The mechanical behavior of polycrystalline hafnium: strain-rate and temperature dependence

The mechanical behavior of polycrystalline hafnium has been investigated over a wide range of strain rates (10−4–105 s−1) and temperatures (298–1023 K). Hafnium specimens were tested in compression and shear using servohydraulic machines, the compression Kolsky bar, and pressure-shear plate impact. The hafnium compression specimens show an unusual strain-hardening behavior, with strong but constant strain hardening for strains <10%, then very strong and increasing hardening up to strains of ≈25%, and finally decreasing hardening until the stress reaches a maximum (in compression). The maximum in the compressive stress–strain curve is associated with the onset of strongly inhomogeneous deformation in the specimen. At larger strains (beyond that corresponding to the maximum stress), the cylindrical hafnium compression specimens fail by localized shearing deformations involving the nucleation, growth and coalescence of microvoids. The flow stress sustained by hafnium is found to be strongly dependent on the rate of deformation over the range of strain rates investigated in this work. Further, the degree of strain hardening is also found to be larger at high strain rates. A 150% increase in flow stress is observed for a change in strain rate over eight orders of magnitude. Twinning is an important deformation mechanism in hafnium and increases with an increase in strain or strain rate. However, at high temperatures deformation occurs entirely by dislocation activity and no twinning is observed. Finally, it is shown that the compressive behavior of hafnium up to a strain of 10% can be modeled reasonably well using a modified form of the Zerilli and Armstrong model (for f.c.c. materials) for a broad range of temperatures (298–1023 K) and strain rates (10−4–103 s−1).