Investigations On Martensitic Transformation And High-Temperature Shape Memory Effect Of Co-V-Si Based Alloys

Shape memory alloys(SMAs)are one of the earliest intelligent materials,with superelasticity(SE)and shape memory effect(SME).In order to meet the requirements of engineering application and expand the service temperature of SMAs,the researchers began to study the SMAs with high transformation temperature.Up to now,Ti-Ni based,Co based,Ni based,Zr based,Ti-Pd based,Ti-Au based and Ta-Ru based alloys have been reported.Therefore,on the basis of the research on the Co-V-Si high temperature SMAs(HTSMAs)by our group,this thesis intend to adopt alloying to optimize the performances of Co-V-Si HTSMAs.In this study,the martensitic transformation(MT)and high temperature SME(HTSME)were studied in the Co-V-Si-X(X=Ge,Cr)quaternary system.Meanwhile,the phase equilibria in the Co-V-Sn system at two isothermal sections of 900? and 1000? were investigated,and the composition range of Heusler Co2VSn was determined,offering the theoretical basis for the subsequent research of Co-V-Sn system in SMAs field.The main contents of this paper are as follows:(1)The microstructure,crystal structure,the characteristics of MT,thermal cycle stability,microstructure evolution during thermal cycle,mechanical properties and HTSME of the Co64V16Si20-xGex(x = 2,4,6,8)alloys were studied in detail.A typical lamellar single martensite was observed in Co64Vi6Si18Ge2 alloy.With the increase of Ge content,the a-Co phase precipitated from the grain boundary.The two-phase microstructure of martensite and a-Co phase,was observed in Co64V16Si16Ge4 and Co64V16Si14Ge6 alloys.Co64V16Si12Ge8 alloy showed the three-phase microstructure,including martensite,a-Co phase and R phase.The reverse MT temperature increased by nearly 50? through adding Ge.The reversible MT peaks were observed during the ten times thermal cycles.The alloys showed a good thermal cycle stability and unchanged microstructure.So the thermal stability of the Co-V-Si HTSMAs can be improved by alloying Ge.In addition,alloying can improve the mechanical properties of the Co-V-Si alloy by the precipitation of the a-Co phase along the the grain boundary.However,with the further precipitation of the a-Co phase and R phase,the mechanical properties deteriorated.Co64V16Si18Ge2 and Co64V16Si16Ge4 alloys respectively possessed the SME of 1.6%and 1.4%under a pre-deformation of 10%.But their shape recovery rates decreased due to plastic deformation.(2)The microstructure,crystal structure,the characteristics of MT and mechanical properties of the Co64V16-xCrxSi2O(x = 1,2,3,4)alloys were studied in detail.Co64V15Cr1Si20 alloy presented a typical lamellar single martensite.The two-phase microstructure of martensite and a-Co phase was observed in Co64V14Cr2Si20 alloy.Co64V13Cr3Si20 and Co64V12Cr4Si20 alloys showed the three-phase microstructure,including martensite,a-Co phase and(V,Cr)3Co5Si2 phase.The L21 parent phase was unstable at high temperature and transformed into(V,Cr)3Co5Si2 phase,a stable phase with A12 hexagonal structure.The MT temperatures of Co64V14Cr2Si20 and Co64V13Cr3Si20 alloys were not significantly altered by alloying Cr.The mechanical properties of the alloys decreased with the addition of Cr.(3)The phase equilibria of the Co-V-Sn ternary system at the 900? and 1000?sections were investigated experimentally,especially the composition range of the Heusler-type ternary compound Co2VSn.It can be seen from two isothermal sections that the ternary compound Co2VSn was in the composition ranges of 36.7?57.6 at.%Co and 11.8?20.9 at.%Sn at 900?,becoming slightly wider at 1000?.The region of the Co2VSn at 900? and 1000? sections can be provided a theoretical basis for the study of Co-V-Sn as SMAs.