J. Li, China university of Petroleum,Beijing, Beijing, China; Y. Zheng, L. Cui, China University of Petroleum, Beijing, China
In-situ shape memory alloys (SMAs) composites had been produced by designing the shape of the original surface curve and cold rolling. If the heterogeneous in a SMA is magnified in such a way that macroscopic distinct domains are aligned regularly, then the SMA has a structure similar to those of composites. As a result, two peaks appeare on the DSC curves in the first heating process, which correspond to different deformation regions in these in-situ SMAs composites respectively. In this paper it has been shown that in-situ composites of TiNi alloys can overlap a desirable (also adjustable) strong negative strain over a moderate negative strain compared to directly cold-deformed TiNi alloys. Composites with a small prestrain level show a larger hysteresis upon heating than those with a large prestrain level. We presumed that the interactions between the dislocation texture and martensite variants were introduced when the TiNi martensite was cold rolled at room temperature might be responsible to the phenomena. All the phenomena above showed it is feasible to the method for controlling the thermal properties of a material by a proper design of the dislocation texture.
Summary: The martensitic transformation and associated thermal expansion behaviors of a TiNi alloy were studied in this paper. The main statements that can be drawn from this study are as follows. Cold deformations higher than the recoverable limit cause massive dislocations in the TiNi alloy. By controlling the ratio of the cold deformation, the density of dislocations can be managed, and therefore the thermal-mechanical behaviors of the TiNi alloy can be tailored due to the interactions between the martensite variants and the dislocation texture. Composites with a small prestrain level show a larger hysteresis upon heating than those with a large prestrain level. The absolute value of the strain rate which a TiNi composite contracts decreases with increasing level of prestrain. As a result of the purposive design, two peaks appear on the DSC curves of all Wave-type samples in the first heating process, which correspond to different deformation regions in these in-situ SMAs composites respectively. And due to the interactions between the domains with different dislocation density, the reverse transformation expands over a large temperature window. This is quite useful to output strains in a large temperature window comprising to conventional SMA composites interface which will be failure in the same temperature. As a example of applications in the so-called temperature memory effect, one do not have to embed SMAs into composites; instead, one just have to cold roll a curved surface TiNi alloy to above its recoverable limit to obtain a material with the ability of temperature memory in a wide temperature range. In conclusion, all the phenomena above show it is feasible to the method for controlling the thermal properties of a material by a proper design of the dislocation texture.
