Development of Processing Maps for Hot Forging of Cast NiTiHf High-Temperature Shape Memory Alloy

A cast Ni50.3Ti29.7Hf20 (at.%) high-temperature shape memory alloy was hot deformed in compression at temperatures from 700 degrees C to 1100 degrees C and strain rates from 0.001 s-1 to 10 s-1. Tests were conducted in vacuum to true strains up to 90% using a Gleeble test system. The flow responses demonstrated a range of behaviors, with most samples exhibiting an initial yield followed by gradual flow softening, ultimately reaching a steady state at a lower flow stress indicative of dynamic recrystallization. In contrast, tests conducted at low temperatures and high strain rates exhibited a nearly flat steady state after yielding, suggesting dynamic recovery. The Prasad Dynamic Material Model was utilized to calculate processing maps from the flow stress data, supported by optical and electron microscopy microstructural observations. The analysis revealed that favorable processing conditions for the cast microstructure lie within a range of 800 degrees C to 1100 degrees C at strain rates between 0.001 s-1 and 0.1 s-1, allowing for optimization of processing parameters for enhanced microstructural control. These results provide valuable insights into the thermo-mechanical processing of NiTiHf high-temperature shape memory alloys, paving the way for future development of wrought processes and products for this alloy.