Design,Preparation And Properties Of Cu-Zn-Sn-Fe Based Shape Memory Single Crystal Alloys

Shape memory alloys(SMAs)are a kind of smart metal materials with the response and drive functions.The SMAs are sensitive to stress and temperature,so they have received extensive attentions.Since the development of SMAs,three alloy systems with higher application value are Ni-Ti-based,partly Cu-based and Fe-based SMAs.The Ni-Ti-based SMAs have been used in many fields due to their excellent shape memory effect(SME),superelasticity(SE),high damping and biocompatibility,but the high manufacturing costs limit their own development.The Cu-based SMAs have always been one of the most popular researches in the field of SMAs due to the advantages of low cost,easy processing and excellent electrical conductivity.However,the polycrystalline brittleness caused by high anisotropy factor and large grain size severely limits the development and applications of Cu-based SMAs.Our research group introduced the nanoprecipitate into Cu-Al-Mn and Cu-Mn-Ga-based SMAs through alloying,which promotes abnormal grain growth(AGG)of cast alloys during high temperature annealing,and a few centimeters of single crystals were obtained.After a large number of early stud ies by research group,it has been found that the AGG in Cu-Al-Mn-based and Cu-Mn-Ga-Fe alloys is closely related to nanoprecipitate existing in cast alloys.When alloys are heat-treated at high temperature,these nanoprecipitates solid-dissolve back to parent phase.Because of a large number of defects at the interface between nanoprecipitates and parent,the solid solution process will cause the orientation difference in parent grains,which provides driving force for the AGG of individual grains.Based on the above-mentioned research ideas,this paper takes Cu-Zn-Sn-based SMAs as research object,and three kinds of Cu-Zn-Sn-based shape memory single crystals over centimeter scale were successfully prepared through optimized design of alloy composition,including Cu-Zn-Sn-Fe,Cu-Zn-Sn-Fe-Mn and Cu-Zn-Sn-Fe-B.The microstructure,reversible martensitic transformation behavior,SME,SE and other functional properties of these Cu-Zn-Sn-based shape memory single crystals were investigated in detail.The research results of this paper mainly include the following aspects:(1)The Cu65-xZn18.5+xSn14Fe2.5(x=0,0.5,1,1.5,2.5)(wt.%)centimeter-scaled single crystals were prepared by optimized design of alloy composition.When x=0,the martensite structure and austenite structure coexist in this single crystal.When x=0.5,1,1.5,2.5,all these single crystals show austenite structure.The characteristic temperature of martensitic transformation of Cu-Zn-Sn-Fe single crystals gradually decrease with the increase of Zn content.When the Zn content is 21 wt.%,the reversible martensitic transformation disappears.The research results of functional characteristics of single crystals show that the SMEs of Cu65Zn18.5SnI4Fe2.5 single crystal are 3.34%,4.28%,5.39%and 6.14%with the shape recovery rate of 100%when the pre-strain increases from 4%to 7%,which indicates that the single crystal has excellent SME properties.The Cu64.5Zn19Sn14Fe2.5 single crystal with the orientation close to[311]and the Cu64Zn19.5Sni4Fe2.5 single crystal with the orientation close to[421]have complete superelasticity of 5%.The Cu62.5Zn21Sn14Fe2.5 single crystal has complete superelasticity of 11%when the orientation is close to[401],showing excellent SE property.However,the Cu63.5Zn20Sn14Fe2.5 single crystal fails to show SE property,which may be related to its single crystal orientation.(2)Based on the results of Cu-Zn-Sn-Fe SMAs,the Mn was further alloyed to prepare Cu65.5-xZn16+xSn14Fe2.5Mn2(x=0,1.5,2,2.5,3)(wt.%)centimeter-scaled single crystals.When x=0,the martensite structure and austenite structure coexist in this single crystal.When x=1.5,2,2.5,3,all these single crystals show austenite structure.The characteristic temperature of martensitic transformation of Cu-Zn-Sn-Fe-Mn single crystals also gradually decrease with the increase of Zn content.When the Zn content is 19 wt.%,the reversible martensitic transformation disappears.At the same time,the introduction of Mn can reduce martensitic transformation temperature of single crystals.The research results of functional characteristics of single crystals show that the SME of Cu65.52Zn16Sn14Fe2.5Mn2 single crystal can be up to 6.5%with the shape recovery rate of 100%when the pre-strain is 8%.The Cu64Zn17.5Sn14Fe2.5Mn2 single crystal with the orientation close to[123]has complete superelasticity of 6%.The Cu63.5Zn18Sn14Fe2.5Mn2 single crystal with the orientation close to[441]has complete superelasticity of 10%.The Cu63Zn18.5Sn14Fe2.5Mn2 single crystal with the orientation close to[140]and the Cu62.5Zn19Sn14Fe2.5Mn2 single crystal with the orientation close to[403]have complete superelasticity of 9%.The Cu63.5Zn18Sn14Fe2.5Mn2,Cu63Zn18.5Sn14Fe2.5Mn2 and Cu62.5Zn19Sn14Fe2.5Mn2 single crystals exhibit excellent SE properties.(3)In the Cu66.3Zn16Sn15Fe2.5B0.2 and Cu63.8Zn19.5Sn14Fe2 5B0.2(wt.%)cast alloys,single crystals with the size of about Φ14 mm × 25 mm can be obtained after high temperature annealing treatment.Among them,the microstructure of Cu66.3Zn16Sn15Fe2.5B0.2 single crystal is martensite phase,and the microstructure of Cu63.8Zn19.5Sn14Fe2.5B0.2 single crystal is austenite phase.Through compression testing,it is found that the shape recovery rate of Cu66.3Zn16Sn15Fe2.5B0.2 single crystal is almost 0 when the pre-strain is 4%-6%and 8%due to poor thermal stability,so this single crystal does not exhibit SME property.The Cu63.8Zn19.5Sn14Fe2.5B0.2 single crystal fails to show SE property,which may be related to its single crystal orientation.