IMPURITY ATOM-DISLOCATION INTERACTIONS AND SUBSEQUENT EFFECTS ON MECHANICAL PROPERTIES OF REFRACTORY METALS

Studies were made of (1) the strain-aging behaviour and (2) the effect of pre-straining and high temperature annealing of Mo crystals of two different purity levels prepared by electron beam melting. One batch of crystals contained an average of 20 ppm total interstitial impurities, the other an average of 250 ppm interstitials with the major constituent being carbon. Yield point experiments revealed no strain-aging in either material in the ''as-grown'' condition, but after a high temperature anneal followed by rapid cooling the material containing C showed appreciable aging effects, and the purer material weak effects. The results are explained in terms of the low solid solubility of the interstitial elements in Mo under equilibrium conditions at moderate temperatures. The second series of experiments showed that pre-straining and annealing treatments which produce a substructure in Mo, also result in a strengthening of the material. The strengthening increases with increasing pre-strain and with increasing C content. The results indicate that sub-grain boundaries strengthen Mo in a qualitatively similar way to ordinary grain boundaries. (Author)