Development of Processing Maps for Hot Forging of Cast NiTiHf

There has been increased interest in the Ni-rich NiTi-20Hf (at. %) alloy due to its combination of high temperature transformation capability and good shape memory properties. This interest has driven efforts to scale up production of this alloy, and recently, ingots of NiTi-20Hf have been produced in sizes reaching 500-lb and 9-inches in diameter. Smaller ingots have been successfully processed relatively economically via multi-pass extrusion. However, for the largest ingots, extrusion costs become prohibitively expensive, and thus, new processing routes such as forging need to be developed.

In order to determine appropriate processing parameters for hot working, Gleeble testing was used to evaluate the deformation behavior of cast Ni-29.7Ti-20Hf (at.%) material in compression at strain rates from 0.001 to 10sec^-1 and temperatures between 700ºC and 1100ºC. Flow stresses were measured from each curve with respect to strain and used to calculate related necessary parameters such as activation energy, strain rate sensitivity, and power dissipation. From these parameters, instability maps and processing maps were calculated to determine strain rate-temperature regions where high temperature compression produces either unwanted or desired microstructural changes. These processing maps were correlated with microstructural analysis of the as-deformed samples. Results of this study will be discussed with relation to selecting optimum processing parameters for hot forging of this alloy.