Low temperature peculiarities of plastic deformation in titanium and its alloys

Abstract

A number of unexpected phenomena of the mechanical behaviour of metals, due to the specific character of dislocation motion and interaction are analysed using the strength and plasticity characteristics of titanium of different grades of purity and its binary alloys with aluminium, vanadium, niobium and oxygen at 4.2–300 K. The non-monotonic temperature dependence of deforming stresses, which is inconsistent with the conventional thermal fluctuation concepts of the plastic deformation process, is considered along with the plastic flow instability appearing as drops of load in the stress-strain curve, and a low temperature increase in plasticity of the metals studied. The analysis of the effects of alloying, preliminary strain and sample sizes on the plastic flow instability parameters (onset temperature, amplitude and number of load drops) has shown that this may be accounted for by a combination of hypotheses of the dislocation array overcoming a barrier and the following local heating which results in a drop of load in the stress-strain curve. It has been found that substitution elements at concentrations corresponding to a homogeneous solid solution do not affect the usual behaviour of temperature dependence of yield stress O 0.2 typical of high purity titanium. The presence of the second phase due to high concentrations of the above elements reverses the sign of the do 0.2/dT derivative. Abov certain concentrations of oxygen atoms forming an interstitial solid solution, it becomes temperature independent. The comparison between the derived experimental results and theoretical concepts available shows that the former make allowance for increasing importance of the inertial mechanism of overcoming pinning centres by dislocations at low temperatures. The observed increase in plasticity of titanium and its alloys based on solid solutions at 40–120 K is attributed to a combined action of two deformation mechanisms - slip and lower temperature activated twinning.