Chaoying Xie

Effects of second phases on mechanical properties and martensitic transformations of ECAPed TiNi and Ti–Mo based shape memory alloys

Abstract TiNi and Ti-based shape memory alloys were processed by equal channel angular pressing (ECAP) at 673-773 K along B c route to obtain ultrafine grains for increasing the strength of parent phase and improving the functional properties. The effects of both thermodynamically stable and metastable second phases on the mechanical properties and martensitic transformations of these alloys were investigated. It is found that thermodynamically stable Ti 2 Ni phase has no effect on martensitic transformation and superelasticity of Ti-rich TiNi alloy, thermodynamically stable α phase is harmful for ductility of Ti–Mo–Nb–V–Al alloy, but metastable Ti 3 Ni 4 phase is effective for R phase transformation, martensitic transformation and superelasticity of Ni-rich TiNi alloy. The mechanisms of the second phases on the martensitic transformations and mechanical properties were discussed.

Effect of Aging on Superelastic Behaviors of a Metastable β Ti-Mo-based alloy

Effect of aging on the tensile and superelasticity (SE) behaviors of Ti-9.8Mo-3.9Nb-2V-3.1Al (wt.%) alloy have been investigated. It is found that the ultimate tensile strength (UTS) and yield strength of aged alloy change with the aging temperature following a nonlinear relationship due to ω-phase and α-phase precipitates. The SE, especially elastic recovery behavior, is closely related with the morphology of α-phase precipitates, as well as ω-phase formed during aging. At a maximum tensile strain of 4%, the specimen aged at 673xa0K with ωxa0+xa0α-phase precipitates shows a superelastic recovery strain of 3.2%. Lots of α-phase precipitates, distributing homogeneously in specimen aged at 773xa0K, result in the lost of SE. A maximum recovery strain of 3.3% is obtained for specimens aged at 873 or 973xa0K for 30xa0min, with coarse short bar-like α-phase precipitates.

Damping behavior of polymer composites with high volume fraction of NiMnGa powders

Polymer composites inserted with high volume fraction (up to 70 Vol%) of NiMnGa powders were fabricated and their damping behavior was investigated by dynamic mechanical analysis. It is found that the polymer matrix has little influence on the transformation temperatures of NiMnGa powders. A damping peak appears for NiMnGa/epoxy resin (EP) composites accompanying with the martensitic transformation or reverse martensitic transformation of NiMnGa powders during cooling or heating. The damping capacity for NiMnGa/EP composites increases linearly with the increase of volume fraction of NiMnGa powders and, decreases dramatically as the test frequency increases. The fracture strain of NiMnGa/EP composites decrease with the increase of NiMnGa powders.

Effects of ECAP and Aging on Mechanical and Superelastic Behaviors of Ti-Mo-Based and Ti-25at.%Nb SMAs

Ni-free Ti-based shape memory alloys (SMAs) are increasingly recognized as promising functional materials for medical applications. The mechanical properties of these metastable Ti-based SMAs are sensitive to aging and thermomechanical treatment. Effects of severe plastic deformation (SPD)-equal channel angular pressing (ECAP) and aging on superelastic behavior of Ni-free Ti-based SMAs, Ti-9.8Mo-3.9Nb-2V-3.1Alxa0wt.% (TMNVA) and Ti-25at.%Nb, have been investigated. The results show that the yielding strength of TMNVA alloy increases sharply with the number of ECAP processes—to greater than 1,400xa0MPa after two passes ECAP—but elongation of TMNVA alloy decreases severely and the plasticity is lost completely after two passes ECAP. Both ECAP process and flash annealing treatment have weak contribution to the superelastic recoverable strains of Ti-Mo-based alloy. For Ti-25at.%Nb alloy, after one pass ECAP process at 400xa0°C, the yielding stress increases obviously, and the recovery strain increases a little. With the further increase in the number of ECAP processes, the yielding stress and the recovery strain change little. Aging treatment at low temperature after ECAP process is in favor of superelasticity of Ti-25at.%Nb alloy. An almost completely recoverable strain of 1.5% is obtained in Ti-25at.%Nb alloy after two passes ECAP and aging at 300xa0°C for 1xa0h. The mechanisms of the effects of SPD and aging are also discussed.

Mechanical deformation and tensile super-elastic behaviors of a Ti-Mo based shape memory alloy

Ni-free shape memory alloys are promising functional materials for medical applications. A newly developed Ti-Mo based shape memory alloy shows superelasticity after thermomechanical treatment. However, the microstructure evolution and precipitation during thermomechanical processes are still not well understood. In the present paper, compressive deformation behavior at a series of temperatures of 298K - 973K and tensile deformation behavior of the alloy after aged at 573K - 973K have been investigated systematically. It is found that the compressive yield stress and ultimate compressive strength change with the deformation temperature. The ultimate tensile strength and yield stress of aged specimens also change with the aging temperature following a non-linear relationship. Microstructures of aged specimens as well as effects of lattice softening and aging-induced precipitates on the deformation behavior have been investigated and discussed.