Study On The Negative Thermal Expansion Behavior Of Ti-rich Ti-Ni Alloys And Their Stability Under Thermal Cycling Conditions

In order to enrich current limited experimental and theoretical understandings of the negative thermal expansion(NTE)behavior of Ti-Ni alloys,and to provide solutions for controlling the thermal expansion reponse of materials in engineering applications,the NTE behavior and and its influence factors in Ti-Ni alloys were systematically studied.In this thesis,conventional cast and suction-cast Ti100-xNix(x=33.3~51.0)alloys were successfully fabricated by vacuum arc melting process combining with rapid solidification technique.Phase analysis(SEM),microstructural analysis(XRD)and thermal analysis(DSC)techniques were employed to characterize the microstructure,phase structure and phase transformation features of Ti-Ni alloys.The effects of phase and microstructure characteristics on the behavior of NTE were investigated.The intrinsic correlation between the martensitic transformation and the thermal expansion behavior of Ti-Ni alloy were clarified.Furthermore,the thermal cycling stability of NTE behavior of Ti-Ni alloys with different composition was studied.The results show that Ti-rich Ti-Ni alloys mainly consist of NiTi(B19?)phase and Ti2 Ni phase.The volume fraction of Ti2 Ni phase shows a linear dependence on the Ni content,which agrees with the lever rule.The solidified structure of both conventional cast and suction-cast alloys exhibits obvious preferential growth characteristics.The dendritic NiTi phases distribute along the direction of the maximum cooling rate in conventional cast alloys with less than 46.0at.% Ni,while the Ti2 Ni phases orient along the strip direction when Ni content is more than 46.0at.%.The NiTi phases and the Ti2 Ni phases in suction-cast Ti-Ni alloys are radially distributed along the maximum cooling rate direction.Conventional cast and suction-cast Ti-rich Ti-Ni alloys exhibit B19? ? B2 single step martensitic transformation.In addition,the martensite and austenite phase transformation enthalpy and thermal hysteresis of conventional cast alloys are larger than that of suction-cast alloys with the same composition.Ti-Ni alloys exhibit NTE behavior in a certain temperature range during heating and cooling processes.Due to the influence of the microstructure and composition of Ti-Ni alloys,the NTE of Ti-rich Ti-Ni alloys exhibit significant anisotropy.With increasing the Ni content,the absolute values of negative thermal expansion coefficients(NCTEs)in conventional cast and suction-cast Ti-Ni alloys increase along the maximum cooling direction,while the NCTEs perpendicular to the maximum cooling direction increase initially and then decrease,and the maximum value of NCTE during heating exists in Ti54Ni46.The NTE behavior also exists in the Ni-rich Ti-Ni alloys after aging treatments,and their NTE behavior originates from the volume change accompanying the B19? ? R phase transformation.With the increase of the aging time,the absolute values of NCTEs in conventional cast Ti49Ni51 decrease.Ti-rich Ti-Ni alloys awalys exhibit NTE behavior after the thermal cycle treatment.The NCTEs and the negative thermal expansion temperature interval(?TNTE)become stablised after a certain number of thermal cycle treatments.On the other hand,with the increase of the number of thermal cycles,the thermal expansion behavior of equiatomic Ti-Ni alloys experiences NTE?nearly zero expansion(ZNTE)?positive expansion(PTE)changes during heating process.However,the NTE response always exists during the cooling process after thermal cycle treatments.