Study On Large-size Single Crystals Of Shape Memory Alloy And Their Preparing Technique

The investigation of copper-based shape memory alloys has confirmed that, the maximal recoverable strain and fatigue property of shape memory alloy (SMA) can be significantly improved by means of preparing the anisotropic poly-crystal into the columnar crystal or the single crystal. Although the elastic anisotropy of TiNi alloy (A≈2) is much smaller than that of the copper-based alloy (A≈13), its shape memory property and mechanical property are also strongly dependent on the asymmetric and orientation.In this thesis, the unidirectional solidification equipment based on Bridgman method with high temperature gradient was designed, and the single crystal and/or the columnar crystal of Ti-50.0at%Ni and Cu-based alloy were successfully fabricated by this equipment as well as a selective growing zigzag-shaped crystallizer and a steady growth container that were made of electro graphite(Ф15-20×200mm ) . In addition, a novel quarternary Cu-based alloy Cu13.5Al3.5Ni0.5Be was designed. The heat treatment process was studied for these alloys. The microstructures of single crystal sample were studied by means of OM, SEM and EDS; the orientation of single crystal was measured by X-ray technology; the phase transformation points were determined by DSC. This may find a way to explore the preparation of the single crystal SMA with the excellent performance, especially the better shape memory effect and superelasticity. Main conclusions were obtained as follows:1. The unidirectional solidification equipment based on Bridgman method with high temperature gradient is compact, rational and systematical. The equipment can fulfill the requirement to preparing the single crystal of shape memory alloys. The single crystal of TiNi and Cu-based alloy is successfully fabricated by this equipment with a selective growing zigzag-shaped crystallizer. With the analysis of the interaction between the refractory and the melt of TiNi, it is showed that the electro graphite can be served as the material of a selective growing zigzag-shaped crystallizer and a steady growth container, but the domestic BN can not.2. The macro- and micro-structures of the ingot of TiNi and Cu-based alloy prepared by this method were verified as the single crystals. For the Cu-based ingot, there is no the grain boundary, the direction of growth is the preferred orientation with the better shape memory property. For the TiNi alloy, the microstructure is dendritic, there exists Ti2Ni intermetallic between the dendrites, the angle between the orientation of single crystal and [111] plane is about 15 degree, which is closing to the orientation of crystal with the best performance ([223] and [111]). In addition, a small amount of the carbon dissolved into the melt of the TiNi alloy and the carbides TiC formed. These carbids TiC particles can not become the nucleus of new grain, thus they almost have no influence on the further direction solidification growth of crystal grain, and also no effect to the shape memory property.3. The phase transformation points of TiNi and Cu-based alloys were determined by DSC. It is found that, all these alloys exhibit the typical feature of phase transition of SMA, all peaks of transition points, including Ms, Mf, As and Af are obvious in the DSC curve, the phase transition points vary around the expectation predicted by their chemical composition. All these data can support the further investigation for the property of alloys.4. Bending shape memory test, tensile test and thermal circle fatigue test were carried out, it is obtained that, the novel CuAlNiBe single crystal possesses the excellent shape memory properties, its maximum recoverable strain reaches about 10%, and the fatigue life and mechanical properties are also better than that of the poly-crystal alloys. For the TiNi alloy, the specimen with 500℃treatment exhibits the best shape memory property, the maximum recoverable strain is 10.8%, which is very close to the theoretical value (10.7%), and 25% higher than that of the poly-crystals. Simultaneously, the memory fatigue property is improved obviously. However, the plastic deformation yield strength of the alloy is very low due to no strengthening phases in the matrix of the equatomic TiNi single crystal.