Martensitic Transformation And Shape Memory Effect Of In-situ（TiB+La₂O₃）/TiNi Matrix Shape Memory Composites

TiNi shape memory alloys（SMAs）have has been widely used in aerospace,mechanical engineering and instruments due to their unique shape memory characteristics and superelasticity.However,the mechanical processing of TiNi alloy complex components is difficult,and its martensite yield strength is only 50²00 MPa.Also,its bearing capacity is insufficient.（TiB+La₂O₃）/TiNi composites were fabricated by hot-pressing sintering method based on the system of large and spherical TiNi powders and fine LaB₆ powders and expected to receive the good mechanical properties and strain recovery features.The results show that different grinding time and rotating speed have certain influence on the morphology of the mixed powder,and the sintering temperature also affects the density of the product.The optimum ball milling parameters are proved to be2 h and 200 rpm and Hot pressing sintering parameters have been optimized to be1020℃.TiB whiskers（about 600 nm）and La₂O₃ particles（about 300 nm）were obtained from in situ reaction by the low energy ball milling and vacuum hot press sintering between LaB₆ and TiNi matrix,scattered along the boundary of primary TiNi powders.Some long TiB whiskers has grown into the TiNi particle,effectively connecting adjacent matrix.For the same LaB₆ fraction of composites,the local reinforcement fraction increased with increasing the average matrix particle size.Otherwise,there were some agglomerate reinforcements existing in rich-TiB area with a irregular distribution.The addition of LaB₆ has a significant influence on the transformation temperature of（TiB+La₂O₃）/TiNi matrix shape memory composites.For the same substrate particle size of composites,only one endothermic and exothermic peak appears in the heating and cooling DSC curves except the composites with 0.5 wt.%LaB₆,indicating most composites undergo the only one-step transformation.With the increase of LaB6 addition,all transformation characteristic temperatures of composites with added LaB₆have been decreased and reduced to the minimum with 0.5 wt.%LaB₆.For the same LaB₆ fraction of composites,transformation characteristic temperatures increased with increasing the average matrix particle size.The test of mechanical properties shows that increasing LaB₆ content within a certain range can improve the strength of the composites when the particle size of the matrix is certain.When the additive amount of LaB₆ was 0.7 wt.%,the compressive strength and fracture strain of TiNi composite reached the maximum value,respectively2475.8MPa and 35.9%.For the same LaB₆ fraction of composites,the recovery strain increased with decreasing the average matrix particle size when the deformation does not exceed 10%,in reverse,the larger matrix particle size had greater recovery strain.When the size of the matrix particle is certain,the addition of LaB₆ within a certain range can increase the recoverable strain of the material.With 12%compress variation,the strain of composites added 0.7 wt.%LaB₆ could be recovered 9.36%after being heating up.