Impact response of nickel in the 150-1150 K temperature range

Samples of polycrystalline nickel of high (99.9945%) and commercial (99.5%) purity were studied in a series of planar impact experiments with initial, preshot, sample temperature ranging from 150 to 1150 K. The free surface velocity of the impacted sample was monitored by velocity interferometer. On the base of the recorded waveforms, the temperature dependences of the dynamic tensile (spall) and the dynamic yield strength of the two kinds of nickel were determined. Both strength parameters of the two types of nickel decline gradually with temperature. The trend, however, is broken by strong anomalies of the yield strength near the Curie point of nickel, 630 K. While the yield strength of the high purity nickel shows a twofold increase over a narrow interval in close vicinity of the magnetic transformation, the strength of commercially pure material experiences, in the same temperature interval, a threefold drop approaching the strength level characteristic for high-purity nickel. The possible mechanisms of the observed phenomena are discussed.