Fabrication Of Ti-rich Ti-Ni Alloys And Characterization Of Their Negative Thermal Expansion Performance And Phase Transformation Behavior

The main purpose of the present work is to investigate the negative thermal expansion(NTE) behavior of Ti-rich Ti-Ni alloys, to explain the reasons for NTE behavior, and to fabricate Ti-Ni based composites with controllable coefficient of thermal expansion(CTE). At first, Ti-rich Ti-Ni alloys with various Ni concentrations ranging from 33.3 at.% to 50 at.% have been successfully fabricated using vacuum arc melting method and suction casting technique. The dependence of NTE behavior on Ni concentration for as-casted, as-quenched, as-annealed Ti-rich Ti-Ni alloys was studied in detail. Further, according to the study on phase transformation behavior and phase compositions of Ti-rich Ti-Ni alloys under above three heat-treating conditions, the rule and the fundamental reasons for NTE behavior of Ti-rich Ti-Ni alloys have been explained successfully. Finally, mechanical properties of Ti-rich Ti-Ni alloys under cyclic compressive loadings, and the dependence of CTE of porous Si C/Ti-Ni and Pb Ti O3/Ti-Ni composites which were fabricated by powder metallurgy process have been studied.In this study, it is shown that Ti-rich Ti-Ni alloys with certain Ni concentration exhibit NTE behavior during heating and cooling processes. The fundamental reason for the presence of NTE behavior is the occurrence of thermoelastic forward and reverse martensitic transformations, although the measured NTE strains are far lower than the maximum NTE strain caculated theoreitcally according to the lattice transition correspondence during phase transformation. During the heating process from 30 to 200 ℃, Ti-rich Ti-Ni alloys with Ni concentration ranging from 38 at.% to 50 at.% exhibit clear NTE behavior, meanwhile, the absolute value of CTE in the NTE temperature range and NTE strain increases firstly and then decreases with increasing Ni concentration. On the other hand, Ti-Ni alloys possessing Ni concentrations beyond the above Ni concentration range do not show NTE behavior. There exist certain releationship between the NTE and phase transformation behavior, where the temperature interval of NTE is shifted to higher temperature region by about 25℃ compared to the temperature interval of martensitic phase transformation. The dependence of NTE and phase transformation behavior on Ni concentration for as-casted, as-quenched and as-annealed Ti-rich Ti-Ni alloys is similar. However, for Ti-rich Ti-Ni alloys with same Ni concentration and subjected to different heat treating conditions, their NTE start temperature(Ts), NTE finish temperature(Tf), phase transformation start temperature(As), phase transformation finish temperature(Af) show the following rule: as-casted>as-quenched>as-annealed, but the NTE strain follows that as annealed<as-casted<as-quenched.Studies on the mechanical performance of Ti-rich Ti-Ni alloys have been done, and results show that the maximum stress under cyclic compression under a 4% strain level firstly increases and then decreases with increasing Ni concentration, while the residual strain consistently increases with increasing Ni concentration. The absolute value of CTE in NTE temperature range of Si C/Ti-Ni composites has negative linear relationship to the Si C content, and the relationship conforms to the revised ROM(Rule of Mixtures). However, the dependence of CTE of Pb Ti O3/Ti-Ni composites on sccond-particles is similar to Si C/Ti-Ni composites, which means that the anticipated target of broadening the NTE temperature interval of Pb Ti O3/Ti-Ni composites has not been achieved.