Microstructures and Mechanical Properties of Shape Memory Alloy Using Pre-mixed TiNi Powders with TiO₂ Particles

Abstract

In this study, microstructural and mechanical properties of the extruded and heat-treated TiNi alloys by sintering the mixture of TiNi pre-mixed powder with titanium dioxide (TiO 2) particles were investigated. Pure Ti and pure Ni powder with TiO 2 particles were mixed and consolidated by spark plasma sintering (SPS). SPSed TiNi alloy compacts were extruded and heat-treated subsequently. SPSed TiNi alloy compacts had TiNi matrix and Ti 4 Ni 2 O phase. Ti 4 Ni 2 O phase was formed during SPS by reaction between TiNi matrix and oxygen atoms originated from additive TiO 2 particles. Consequently, the heat-treated Ti-50.5 at.%Ni alloy using pre-mixed powder with 1.0 vol.% TiO 2 particles showed a high plateau stress of 630 MPa and a good shape recovery of 79.7% in 8% strain applied. The heat-treated TiNi alloy with 1.0 vol.% TiO 2 particles revealed the high strength and good shape memory properties. The high strengthening mechanism of the TiNi alloy using pre-mixed powder with TiO 2 particles was mainly due to a decrease martensitic transformation temperature by an increase soluted Ni content in TiNi matrix after reaction between TiNi and TiO 2 .

Effect of Shape Memory Heat Treatment on Microstructures and Mechanical Properties of Powder Metallurgy TiNi Shape Memory Alloy

“…Figure 1 shows the dependence of plateau stress and shape recovery rate on the holding time of the SMHT. After the SMHT temperature at 773 K for 60 min which was the same condition in previous report, 9) the plateau stress was 370 MPa. The SMHT condition of the holding time over 60 min led to decrease the plateau stress.…”

“…In the previous study, 9) PM TiNi alloy fabricated by using the elemental mixture of pure Ti and pure Ni powders, having TiNi matrix phases with fine Ti 4 Ni 2 O dispersoids, showed dense microstructures without any large pores. In addition, the heat-treated TiNi alloy indicated an enough plateau stress to be possibly used as a shape memory alloy.…”

“…In addition, the heat-treated TiNi alloy indicated an enough plateau stress to be possibly used as a shape memory alloy. According to these results, this study also employed the same process conditions used in the previous study 9) for spark plasma sintering (SPS), hot extruding, homogenization and solution heat treatment to fabricate the same procedure. The temperature and holding time in the shape memory heat treatment were, however, changed to control the microstructures by intermetallic compounds precipitation in the matrix.…”

“…In the previous reports, TiNi shape memory alloys were manufactured from pure Ti powder and pure Ni powder by powder metallurgy process. 9) In that study, the improvement of mechanical properties such as the plateau stress was reported, and the mechanisms of the high-strengthened expression was elucidated. 9) Specifically, the homogenization and solution heat treatment led to more homogeneous structures and compositions, and the shape memory heat treatment resulted in fine Ti 3 Ni 4 precipitates.…”

“…9) In that study, the improvement of mechanical properties such as the plateau stress was reported, and the mechanisms of the high-strengthened expression was elucidated. 9) Specifically, the homogenization and solution heat treatment led to more homogeneous structures and compositions, and the shape memory heat treatment resulted in fine Ti 3 Ni 4 precipitates. It was confirmed that precipitation strengthening inhibited slip deformation in TiNi matrix phases during the martensitic transformation, which improved the shape recovery rate compared with TiNi alloys without shape memory heat treatment while the plateau stress decreased comparing with the solution heat-treated materials.…”