A study of TiNiW ternary shape memory alloys

• Main Authors: CHUANG CHENG AN, 莊正安
• Other Authors: HSIEH SHIH FENG
• Format: Others
• Language: zh-TW
• Online Access: http://ndltd.ncl.edu.tw/handle/33149226202413718116

碩士 === 國立高雄應用科技大學 === 模具工程系 === 100 === In this study, the phase transformation behavior, microstructures, shape memory recovery and electrical resistivity of the thermo-mechanical treatment of the ternary Ti50Ni49W1, Ti50Ni48W2 and Ti49Ni50W1 alloys were investigated. Several second phase particles Ti2(Ni,W) and -W located around grain boundaries are observed. The lattice parameters of B19’martensite of the Ti50Ni49W1 alloy are calculated from the XRD profiles, a=2.901Å、b=4.122Å、c=4.652Å and β=97.58∘and all of them are larger than those of Ti50Ni50 alloy. The transformation sequence of TiNiW ternary alloys has a one-stage B2B19’ transformation. The shape memory effect of these alloys can be improved by the W solid-solution hardening. The transformation sequence of the Ti49Ni50W1 alloy aged at 300℃ can exhibit the two-stage B2RB19’ transformation. The R-phase transformation can be induced due to the Ms temperature being deeply depressed by the coherent stress of Ti3Ni4 precipitates, and the electrical resistivity initially increases and up to level during the aging process. Compared with nickel-rich TiNi binary alloy, the nucleation and growth rate of Ti3Ni4 precipitates can be retarded on account of the W atoms solid solution in TiNi alloys. However, the nucleation rate of Ti3Ni4 precipitates of the cold-rolled Ti49Ni50W1 alloy is faster in the initial aging stage due to much more nucleate sites formed in dislocations, resulting in the electrical resistivity decrease sharply. Precipitation strengthening can also improve the SME characteristics. Ti50Ni49W1 and Ti50Ni48W2 alloys specimens with and without cold-rolling are subjected to thermal cycling. The transformation sequence of these alloys can be changed from B2→B19’ to B2→R→B19’ by increasing the thermal cycling number. These results arise from thermal stress introduced dislocations. The hardness increment and transformation temperature depression of Ti50Ni48W2 alloy are more than those of Ti50Ni49W1 alloy under the same thermal number due to the former alloy having a higher inherent hardness from 2 at.% W atoms solid-soluted in TiNi alloy.