Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C9/00—Alloys based on copper
  • C22C9/04—Alloys based on copper with zinc as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/08—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of copper or alloys based thereon

Definitions

• the present invention relates to Cu-Zn-Al(6%) shape memory alloy having a low martensitic transformation temperature and a process of lowering the martensitic transformation temperature.
• BACKGROUND ART Cu-Zn-Al Shape Memory Effect (SME) alloys are promising smart and intelligent engineering materials. (Wayman CM., 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.
• nitinol has superior properties, long fatigue life and is biocompatible. There are about twenty elements in the central part of the periodic table Golestaneh A.
• Shape memory alloys 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 below a 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 martensitic structure 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.
• the alloys can be incorporated into range of temperature sensitive devices for warning, control, detection, regulation etc .
• the actuators can be calibrated to operate within a narrow temperature range by incorporating a compensating bias spring.
• the recoverable strain is 2-8% and is dependent upon one or two way memory.
• Copper based shape memory alloys in addition to one-way memory also exhibit two-way memory behavior, after undergoing a suitable thermal-mechanical processing called training (Wayman CM., Journal of Metals, 32 (June 1980), p-129-137 and Michael A.D & Hart W.B Metal Material Technol., 12(1980), p- 434-440.
• Martensitic transformation temperature is extremely sensitive to composition. A slight variation of either of the elements, Zinc or Aluminum (say ⁇ 0.5%) shifts the transformation temperature by + 50°C Therefore close control of composition is utmost essential to get the desired transformation temperature for the actuator to work at a specific temperature. Loss of low melting and volatile elements like Al, Zn etc. while melting cannot be avoided in air melting furnaces. Vacuum melting furnaces, in which close control of composition is possible but their installation is extremely costly and are unaffordable to the small and medium scale melting units/industries. In air melting furnaces, there is always a danger of loss of such elements inspite of compensating these losses and following the necessary precautions rigidly during melting.
• the alloy with off-composition and undesired martensitic transformation temperature has to be rejected or remelted.
• the efforts & inputs, thus put in, go waste. It was also observed that loss of zinc or aluminum raises the martensitic transformation temperature whereas increase of these elements lowers the transformation temperature.
• the present invention is directed towards increasing or decreasing of martensitic transformation temperature.
• 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 SMA's are no exceptions to these.
• the martensitic transformation 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 martensitic transformation temperature by + 50°C The material thus cast and processed reduces to a scrap & has to be remelted thereby resulting in wastage of efforts, manpower and machinery.
• 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 the 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.
• the present invention relates to a shape memory alloy having a composition of Cu-Zn-Al (6%) with lower martensitic temperature.
• the present invention also relates to a process of lowering martensitic temperature of said alloy by selecting material of composition 74.4% copper, 19.5% Zinc & 6.1% Aluminum and having an 'As' temperature of 110°C-112°C is selected. In this process the previously high temperature betatised material has been subjected to re-betatising at lower temperature in order to utilize the material suitably.
• the present invention provides for a 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% of Aluminum.
• a shape memory alloy having a martensitic transformation temperature lowered by about 80°C
• said alloy displays good shape memory at a re-betatising temperature of about 575°C
• Still another embodiment of the present invention wherein said alloy once processed can be utilized for some other temperature device or application. Further another embodiment of the present invention, wherein said alloy having good shape memory response properties.
• the present invention also provides for a process for lowering the Martensitic Transformation Temperature(As) of shape memory alloy as claimed in claim 1, by a re- betatising treatment of previously high temperature betatised material, said process comprising the following steps of:
• Yet another embodiment of the present invention wherein the two-step betatising and resultant lowering of transformation temperature is valid for higher Aluminum content of 6-10 % shape memory alloys.
• 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 & 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 & 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 ten minutes 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 re- betatising 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.
• 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 fiat sheets with number of reheating in- between the reduction passes.
• Martensitic structure prevailed between 500°C to 700°C
• 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).
• Cu-Zn-Al is a ternary alloy system. It is basically a Cu-Zn alloy system with an addition of 3 rd element Aluminum.
• the rebetaised 500°C material was soft but had no SME. Its structure was ⁇ + ⁇ very little martensite. It had very thin ⁇ -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 ⁇ -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.
• shape memory effect in this material was between 550°C to 700°C
• 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 ⁇ -phase in ⁇ and very little visible martensite. Grain boundary ⁇ -envelop was also thick.

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• 2001
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