The present work aimed to investigate the transformation behavior and strain recovery characteristics of Ni50.2Ti/Ni51Ti shape memory alloys （SMAs） prepared by explosive welding. The differential scanning calorimetry （DSC） results showed that the reverse transformation temperatures and the temperature range of NiTi-NiTi alloys increased with increasing prestrain level. Meanwhile, a two-stage strain recovery over a wide temperature range was obtained.
The study is a first attempt to prepare bulk NiTi/NiTi shape memory alloy （SMA） laminates with a macroscopic heterogeneous composition by explosive welding and investigate their microstructures and martensitic transformation behaviors. After explosive weld- ing, a perfect interfacial bonding between the two components and a reversible martensitic transformation are realized in the tandem. Results show achievement of a fine granular structure and the maximum value of microhardness near the welding interface because of the excessive cold plastic deformation and the high impact velocity during the explosive welding. Meanwhile, the effects of aging on the transformation of the welded tandem are investigated by differential scanning calorimeter （DSC） and subject to discussion. The trans- formation temperatures of NiTi/NiTi SMAs increase with the rise of the aging temperature. The experimental results indicate the shape memory properties of NiTi/NiTi SMA fabricated by explosive welding can be improved by optimizing the aging technology.
The effects of severe plastic deformation and heat treatment on the transformation behavior of explosively welded duplex TiNi-TiNi shape memory alloys （SMAs） were investigated by differential scanning calorimeter （DSC） measurements. The explosively welded duplex TiNi-TiNi plate of 0.7 mm thickness was cold-rolled at room temperature to a 60% reduction in thickness and then annealed at different temperatures for different durations. The results showed that low temperature （623-723K） heat-treatment led to the crystallization of the amorphous region, and re-crystallization occurred in the specimens annealed at higher temperatures （over 873 K）. Research indicated that the change of martensitic transformation temperature is due to the change of internal stresses with increasing heat treatment temperature. The change of annealing time also led to a change in martensitic transformation temperature, which was associated with the precipitation and decomposition of Ti3Ni4 in TiNi-1.
By designing the shape of the original surface curve(sine-wave and square-wave),macroscopic domains with different dislocation density were introduced into the TiNi alloys and materials were endued with the characteristics of composites. Dilatometer(DL) and the differential scanning calorimeter(DSC) were used to study the properties of in-situ composites of TiNi alloys. The results show that the reverse transformation temperature range of in-situ TiNi composites with sine-wave surface is significantly enlarged. However,two endothermic peaks appear on DSC curves in the first heating process for these samples with square-wave surface. It is presumed that the interactions between the dislocation texture and martensite variants introduced when the TiNi martensite is cold rolled at room temperature might be responsible to the phenomena,which are compared with cold-rolled sheets with flat surface. All the phenomena above show it is feasible to control the thermal properties of a material by a proper design of the dislocation texture.
A TiNi-alloy is solution-treated in a vacuum furnace of 6.7×10^-3 Pa at 1 223 K for 3.6 ks, and then aged at 773 K for 1.8 ks. The differential scanning calorimetry （DSC） measurements show that after the heat-treatment, takes place a two-step reverse Martensitic transformation, from which results the two-stage recovery strain in a prestrained Martensitic TiNi-alloy wire during heating. The Vickers microhardness indentation test in cross-sectional areas of the TiNi-alloy wire indicates the compositional heterogeneity between its surface layers and its inside. The fact that the sizes of the indentation in surface layers smaller than those in the inside bears witness to the existence of slightly harder surface layers. It is believed that these phenomena are related to the compositional fluctuation caused by the evaporation and oxidization of Ti-element during the solution-treatment and heterogeneity formed during the subsequent aging treatment.
JIANG Da-qiang CUI Li-shan ZHENG Yan-jun JIANG Xiao-hua
