T.S. Chou

The effects of cold rolling on the martensitic transformation of an equiatomic TiNi alloy

Abstract The effects of cold rolling on the martensitic transformation of an equiatomic TiNi alloy have been studied by internal friction and shear modulus measurements, hardness test and TEM observation. The martensite stabilization can be induced by cold rolling at room temperature. A variety of deformed martensite structures has been observed. Both deformed martensite structures and deform-induced dislocations/vacancies are considered to be related to the martensite stabilization. The hardness test results also support this viewpoint. After the occurrence of the first reverse martensitic transformation of B19′ → B2, the martensite stabilization dies out and the transformation temperatures are depressed by retained dislocation on subsequent thermal cycles. Experimental results indicate that the martensite stabilization can depress the rate of martensitic transformation in the equiatomic TiNi alloy.

A study of electrical resistivity, internal friction and shear modulus on an aged Ti49Ni51 alloy

Abstract The 400°C aged Ti 49 Ni 51 alloy can exhibit the transformation sequence of B2 →r premartensite R-phase →r martensite. In the early aging stage, only the premartensitic transformation is observed due to the M s point being deeply depressed by the coherent stress of Ti 11 Ni 14 precipitates. In the later aging stage, internal friction peaks associated with premartensitic and martensitic transformations are all observed on both heating and cooling. The sharp peaks associated premartensitic transformation on heating is believed to be related to the “bias” effect of the coherent stress induced by the Ti 11 Ni 14 precipitates. The serrations of internal friction appearing significantly in the temperature around −30 to −80°C are found to be caused by the stress induced accomodation of R-phase or martensite variants, and are not associated with the transformation. The Ti 11 Ni 14 precipitates can enhance the amount of martensite formed by unit of temperature or time during the martensitic transformation.

Aging effect on the low temperature internal friction relaxation peak in a Ti49Ni51 alloy

The internal friction relaxation peak appearing around 200 K in TiNi alloys is associated with the interaction of twin-related dislocations with the pinning agents. The 200 K relaxation peak in the 400 °C aged Ti 49 Ni 51 alloy appears at the R-phase and martensite but not at the B2 phase, since there is no twin-dislocation available in the latter. The sources of the pinning agents are due to the quenched-in vacancies or Ti 11 Ni 14 precipitate-vacated lattice sites in the matrix. It is found that 400°C aging will significantly affect the relaxation characteristic in Ti 49 Ni 51 alloy. These aging effects on the 200 K relaxation peak in Ti 49 Ni 51 alloy will be systematically investigated in this study.

The welding characteristics of Fe–30Mn–6Si and Fe–30Mn–6Si–5Cr shape memory alloys

Abstract After tungsten-inert gas (TIG) and laser welding, the microstructure, shape memory effect and chemical corrosion resistance of Fe–30Mn–6Si and Fe–30Mn–6Si–5Cr shape memory alloys have been investigated. Experimental results show that the dendrite structures in the laser-welded zones are finer than in the TIG-welded zones for both Fe–30Mn–6Si and Fe–30Mn–6Si–5Cr specimens. The Mn content in the TIG-welded zone of Fe–30Mn–6Si specimen is obviously reduced due to its vaporization during welding. The as-welded alloys still exhibit an excellent shape memory effect, with a variation of ±10% due to welding. The corrosion resistance of welded zones is worse than that of base material for both Fe–30Mn–6Si and Fe–30Mn–6Si–5Cr specimens. The degradation of corrosion resistance is ascribed to the micro-segregation and welding stress occurring within the welded zones. Meanwhile, the Cl − is more corrosive than the NO 3 − for Fe–Mn–Si shape memory alloys. After annealing treatment of 1100°C×2 h for these welded alloys, the dendrite structures in the welded zones disappear. The welded-then-annealed Fe–Mn–Si specimens still maintain the excellent shape memory effect, and their corrosion resistance is improved due to the annihilation of micro-segregation and welding stress.

A study of Fe-30Mn-6Si shape memory alloys prepared from different melting techniques

Newly developed shape memory alloys Ge-Mn-Si, governed by the stress-induced {gamma} {yields} {epsilon} martensitic transformation, are attracting attention due to their low cost and excellent workability. The shape memory effect (SME), although only a one way type, can be nearly perfect for alloys containing suitable amounts of manganese and silicon. However, there are few papers reporting the effect of the melting process, although it is an important factor affecting the material properties. Therefore, in the present study, the authors aim to investigate the casting morphology, shape memory effect, tensile property and fracture behavior for the Fe-30Mn-6Si alloys which are prepared by two melting techniques.

Characterization of internal friction of Fe-30Mn-6Si-5Cr

Fe-Mn-Si iron-based shape memory alloys have attracted much attention due to their low cost and excellent workability. Owing to its high sensitivity on constitutional structure, internal friction measurement has been used to investigate the martensitic transformations for thermoelastic alloys and ferrous alloys. In their previous paper, the internal friction characteristics due to {gamma}{yields}{epsilon} martensitic transformation have been examined by simple heating and cooling cycles. In the present study, the characterization of internal friction with the change in deformation states on an Fe-30Mn-6Si-5Cr shape memory alloy is investigated. In addition, the effect of thermo-mechanical training on the SME of this allow is also discussed.