The Correlation of the DSC Response and Fine Structure of Hot Worked NiTi Alloy

The transformation temperature of hot worked NiTi alloy products such as bar and large diameter wire are commonly reported after heat treatment. The samples are solution annealed in the temperature range of 800oC to 950oC which may be considered a full anneal. This is in contrast to the temperatures used to shape set cold worked products and components which are heat treated in the range of 350oC to 550oC. Heat treatment in this temperature range results in precipitation of Ni₄Ti₃ with high residual dislocation density. Hot worked samples, annealed at 850oC, sometimes show an unusual calorimetric response with wide hysteresis and depressed A_s temperatures. This paper attempts to explain the source of the unusual transformation temperature behavior.

Samples of hot worked bar and wire were heat treated at 850^oC and evaluated by calorimetry and thin foil electron microscopy. No precipitates were observed in any sample. However, differences in the DSC response were correlated to differences in the fine substructure of the product. A substructure with higher dislocation density and residual martensite correlated to a good DSC response. A substructure with lower dislocation density and no residual martensite resulted in heterogeneous nucleation of martensite and austenite and wide hysteresis. These differences are important for the control of properties in hot worked products.