A study on a Ti52Ni47Al1 shape memory alloy

The Ti52Ni47Al1 alloy has 16% volume fraction Ti2Ni particles in the B2 mat rix with Ti2Ni particles having a higher Al content than the B2 matrix. Tra nsformation temperatures M* and A* of this alloy are lower than those of th e Ti51Ni49 alloy due to the solid solution of the Al atoms. M* and A* decre ase with increasing aging time at 400 degrees C because the Al atoms diffus e slightly from the Ti2Ni to the B2 matrix. The hardness increment of this alloy is more than that of the Ti51Ni49 alloy under the same degree of cold rolling. At the same time, M* and A* of this alloy can be more depressed b y thermal cycling than those of the Ti51Ni49 alloy, especially in the first ten cycles. All of these features result from the fact that this alloy has a higher inherent hardness due to the solid solution of the Al atoms. This also causes the R-phase transformation to be more easily promoted by both cold rolling and thermal cycling in this alloy. The strengthening effects o f cold rolling and thermal cycling on the M* (Ms) temperature of this alloy follows the expression Ms = T-0 - K Delta sigma(y), in which K values are affected by different strengthening processes. It is found that the higher the inherent hardness of the TiNi and TiNiX alloys, the higher the K values they have. (C) 1999 Kluwer Academic Publishers.