| As a kind of metallic intelligent materials,shape memory alloys have got more and more attentions and applications in daily life and military fields.Among the discovered shape memory alloys,the Cu-based shape memory alloys are the most widespread,which can be mainly divided into the Cu-Al and Cu-Zn systems.Among them,Cu-Zn-Al,Cu-Al-Ni and Cu-Al-Mn shape memory alloys are considered to have practical value.The Cu-based shape memory alloys have the advantages of excellent shape memory effect,superelasticity,low cost,good electrical and thermal conductivity and wide adjustable range of phase transition temperature,which makes them suitable for the production of various low demand execution and drive components.However,the parent phases of Cu-based shape memory alloys are all ordered body-centered cubic structure,so the problem of polycrystalline brittleness has become one of the biggest obstacles restricting their development.Up to now,the methods to improve polycrystalline brittleness mainly include grain refinement,reducing the order degree of the parent phase and preparing single crystals.Our research group found when adding a small amount of Fe into the Cu-Al-Mn alloys,the abnormal grain growth can occur through directly annealing cast alloys without deformation,and the large grains over centimeter level can be obtained.But up to now,the abnormal grain growth are only observed in few Cu-Al-Mn-Fe alloys,and the range of alloy composition where the abnormal grain growth can happen and the optimization design of the alloy composition need to be further studied.As a result,the phase equilibrium of the Cu-rich side in the Cu-Al-Mn-Fe alloy system,including the phase equilibrium of the bcc phase region and the bcc two-phase separation region,plays a guiding role in the subsequent optimization design of the alloy composition and the control of the microstructure.In this paper,the isothermal sections of(90.86Cu-9.14Mn)-Al-Fe(at.%)((92Cu-8Mn)-Al-Fe,wt.%,since most phase diagrams are expressed in atomic ratio,the mass ratio is converted into atomic ratio)system at 700～900℃ were determined by using alloy method,electron probe(EPMA),X-ray diffraction(XRD)and other techniques.In this paper,two Cu-Al-Mn-Fe polycrystalline alloys were designed and prepared on the basis of phase equilibrium,and their microstructure,phase transformation characteristics and functional characteristics were investigated in detail by metallography,EPMA,compression test,TMA,DSC and TEM.Additionally,in this paper,the Cu-Al-Mn-Fe-Ga single crystal alloys were prepared by adding Ga to Cu-Al-Mn-Fe single crystal alloys,and their microstructure,reversible martensitic transformation and mechanical properties were investigated.The main contents of this paper are as follows:(1)On the basis of the known phase equilibrium of the Cu-Al-Fe ternary system,the phase equilibria in the Cu-rich portion of(90.86Cu-9.14Mn)-Al-Fe(at.%)system at 700～900℃ was determined experimentally,including the bcc phase region and the bcc two-phase separation region.The results show that at 700℃,800℃ and 900℃,the composition range of(Cu)phase is very narrow,and the maximum solid solubility of Fe is 2,64 at.%,2.2 at.%and 5.05 at.%,respectively.The composition range of the bcc phase is very wide,extending from the Cu-Mn rich side to Fe-rich side.(2)Two polycrystalline alloys of Cu-12.9Al-4.5Mn-2.6Fe and Cu-12.9Al-3.7Mn-3.6Fe(wt.%)were prepared.Their microstructure is composed of 18R martensite,2H martensite and bcc β(FeAl)nanoparticles.Among them,the bccβ(FeAl)nanoparticles are Fe-rich,Mn-rich and Cu-poor,while the martensite is Cu-rich,Fe-poor and Mn-poor.The results of the reversible martensite transformation of Cu-12.9Al-4.5Mn-2.6Fe and Cu-12.9Al-3.7Mn-3.6Fe polycrystalline alloys show that the forward martensitic starting(Ms)and finishing(Mf)temperatue are respectively 315K and 310K for Cu-12.9Al-4.5Mn-2.6Fe alloy,378K and 357K for Cu-12.9Al-3.7Mn-3.6Fe alloy.The reverse martensitic starting(As)and finishing(Af)temperature are respectively 341K and 352K for Cu-12.9Al-4.5Mn-2.6Fe alloy,381K and 395K for Cu-12.9 Al-3.7Mn-3.6Fe alloy.The reversible martensitic transformation temperature of the studied alloys is between room temperature and 373K,which are suitable for practical applications.In addition,the martensite transformation temperature of the alloy increases with the decrease of Mn content,which is consistent with the previous reports.The research of the functional characteristics of Cu-12.9Al-4.5Mn-2.6Fe alloy shows when the applied pre-strain ranges from 5～8%,the shape memory effect is 2.5%,3.3%,4.1%,and 4.4%,respectively,along with a shape recovery rate of 100%.It can be seen that the polycrystalline alloy exhibits excellent shape memory effect.The research of the functional characteristics of Cu-12.9Al-3.7Mn-3.6Fe alloy shows when the applied pre-strain is 5～6%,it also reveals a shape recovery rate of 100%and the shape memory effect is 2.4%and 2.7%,respectively.When the pre-strain reaches 7%,the shape recovery rate of Cu-12.9Al-3.7Mn-3.6Fe alloy drops to 88%,accompanied by shape memory effect of 3.0%.When the deformation level is up to 8%,the shape recovery rate of Cu-12.9Al-3.7Mn-3.6Fe alloy further decreases to 67%,with shape memory effect of 3.1%.From the stress-strain curve,it can be seen that when large deformation is applied,the Cu-12.9Al-3.7Mn-3.6Fe alloy undergoes plastic deformation due to high external stress,which leads to a significant decrease in the shape recovery rate.(3)Based on the results of the previous studies on Cu-Al-Mn-Fe single crystal alloys,Cu-10Al-6Mn-3Al-4Ga,Cu-12Al-2Mn-3Fe-4Ga and Cu-12Al-3Fe-4Ga single crystal alloys were prepared through alloying Ga.The Cu-10Al-6Mn-3Al-4Ga single crystal alloy consists of martensite and bcc nanoparticles.The nanoparticles are the Fe-rich,Mn-poor and Cu-poor,while the martensite is Cu-rich,Fe-poor and Mn-poor.The results of functional characteristics of Cu-10Al-6Mn-3Fe-4Ga single crystal alloy show when the applied pre-strain is located at 6-11%,the shape memory effect is 5.6%,6.4%,7.3%,8.2%,8.6%and 9.2%,respectively,with a shape recovery rate of 100%.The results of functional characteristics of Cu-12Al-2Mn-3Fe-2Ga single crystal alloy show when the pre-strain does not exceed 7%,the alloy exhibits perfect superelasticity.That is,the stress-induced martensite leads to stress platforms in the curves,and the martensite transform back to the parent phase through reverse martensite transformation after unloading.When the pre-strain is in the range of 8～12%,the alloy exhibits shape memory effect of 7%,with a shape recovery rate of 100%.The deformed alloy can restore to its original shape when placed at room temperature for a period of time. |
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