The temperature dependence of the flow stress of hafnium having a bamboo structure has been investigated from 77 to 500 K for selected oxygen concentrations and the activation parameters have been evaluated. In addition, the dependence of the flow stress on nitrogen concentration at testing temperatures of 77 and 500 K has been determined. Nitrogen was found to increase the flow stress more than oxygen. The corresponding hardening coefficients Δσ/Δc at 300 K are 12 kg mm−2 (at.% O)−1 for Hf-O and 38 kg mm−2 (at.% N)−1 for Hf-N. Special attention has been paid to a perturbation or break in the flow stress-temperature curves of Hf-O alloys. Electron microscopy and electrical resistivity served as additional means of characterizing the nature of the obstacles in the Hf-O specimens. p]The onset of plastic deformation between 77 and 500 K is described using two mechanisms: interstitially dissolved oxygen atoms (weak obstacles) and oxygen clusters (strong obstacles). The transition occurs at about 350 K. Below this temperature thermally activated deformation is controlled by the interaction between dislocations and single interstitial oxygen atoms. In the low temperature region the flow stress is composed of a temperature and concentration dependent component and a second, concentration dependent but temperature independent, component which is due to the oxygen clusters. Above about 350 K single interstitial oxygen atoms are essentially transparent to the moving dislocation, and the observed decrease in flow stress with temperature is attributed to cluster redistribution.

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