Investigation On Martensitic Transformation And Shape Memory Effect At High Temperature In Co-V-X(X:Si,Al) Alloys

Due to its shape memory effect?SME?and superelasticity?SE?,shape memory alloys?SMA?have been paid attentions on as a kind of smart materials with high response frequency and large recoverable strains for the use of drive element.But in traditional SMA,the martensitic transformation?MT?temperatures are relatively low,not suitable for some application at evaluated temperatures.Therefore,researchers have found some SMA with high transformation points,namely high-temperature shape memory alloys?HTSMA?.However,some challenges are faced in the existing HTSMA systems:?1?At high temperatures,the thermal stability of martensite is low;?2?In the polycrystalline alloy,the inner brittleness of martensite transformed from intermetallics will deteriorate the ductility of HTSMA;?3?In some HTSMA having good overall properties,the costs of raw materials are relatively high.Therefore,continuing in exploring new HTSMA with good combination properties is significant.On the other hand,there are only a few reports about MT at high temperature in Co based alloys,which still need be developed further,especially in SME.Depending on this,based on the pre-studied thermodynamics information,phase diagram and first-principle calculations,in this paper,by focusing on the Co-V-X?X:Si,Al?alloy in the field of MT and SME at high temperature,the potential of these alloys as HTSMA is studied.The details in this research are as below:?1?The compositions of Co2VSi phase in Co-V-Si ternary system were determined by EMPA and XRD.Corresponding microstructures,crystal structures,phase transformation behaviors,mechanical properties and shape memory effect at high temperatures were experimentally investigated.It is found that a high MT temperature of 700 0C is obtained in CO63.5V17.0Si19.5 single martensite phase alloy,which shows good strength and plasticity with a max recoverable strain of 3.1%.However,during the heating process,parent phase transformed from martensite decomposes to A12 structure phase,deteriorating the thermal cycle stability of alloys.?2?The alloying effect of Al and Ga on the Co-V-Si ternary system is studied,mainly through the aspects of microstructures,crystal structures,phase transformation behaviors,mechanical properties and shape memory effect at high temperatures.The thermal cycle stability and microstructure evaluation during thermal process are mainly focused.It is found that the alloying elements lower the MT temperatures of Co-V-Si alloys,avoiding the bad effect of A12 phase.So the thermal cycle stability is enhanced especially in Co₆₄V₁₅Si₁₇Al₄ and Co64V15Si17Ga4 alloys.With the increase of alloying elements' contents,R phase precipitates from martensite.At the same time,the thermal process is along with?sCo?generating.In addition,the alloying deteriorates the mechanical properties,while increases the SME.?3?Rare earth elements Dy and Gd are doped into the modified Co-V-Si-?Al,Ga?alloys,corresponding microstructures,phase relationships,crystal structures,phase transformation behaviors,mechanical properties and shape memory effect at high temperature were investigated for the explanation of the Dy/Gd doping effect.It is found that after adding O.lat.%Dy/Gd into Co₆₄V₁₅Si₁₇Al₄ and Co64V15Si17Ga4 alloys,respectively,grains refinement in the microstructures was observed.Under the premise of maintaining the thermal cycle stability,the MT temperatures were increased slightly.Furthermore,the mechanical properties were also developed by doping effect.All the results suggest that these alloys have excellent properties.?4?The MT in Co-V-Al ternary system is studied experimentally and the microstructures,crystal structures,phase transformation behavior,mechanical properties and shape memory effect were investigated.Possible martensite stabilization was also studied via in-situ technologies.It is confirmed that a B2/L10 MT happens in Co-V-Al alloys,and the MT temperatures decrease with the rise of Al contents.At the same time,the mechanical properties were lowered,hindering the further expression of SME.On the other hand,martensite stabilization was proven in Co60V32Al8 and Co60V30Al10 alloys.Especially in Co60V30Al10 alloys,a special two-step reverse MT and two-stage SMEa at high temperature were obtained.