Effects of severe plastic deformation and heat treatment on transformation behavior of explosively welded duplex TiNi−TiNi

Abstract

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 Ti 3Ni4 in TiNi-l. Table 1 Transformation temperatures of original materialsSpecimens of 10 mmx l S mm were spark cut from the 0.7 mm thick sheet of explosively welded duplex TiNi-TiNi SMAs. They were cold rolled at room temperature to a 60% reduction in thickness. During the cold rolling, the duplex TiNi-TiNi sheets were sandwiched by stainless steel sheets to prevent fracture. After being cold rolled, the duplex sheet was submitted to various annealing treatments. A large range of annealing temperatures (from 573 to 973 K) and time durations (from 15 min to 10 h) were selected for the vacuum annealing under a pressure lower than 1.0xlO-2 Pa. The ExperimentalThe specimens used were explosively welded duplex TiNi-TiNi shape memory alloys. The two TiNi plates used for welding had dimensions of 200 mmx 100 mmx l mm. The chemical composition of TiNi-l was Ni-50.2 at% Ti and TiNi-2 was Ni-50 at% Ti. The transformation temperatures of these two TiNi plates measured with a differential scanning calorimeter (DSC) before welding are listed in Table 1, where M; M r , M p , As, A r and A p are the starting, finishing and peak temperatures of the martensitic and reverse martensitic transformation, respectively.

Welding and Joining of NiTi Shape Memory Alloys: A Review

“…Explosive welding for joining NiTi to itself was also studied by several authors [50][51][52][53][54][55]. In these studies, two different NiTi alloys, with different compositions and transformation temperatures, were used.…”

“…The objective was to obtain a NiTi/NiTi composite with macroscopic heterogeneities in chemical composition. In [50][51][52]54,55], after explosive welding of NiTi to itself a controlled thickness reduction was performed and the effect of heat treatments on the transformation temperatures to the welded joints was analysed. As expected, by a proper selection of the heat treatment temperature, the martensitic transformation could be adjusted so that the hysteresis of the transformation varied depending on the foreseen application.…”