Investigations On The Abnormal Grain Growth And Properties Of Cu-Mn-Al/Ga Based Shape Memory Alloys

Cu-based shape memory alloys(SMAs)have good shape memory properties,excellent superelasticity,high electrical and thermal conductivity,and they are also easy in fabrication with low cost,thus Cu-based SMAs have attracted the attentions of many researchers and been widely used in varnous fields.However,Cu-based SMAs usually tend to grow into coarse grains,so the polycrystalline brittleness is one of the biggest obstacles to the development of Cu-based alloys.At present,this problem is mainly solved by refining grains,reducing the order degree of the parent phase,and preparing for single crystals.In this paper,Cu-Mn-Al/Ga-based cast alloys with a unique microstructure of the L21 parent phase and the bcc nano precipitates completely coherent with the parent phase were designed by bcc phase separation.The abnormal grain growth(AGG)can occur when directly annealing cast alloys,and the single crystals over cm-scale were obtained.In this paper,Cu-Al-Mn-V alloys and Cu-Mn-Ga-Fe(-Al)alloys were prepared.Their microstructure,phase transition characteristics and functional properties,and the corresponding mechanism of the AGG of the alloys were studied in detail by means of metallography,EPMA,DSC,TEM,EBSD,compressive tests and so on.On the other hand,this paper investigated the microstructure,martensitic transformation characteristics and mechanical properties of alloys by adding trace amounts of Zr/Nb in Cu-Al-Mn-Fe alloys to analyse the relationship amorng precipitates,martensite stabilization and shape memory effects.The main contents of this thesis are as follows:(1)The Cu-Al-Mn-V cast alloys containing L21-CU2AlMn parent phase and the A2(V)phase which was completely coherent with the parent phase were annealed at 1173K,then the grains grew abnormally.After quenching,the single crystals over cm-scaie were obtained.During the annealing process,the A2(V)nanoparticles dissolved into the matrix phase,resulting in a continuous misornentation gradient with a deviation below 2,which provided a driving force for the rapid migration of the grain boundaries leading to the AGG.The single crystals exhibited excellent superelasticity.When the orientation of the single crystal was close to[901]:Cu-13.7Al-8.6Mn-1.4V single crystal showed the largest full superelasticity strain of 7.0%,and Cu-13.2Al-9.8Mn-0.7V single crystal owned the largest superelastic strain of 5.4%.When the orientation of the single crystal was close to[052],the superelasticity was weakened,and when the orientation of the single crystal was close to[523],the superelasticity substantially disappeared.(2)The Cu-Mn-Ga-Fe cast alloys contained the L2,-Cu2MnGa parent phase and the a(Fe)phase which was completely coherent with the parent phase.After annealing at 973K,the grains growed abnormally in the alloys.The dissolution of the a(Fe)particles played a key role in the AGG.Although a large stress platform according to the reorientation of martensite variants was observed in Cu-13.3Mn-20.7Ga-1.5Fe and Cu-14.3Mn-19.9Ga-l.5Fe single crystals,these single crystals did not show any shape memory effect.It indicated that thermal stability of these single crystals was very poor.In this study,Al was added into Cu-Mn-Ga-Fe alloys to improve the thermal stability.The bcc phase separation also occured in Cu-Mn-Ga-Fe-A1 cast alloys.After annealing at 973K,the AGG happened,and the single crystals over cm-scale were obtained.The single crystal orientations of Cu-13.9Mn-16.1Ga-4.4Al-2.8Fe alloy and Cu-13.6Mn-12.2Ga-8.6Al-2.6Fe alloy were both cdose to the[100],and they both showed excellent superelasticity and good fatigue resistance.Cu-13.9Mn-16.1Ga-4.4Al-2.8Fe single crystal showed the largest full recovery of 10%and the largest superelastic strain of 6.5%,Cu—13.6Mn-12,2Ga-8.6Al-2.6Fe single crystal owned the largest full recovery of 11%and the largest superelastic strain of 7.2%.These single crystals almost remained full superelastic characteristics during 230 and 500 times under a fixed pre-strain of 9%,showing good fatigue resistance.In addition,it was confirmed that the Cu-Mn-Ga-Al-Fe single crystals were a kind of ferromagnetic shape memory alloys with a certain soft magnetic properties.(3)Cu—AI-Fe—Mn-Zr/Nb cast alloys also exhibited bcc phase separation.After annealing at 1173K,the microstructure of the alloys consisted of L2,-Cu2AlMn parent phase,a small amount of 2H martensite produced by quenching and?(Fe,Al,Zr)/?(Fe,Al,Nb)phase,which was completely coherent with the parent phase.The results showed that the anmounts of p(Fe,Al,Zr)or ?(Fe,Al,Nb)phase increased obviously with Zr/Nb additions.Thus when annealing their cast alloys,the AGG did not happen.Although the alloys showed L21 arent phase structure at room temperature,the stress-induced martensite stabilization can easily occur even if under the compression pre-strain of 6%by adding trace amounts of Zr/Nb in Cu-Al-Mn-Fe alloys.This martensite stabilization process resulted in the shape memory effect of the alloys.In(Cu78Al2Fe4FMn6)100-xZrx(x=1and 2)alloys and(Cu78Al12Fe4Mn6)100-yNby(y= 1 and 2)alloys,the obtained maximum values of shape memory effects were 4.1%,4.4%,4.4%and 4.0%respectively.