strip, shape memory response was seen by hot air blower. Transformation temperature was determined using hot & cold water and temperature indicator. Its Shape Memory response was good and transformation temperature was around 110°C-112°C (Fig.-l) Six more memorized (betatised) sheets were then heated at 200°C, 300°C 400°C, 500°C, 600°C & 700°C for ten minutes & cold (ordinary) water quenched. These were deformed to check the shape memory response and their microstructures were analyzed (Fig.-2&3). The samples heated to 200°C, 300°C, 400°C were very stiff as such memory could 60not be noticed. Sample heated at 500°C was soft but without shape memory. Their microstructures were seen. Samples heated at 600°C and 700°C were soft and showed shape memory at low temperatures & high temperatures respectively. Their microstructures were also seen. Martensitic structure prevailed between 500°C to 700°C. Thus seven more betatised samples were further reheated (rebetatised) at 550°C, 575°C, 600°C, 625°C, 650°C, 675°C, 700°C (increments of 25°C) for ten minutes and cold water (room temperature) quenched. Their microstructures were observed (Fig.-4) These were deformed and their S.M. response & transformation temperatures were determined (Fig.-5,6). A betatised sample, rebetatised at 575°C for ten minutes and water quenched was martensitic with enough alpha at the grain boundaries & within the grains, showed good shape memory response and its transformation temperature (As) was around 30°C. An initial temperature of 110°C was thus lowered to 30°C by this two-step treatment, a drop of temperature of 80°C by this process. Any intermediate transformation temperature can be achieved by selecting appropriate re-betatising temperature. The process of re-betatising was repeated number of times, to ascertain the reproducibility and for the confirmation of results. EXAMPLE 2 In the Cu-Zn binary phase diagram Higgins R.A, Engineering metallurgy Vol.1 (1971), P- 312-339, alpha solid phase (a) exists up to 39% zinc content. This a-phase has face centered cubic structure (FCC). It is ductile, malleable and cold workable. Above 39% Zinc to 50% zinc content a beta phase (P) appears. It has body centered cubic structure (BCC). It is a hard phase and can only be hot worked. Above 50% zinc content a complex, brittle and undesirable gamma y phase structure is formed. Cu-Zn-Al is a ternary alloy system. It is basically a Cu-Zn alloy system with an addition of 3rd element Aluminum. The zinc equivalent of Aluminum is six that is. 1% Aluminum has an effect similar to 6% zinc (1A1 = 6 Zn) West E.G. Copper & its alloys (1982), P-98-105. We can thus calculate the equivalent of zinc for the shape memory alloy of composition (74.4% Cu -19.5% Zn - 6.1% Al) by applying the following formula Greaves R.H. & Wrighton H., Practical Microscopical Metallography (1971), P-159-177. The equivalent of Zinc is calculated to be 43%. Equivalent of Zinc = % of Zn + 6 X % of Al X 100 % of Cu + %of Zn + 6 X % of Al If we examine the binary diagram of Cu-Zn System, the 43% Zinc content lies very close to or almost on the phase boundary regions of a and p i.e. in the p rich, regions of the diagram. Keeping in view their conditions relevant experiments were carried out. On heating, the alloy to 750°C the structural transformation is from martensite to beta, as such, we will designate this heating treatment as betatising and the transformation temperature as As (while heating) & Ms (while cooling) respectively. The betatised material (750°C /3 min./CWQ) was soft & fully martensitic with a good SM response. Its temperature was 110°C to 112°C. The previously high temperature betatised samples (6 Nos.) were rebetatised at low temperature to 200°C, 300°C, 400°C, 500°C, 600°C and 700°C for ten minutes and cold water (room temperature) quenched. Their microstructures, shape memory response and martensitic transformation temperatures were determined. Rebetatised 200°C material was martensitic. But was very stiff, as quenching from 200°C and as such did not show shape memory. Rebetatised 300°C sample had a + P little martensite. It was stiff and had no memory. The morphology of a was rod or plate type. Rebetatised 400°C sample too was stiff with no SMB. It contained a+p structure and o> Phase was within the grains. The rebetaised 500°C material was soft but had no SMB. Its structure was a+p very little martensite. It had very thin a-phase rim at the grain boundaries, which had tendency towards globular form. The samples heated at 600°C and 700°C were soft and deformable and showed memory of low & high temperatures respectively. These materials were fully martensitic but 600°C rebetatised sample had little a-precipitated at the grain boundaries and within the grains as compared to 700°C sample. The 700°C sample was comparatively fine grained. These materials did not crack even on cold water quenching. By these specific experiments it was ascertained that shape memory effect in this material was between 550°C to 700°C. Thus, for further experiment seven betatised strips were taken and were subjected to re-betatising treatments at 550°C, 575°C, 600°C, 625°C, 650°C 675°C and 700°C (an increment of 25°C) for ten minutes and then cold (room temperature) water quenched. Microstructures, shape memory response, and transformation temperatures were evaluated. The 550°C betatised sample was soft and its transformation temperature had dropped from 110°C to 22°C The sample had a feeble memory mainly because of separation of sufficient volume fraction of a-phase in p and very little visible martensite. Grain boundary a-envelop was also thick. Sample rebetatised at 575°C was soft and had good shape memory 30°C. Its microstructure was martensitic with enough volume fraction of a phase-streaks at the grain boundaries and within the grain, a- Phase had tendency towards globular or lenticular shape formation. The precipitation of a-phase from the matrix has enriched the remaining beta phase in zinc content & shifted the composition towards right in binary diagram. This zinc rich beta on quenching transforms to zinc rich martensite and thus lowers the martensitic transformation temperature considerably i.e. from 110°C to 30°C a drop of 80°C. On rebetatising at 600°C the material was soft with good SME and the transformation temperature was around 45°C. The sample was fully martensitic with little a-phase precipitated at grain boundaries & within the interior of grains. Samples rebetatised at 625°C was also soft and has good SME around 61°C. Structure was martensitic with unresolved a-phase at the grain boundaries. The sample rebetatised at 650°C) 675°C and 700°C were all soft and had shape memory at 79°C, 100°C & 110°C respectively (Fig.- 7,8). These were fully martensitic and there was hardly any a visible at grain boundaries & within the grains. In other wards a-phase was not resolvable. Since very little or negligible a-phase has separated from the martensite matrix, these samples showed shape memory at high temperatures. The results, therefore, indicate that the sample rebetatised at 575°C gave an optimum value i.e. its martensitic transformation temperature was around 30°C i.e. a drop of As from 110°C to 30°C which is a drop of 80°C by this particular rebetatising treatment. EXAMPLE 3 Cu-Zn-Al Shape Memory alloys (4% Al & 6% Al) (a) 74.4% Cu-19.5% Zn-6.1% Al (Melt No. 7) BETATISED (As): 110°C, REBETATISED (As): 30°C (b) 74.1% Cu-19.5% Zn-6.4% Al (MeltNo.5) BETATISED (As): 130°C, REBETATISED (As); 50°C (c) 73.6% Cu-20.2%Zn-6.2% Al (Melt No 6) BETATISED (As): 83°C, REBETATISED (As)-10°C (d) 71.0% Cu-24.8% Zn-4.2%Al (Melt No35) BETATISED (As): 65°C, REBETATISED (As): 45 °C EXAMPLE 4 It is observed that in Cu-Zn-Al (6%) shape memory alloys lowering of martensite transformation temperature is substantial i.e. 70°C-80°C. It is also observed during experimentation that martensitic transformation temperature could be raised by 15°C-20°C by incorporating a bias or by suitable thermal treatment for the selective etching/loss of Zinc. 10 Martensite transformation temperature (As) can be lowered substantially by about 80°C in Cu-Zn-6% Al alloys by specific thermal treatment i.e. by low temperature re-betatising of the previously high temperature betatised material. The decrease of temperature was mainly due to the separation of small quantities of alpha (a) from the matrix of martensite. This retained a-phase does not affect the shape memory response but in turn, it assists in cushioning the grain boundaries & thereby preventing the material from cracking, even on quenching in cold water. ADVANTAGES OF THE INVENTION (1) In Cu-Zn-4% Al alloy Adnyana D. N., Wire Journal International, (1984), P-52-61, lowering of martensitic transformation temperature has been comparatively low i.e. around 20°C-25°C whereas in Cu-Zn-6% Al Shape memory alloys it was found to be substantial i.e. 70°C-80°C. (2) The present improved process has no adverse effect on shape memory response, recovery, fatigue life etc. rather the precipitation of a-phase in the martensite phase assists in cushioning the matrix and generally associated with shape memory alloys. (3) The present improved process is likely to assist, the small & medium scale melting units, to accept the risks and challenges faced, in the melting of shape memory alloys by way of savings in the form of cost, manpower and machinery. (4) The process is novel, simple & needs no additional manpower or equipment. (5) Cold water quenching gives better shape memory response. We claim: 1. A process for lowering the Martensitic Transformation Temperature(As) of shape memory alloy having a low martensitic transformation temperature, said alloy comprising Copper and Zinc in the range of 62-86% of Copper and 10 -28% of Zinc along with 6% to 10% of Aluminum, by a re-betatising treatment of previously high temperature betatised material, said process comprising the following steps of: (i) selecting an alloy composition comprising Copper and Zinc in the range of 62-86% of Copper and 10-28% of Zinc along with 6% of Aluminum; (ii) melting the alloy composition in an induction furnace operating in air under charcoal cover followed by casting intodesired shapes to form a shaped material; (iii) homogenizing the shaped material at 800° C. for a period of about two hours followed by cooling; (iv) surface machining the shaped material for removing oxide scale formation; (v) re-heating theshaped material at about 575° C. for about three minutes (vi) quenching said shaped material with cold water for obtaining a fully martensitic structure; (vii) recording the temperature and structure of the material. 2. A process as claimed in claim 1, wherein the composition comprises an Aluminum content of 6%. 3. A process for lowering the Martensitic Transformation Temperature(As) of shape memory alloy having a low martensitic transformation temperature substantially as herein described with reference to examples and drawings accompanying this specification.

Full Text

FIELD OF INVENTION
The present invention relates to a process for lowering the Martensitic Transformation Temperature(As) of shape memory alloy having a low martensitic transformation temperature, said alloy comprising Copper and Zinc in the range of 62-86% of Copper and 10 -28% of Zinc along with 6% to 10% of Aluminum, by a re-betatising treatment of previously high temperature betatised material.
BACKGROUND AND PRIOR ART OF THE INVENTION
Cu--Zn—Al Shape Memory Effect (SME) alloys are promising smart and intelligent engineering materials. (Wayman C. M., Journal of Metals, 32 (June 1980), p. 129 137 and Michael A. D & Hart W. B Metal Material Technology, 12(1980), p. 434 440. These have attracted much attention because of their low cost and ease of fabrication relative to nitinol (White S. M., Cook J. M. & Stobbs W. M, Journal De Physique, C4 (ICOMAT-82), P-779 783. But nitinol has superior properties, long fatigue life andis biocompatible. There are about twenty elements in the central part of the periodic table Golestaneh A. A., Physics Today, (April 1984), p-62 70 whose alloys exhibit shape memory like Ag— Cd, Au~Cd, Cu-Al-Ni, Cu~Al~Mn, Cu~Au~Zn, Cu-Sn, Cu-Au~Sn,Cu-Zn, Cu-Zn-Al, Cu~Zn-Sn, Cu-Zn-Ga, Cu-Zn-Si, In-Ti, Ni-Al, Ni-Ti, Fe-Pt, Fe-Pd, etc. (Wayman C. M., Journal of Metals, 32 (June 1980), p-129 137 and Michael A. D & Hart W. B Metal Material Technol., 12(1980), p. 434 440.
Shape memory alloys (SMA) have a unique property, i.e., these materials remember their past shapes/ configurations. The Important characteristics of these alloys are their ability to exist in two distinct shapes or configurations above or belowa certain critical transformation temperature. It undergoes diffusionless martensitic transformation Golestaneh A. A., Physics Today; (April 1984), p. 62 70, which is also thermo elastic in nature, i.e., below the critical temperature a martensiticstructure forms and grows as the temperature is lowered, whereas, on heating the martensite shrinks and ultimately vanishes.
The martensite in shape memory alloys is soft in contrast to martensite of steels. Deformation of these alloys is not by slip, twinning or grain boundary sliding but by growth or shrinkage of self-accommodating, multi-oriented martensiticplates/variant Saburi T., Wayman C. M., Takala K & Nenno S., Acta Metallurgica (January 1980) P-15.
On heating, the strained martensite reverts back to its parent phase, thereby, the original undeformed shape is recovered. The change in structure can be linked with change in shape and dimensions and the alloy exhibits a memory of high and lowtemperature
Hence, the present invention is directed towards increasing or decreasing of martensitic transformation temperature.
In Cu~Zn-4% Al alloy Adnyana D. N., Wire Journal International, (1984), pp. 52 61, lowering of martensitic transformation temperature has been comparatively low, i.e.,

around 20° C. 25° C.
There is always evaporation of volatile and low melting elements like zinc, aluminum, tin, lead etc during the melting of copper base and other alloys, especially in the air melting furnaces. These losses cannot be avoided but can be minimizedby taking all the care during melting, adding precisely weighed quantities of each element, compensating for the elemental losses and rigidly following precautions during melting. Vacuum furnaces precisely control these losses but their installations are costly and are thus unaffordable to the small and medium scale melting/foundry units. Cu—Zn~Al shape memory alloys (SMAs) are no exceptions to these. The martensitictransformation temperature (As) is an important parameter in shape memory alloys and is extremely sensitive to the composition. A slight variation of either zinc or aluminum (±0.5%), as a result of melting losses, shifts the martensitictransformation temperature by ±50° C. The material thus cast and processed reduces to a scrap and has to be remelted thereby resulting in wastage of efforts, manpower and machinery.
Experimental studies show that it is possible to raise As temperature by 15° C. 20° C. by the use of either a compensating bias spring or by selective etching/leaching out of zinc by thermal treatments. But lowering of Astemperature, once obtained poses problems. U.S. Pat. No. 4,634,477 recites about shape memory alloys. However, this patent does not mention about the reduction in martensitic temperature.
OBJECTS OF THE PRESENT INVENTION
The main object of the present invention is to provide a shape memory alloy having a
composition of Cu—Zn—Al (6%) with lower martensitic temperature.
Another object of the present invention relates to provide a shape memory alloy having
good memory response.
Yet another object of the present invention is to provide a shape memory alloy having
good recovery and fatigue life properties.
Still another object of the present invention is to provide a shape memory alloy that can
prevent quench cracks.
Another object of the present invention is to provide a process for lowering the

martensitic transformation temperature (As) of a shape memory alloy having a composition of Cu—Zn—Al (6%)
Yet another object of the present invention is to provide for an improved process, in order to lower the transformation temperature, by a low temperature re-betatising treatment from 110° C. to 30° C. i.e. a lowering of 80° C.
SUMMARY OF THE PRESENT INVENTION
Accordingly the present invention provides a process for lowering the Martensitic Transformation Temperature(As) of shape memory alloy having a low martensitic transformation temperature, said alloy comprising Copper and Zinc in the range of 62-86% of Copper and 10 -28% of Zinc along with 6% to 10% of Aluminum, by a re-betatising treatment of previously high temperature betatised material, said process comprising the following steps of: (i) selecting an alloy composition comprising Copper and Zinc in the range of 62-86% of Copper and 10-28% of Zinc along with 6% of Aluminum; (ii) melting the alloy composition in an induction furnace operating in air under charcoal cover followed by casting intodesired shapes to form a shaped material; (iii) homogenizing the shaped material at 800° C. for a period of about two hours followed by cooling; (iv) surface machining the shaped material for removing oxide scale formation; (v) re-heating theshaped material at about 575° C. for about three minutes (vi) quenching said shaped material with cold water for obtaining a fully martensitic structure; (vii) recording the temperature and structure of the material.
In an embodiment of the present invention said alloy have a martensitic transformation temperature lowered by about 80° C.
In another embodiment of the present invention, wherein said alloy displays good shape memory at a re-betatising temperature of about 575° C. In yet embodiment of the present invention, wherein said alloy having good fatigue properties thereby preventing quench cracking.
In still another embodiment of the present invention, wherein said alloy once processed can be utilized for some other temperature device or application. In further another embodiment of the present invention, said alloy have good shape

memory response properties.
In another embodiment of the present invention, the loss of Zinc or Aluminum raises
the martensitic transformation temperature whereas increase of these elements lowers
the transformation temperature.
In yet another embodiment of the present invention the material once cast and
processed can be utilized for some other temperature device or application.
In still another embodiment of the present invention,the shape memory response
properties are not affected.
In yet another embodiment of the present invention, the two-step betatising and
resultant lowering of transformation temperature is valid for higher Aluminum
content of 6-10% shape memory alloys.
BRIEF DESCRIPTION OF THE ACCOMPANYINGPROCESS FLOW SHEETS, METALLOGRAPHS, BAR CHARTS AND CURVES
FIG. 1 Shows experimental flow sheet of the process of production of Shape memory alloy in the sheet form and its betatising (memorizing) heat treatment. It also depicts its structure, SME response and martensitic transformation temperature.
FIG. 2 Depicts microstructures of material betatised at 750° C./3 min/CWQ.
FIG. 3 Depicts microstructures on heating the betatised material at various temperatures like 200° C, 300° C, 400° C, 500° C, 600° C. and 700° C.
FIG. 4 Shows microstructures of seven more betatised samples, reheated (rebetatised) at 550° C, 575° C, 600° C, 625° C, 650° C, 675° C, 700° C. (increments of 25° C.) for tenminutes and cold water (room temperature) quenched.
FIG. 5 Flow diagram explains in details the condition of material, its microstructure and shape memory response on heating the previously high temperature betatised material at

200°C, 300°C, 400°C, 500°C, 600°C & 700°C.
Fig 6 Flow diagram explains in details the condition of material, its SME response,
martensitic transformation temperature (As) and its microstructure on low temperature rebetatising
of the previously high temperature betatised material at 550°C, 575°C, 600°C,
625°C, 650°C, 675°C & 700°C.
Fig 7 shows bar chart explaining re-betatising temperature Versus martensitic
transformation temperature (As).
Fig 8 shows the curve explaining re-betatising temperature versus martensitic
transformation temperature. It also depicts optimum lowering of martensitic transformation
temperature (As 80°C) on re-betatising at 575° C.
EXAMPLES
The following examples are provided to illustrate further the invention. However, the
same shall not be considered to limit the scope of the invention.
EXAMPLE 1
The charge consisting of commercially pure Copper, Zinc & Aluminum was melted in an
induction furnace under a charcoal cover and cast into sand moulds in plates of sizes 150 x
100 x 12.5 mm. These were then homogenized at 800 °C for two hours and cooled. These
were then surface machined to remove oxidized layer. These homogenized plates were
analyzed for chemical composition. The plates (12mm thick) were reheated at 750°C for
one hour and hot rolled down to one-mm thick flat sheets with number of reheating inbetween
the reduction passes. These sheets were held in fixtures (1.0-mm thick sheets) and
were betatised at 750°C for 3 minutes and then cold (ordinary) water quenched. These
were trimmed to desired dimensions, approximately, 20-25 pieces of size 100mm X 10-12
mm X 1mm. From one flat sheet (betatised strip) a small rectangular piece (10 x 10 mm)
was cut and mounted in a acrylic compound, polished on grades of silicon carbide papers
then on diamond paste impregnated microcloth rotating wheel, etched in potassium
dichromate etchant and its micro structure was seen under the optical microscope. The
structure was fully martensitic. On the remaining strip, shape memory response was seen
by hot air blower. Transformation temperature was determined using hot & cold water and
temperature indicator. Its Shape Memory response was good and transformation
temperature was around 110°C-112°C (Fig.-l) Six more memorized (betatised) sheets
were then heated at 200°C, 300°C 400°C, 500°C, 600°C & 700°C for ten minutes & cold
(ordinary) water quenched. These were deformed to check the shape memory response and
their microstructures were analyzed (Fig.-2&3). The samples heated to 200°C, 300°C,
400°C were very stiff as such memory could 60not be noticed. Sample heated at 500°C
was soft but without shape memory. Their microstructures were seen. Samples heated at
600°C and 700°C were soft and showed shape memory at low temperatures & high
temperatures respectively. Their microstructures were also seen. Martensitic structure
prevailed between 500°C to 700°C. Thus seven more betatised samples were further
reheated (rebetatised) at 550°C, 575°C, 600°C, 625°C, 650°C, 675°C, 700°C (increments
of 25°C) for ten minutes and cold water (room temperature) quenched. Their
microstructures were observed (Fig.-4) These were deformed and their S.M. response &
transformation temperatures were determined (Fig.-5,6). A betatised sample, rebetatised
at 575°C for ten minutes and water quenched was martensitic with enough alpha at the
grain boundaries & within the grains, showed good shape memory response and its
transformation temperature (As) was around 30°C. An initial temperature of 110°C was
thus lowered to 30°C by this two-step treatment, a drop of temperature of 80°C by this
process. Any intermediate transformation temperature can be achieved by selecting
appropriate re-betatising temperature. The process of re-betatising was repeated number of
times, to ascertain the reproducibility and for the confirmation of results.
EXAMPLE 2
In the Cu-Zn binary phase diagram Higgins R.A, Engineering metallurgy Vol.1 (1971), P-
312-339, alpha solid phase (a) exists up to 39% zinc content. This a-phase has face
centered cubic structure (FCC). It is ductile, malleable and cold workable. Above 39%
Zinc to 50% zinc content a beta phase (P) appears. It has body centered cubic structure
(BCC). It is a hard phase and can only be hot worked. Above 50% zinc content a complex,
brittle and undesirable gamma y phase structure is formed. Cu-Zn-Al is a ternary alloy
system. It is basically a Cu-Zn alloy system with an addition of 3rd element Aluminum.
The zinc equivalent of Aluminum is six that is. 1% Aluminum has an effect similar to 6%
zinc (1A1 = 6 Zn) West E.G. Copper & its alloys (1982), P-98-105. We can thus calculate
the equivalent of zinc for the shape memory alloy of composition (74.4% Cu -19.5% Zn -
6.1% Al) by applying the following formula Greaves R.H. & Wrighton H., Practical
Microscopical Metallography (1971), P-159-177. The equivalent of Zinc is calculated to
be 43%.
Equivalent of Zinc = % of Zn + 6 X % of Al X 100
% of Cu + %of Zn + 6 X % of Al
If we examine the binary diagram of Cu-Zn System, the 43% Zinc content lies very close
to or almost on the phase boundary regions of a and p i.e. in the p rich, regions of the
diagram. Keeping in view their conditions relevant experiments were carried out. On
heating, the alloy to 750°C the structural transformation is from martensite to beta, as such,
we will designate this heating treatment as betatising and the transformation temperature as
As (while heating) & Ms (while cooling) respectively. The betatised material (750°C /3
min./CWQ) was soft & fully martensitic with a good SM response. Its temperature was
110°C to 112°C. The previously high temperature betatised samples (6 Nos.) were
rebetatised at low temperature to 200°C, 300°C, 400°C, 500°C, 600°C and 700°C for ten
minutes and cold water (room temperature) quenched. Their microstructures, shape
memory response and martensitic transformation temperatures were determined.
Rebetatised 200°C material was martensitic. But was very stiff, as quenching from 200°C
and as such did not show shape memory. Rebetatised 300°C sample had a + P little
martensite. It was stiff and had no memory. The morphology of a was rod or plate type.
Rebetatised 400°C sample too was stiff with no SMB. It contained a+p structure and o>
Phase was within the grains. The rebetaised 500°C material was soft but had no SMB. Its
structure was a+p very little martensite. It had very thin a-phase rim at the grain
boundaries, which had tendency towards globular form. The samples heated at 600°C and
700°C were soft and deformable and showed memory of low & high temperatures
respectively. These materials were fully martensitic but 600°C rebetatised sample had little
a-precipitated at the grain boundaries and within the grains as compared to 700°C sample.
The 700°C sample was comparatively fine grained. These materials did not crack even on
cold water quenching. By these specific experiments it was ascertained that shape memory
effect in this material was between 550°C to 700°C. Thus, for further experiment seven
betatised strips were taken and were subjected to re-betatising treatments at 550°C, 575°C,
600°C, 625°C, 650°C 675°C and 700°C (an increment of 25°C) for ten minutes and then
cold (room temperature) water quenched. Microstructures, shape memory response, and
transformation temperatures were evaluated. The 550°C betatised sample was soft and its
transformation temperature had dropped from 110°C to 22°C The sample had a feeble
memory mainly because of separation of sufficient volume fraction of a-phase in p and
very little visible martensite. Grain boundary a-envelop was also thick. Sample
rebetatised at 575°C was soft and had good shape memory 30°C. Its microstructure was
martensitic with enough volume fraction of a phase-streaks at the grain boundaries and
within the grain, a- Phase had tendency towards globular or lenticular shape formation.
The precipitation of a-phase from the matrix has enriched the remaining beta phase in zinc
content & shifted the composition towards right in binary diagram. This zinc rich beta on
quenching transforms to zinc rich martensite and thus lowers the martensitic
transformation temperature considerably i.e. from 110°C to 30°C a drop of 80°C. On rebetatising
at 600°C the material was soft with good SME and the transformation
temperature was around 45°C. The sample was fully martensitic with little a-phase
precipitated at grain boundaries & within the interior of grains. Samples rebetatised at
625°C was also soft and has good SME around 61°C. Structure was martensitic with
unresolved a-phase at the grain boundaries. The sample rebetatised at 650°C) 675°C and
700°C were all soft and had shape memory at 79°C, 100°C & 110°C respectively (Fig.-
7,8). These were fully martensitic and there was hardly any a visible at grain boundaries
& within the grains. In other wards a-phase was not resolvable. Since very little or
negligible a-phase has separated from the martensite matrix, these samples showed shape
memory at high temperatures. The results, therefore, indicate that the sample rebetatised at
575°C gave an optimum value i.e. its martensitic transformation temperature was around
30°C i.e. a drop of As from 110°C to 30°C which is a drop of 80°C by this particular rebetatising
treatment.
EXAMPLE 3
Cu-Zn-Al Shape Memory alloys (4% Al & 6% Al)
(a) 74.4% Cu-19.5% Zn-6.1% Al (Melt No. 7)
BETATISED (As): 110°C, REBETATISED (As): 30°C
(b) 74.1% Cu-19.5% Zn-6.4% Al (MeltNo.5)
BETATISED (As): 130°C, REBETATISED (As); 50°C
(c) 73.6% Cu-20.2%Zn-6.2% Al (Melt No 6)
BETATISED (As): 83°C, REBETATISED (As)-10°C
(d) 71.0% Cu-24.8% Zn-4.2%Al (Melt No35)
BETATISED (As): 65°C, REBETATISED (As): 45 °C
EXAMPLE 4
It is observed that in Cu-Zn-Al (6%) shape memory alloys lowering of martensite
transformation temperature is substantial i.e. 70°C-80°C. It is also observed during
experimentation that martensitic transformation temperature could be raised by 15°C-20°C
by incorporating a bias or by suitable thermal treatment for the selective etching/loss of
Zinc.
10
Martensite transformation temperature (As) can be lowered substantially by about 80°C in
Cu-Zn-6% Al alloys by specific thermal treatment i.e. by low temperature re-betatising of
the previously high temperature betatised material. The decrease of temperature was
mainly due to the separation of small quantities of alpha (a) from the matrix of martensite.
This retained a-phase does not affect the shape memory response but in turn, it assists in
cushioning the grain boundaries & thereby preventing the material from cracking, even on
quenching in cold water.
ADVANTAGES OF THE INVENTION
(1) In Cu-Zn-4% Al alloy Adnyana D. N., Wire Journal International, (1984), P-52-61,
lowering of martensitic transformation temperature has been comparatively low i.e.
around 20°C-25°C whereas in Cu-Zn-6% Al Shape memory alloys it was found to
be substantial i.e. 70°C-80°C.
(2) The present improved process has no adverse effect on shape memory response,
recovery, fatigue life etc. rather the precipitation of a-phase in the martensite phase
assists in cushioning the matrix and generally associated with shape memory
alloys.
(3) The present improved process is likely to assist, the small & medium scale melting
units, to accept the risks and challenges faced, in the melting of shape memory
alloys by way of savings in the form of cost, manpower and machinery.
(4) The process is novel, simple & needs no additional manpower or equipment.
(5) Cold water quenching gives better shape memory response.

We claim:
1. A process for lowering the Martensitic Transformation Temperature(As) of shape memory alloy having a low martensitic transformation temperature, said alloy comprising Copper and Zinc in the range of 62-86% of Copper and 10 -28% of Zinc along with 6% to 10% of Aluminum, by a re-betatising treatment of previously high temperature betatised material, said process comprising the following steps of: (i) selecting an alloy composition comprising Copper and Zinc in the range of 62-86% of Copper and 10-28% of Zinc along with 6% of Aluminum; (ii) melting the alloy composition in an induction furnace operating in air under charcoal cover followed by casting intodesired shapes to form a shaped material; (iii) homogenizing the shaped material at 800° C. for a period of about two hours followed by cooling; (iv) surface machining the shaped material for removing oxide scale formation; (v) re-heating theshaped material at about 575° C. for about three minutes (vi) quenching said shaped material with cold water for obtaining a fully martensitic structure; (vii) recording the temperature and structure of the material.
2. A process as claimed in claim 1, wherein the composition comprises an Aluminum content of 6%.
3. A process for lowering the Martensitic Transformation Temperature(As) of shape memory alloy having a low martensitic transformation temperature substantially as herein described with reference to examples and drawings accompanying this specification.

Documents:

01691-delnp-2003-abstract.pdf

01691-delnp-2003-claims.pdf

01691-delnp-2003-correspondence-others.pdf

01691-delnp-2003-description (complete).pdf

01691-delnp-2003-drawings.pdf

01691-delnp-2003-form-1.pdf

01691-delnp-2003-form-18.pdf

01691-delnp-2003-form-2.pdf

01691-delnp-2003-form-3.pdf

1691-DELNP-2003-Abstract-(10-02-2009).pdf

1691-DELNP-2003-Claims-(10-02-2009).pdf

1691-DELNP-2003-Correspondence-Others-(10-02-2009).pdf

1691-DELNP-2003-Description (Complete)-(10-02-2009).pdf

1691-DELNP-2003-Drawings-(10-02-2009).pdf

1691-DELNP-2003-Form-2-(10-02-2009).pdf

1691-DELNP-2003-Form-3-(10-02-2009).pdf

1691-DELNP-2003-Form-5-(10-02-2009).pdf

1691-DELNP-2003-Petition-137-(10-02-2009).pdf

« Previous Patent

Next Patent »

Patent Number

229248

Indian Patent Application Number

01691/DELNP/2003

PG Journal Number

09/2009

Publication Date

27-Feb-2009

Grant Date

16-Feb-2009

Date of Filing

16-Oct-2003

Name of Patentee

COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110 001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	VIJAY RAJARAM HARCHEKAR	CENTRAL SCIENTIFIC INSTRUMENT ORGANIZATION, CHANDIGARH, INDIA.
2	MADANLAL SINGLA	CENTRAL SCIENTIFIC INSTRUMENT ORGANIZATION, CHANDIGARH, INDIA.

PCT International Classification Number

C22C

PCT International Application Number

PCT/IN01/00186

PCT International Filing date

2001-10-22

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	PCT/IN01/001086	2001-10-22	India