Bimetallic shape memory alloy composites produced by explosion welding: Structure and martensitic transformation

The aim of the present work was a study of the influence of explosion welding on the grain structure, chemical composition, phase composition and phase transitions in bimetallic shape memory composites containing the TiNi shape memory alloy as an active layer and one of four alloys having different abilities to react with TiNi alloys as passive layer. Stainless steel was chosen as a non-reacted material, TiNi alloy was used as the alloy with close chemical composition, and Ti6Al4 V alloy and copper beryllium (C17200) alloy were chosen as materials which might react with TiNi alloy to form new phases. The results of the study showed that variations in the grain structure and properties of the layers in the bimetal composite depended on the ability of the alloys used for the production of these composites to react with TiNi alloys. If the alloys did not react with TiNi alloy, as in the case of stainless steel, the explosion welding resulted in significant plastic deformation of the layers. This led to a change in the grain morphology, a deep decrease in transformation temperatures and a suppression of the martensitic transformation (a decrease in the volume fraction undergoing phase transformation). In this case, annealing at high temperatures had to be used for recovery of the grain structure and properties of the composite before application. If the alloys were able to react with TiNi alloy to form the intermediate phases, for instance Ti6Al4 V alloy, the explosion energy was then expended for the formation of the intermediate layer and the plastic deformation of the basic layers was small. This composite retained the parameters of the martensitic transformation and grain structure and should be subjected to annealing at low temperature to decrease the internal stresses appeared during welding.