Effects of thermo-mechanical processing parameters on the special boundary configurations of commercially pure nickel

Abstract: The roles of plastic strain, annealing temperature, and annealing time in affecting the fraction of special boundaries (Fsp), and twin densities of thermo-mechanically processed commercially pure nickel are examined. Different strain levels were achieved by cold rolling the material at different amounts. One-step low strain-recovery processing with strain levels in the range of 3.0–7.5% and annealing temperatures in the range 800–1000°C were conducted in order to ensure that recrystallization did not occur. From orientation image microscopy analysis it was found that the fraction of special boundaries increased from about 30% for the as-received material to almost 80% for plastically deformed and annealed material. This showed that material strained in the range from 3.0% to 7.5% and annealed at 800°C for different times all reached Fsp values in the range 75–80%, a considerable increase over the as-received material. Various multi-processing cycle treatments did not increase the fraction of special boundaries to above the value of 80% achieved in single processing cycles. TEM observations indicated dislocation tangles/cells occurred near grain boundaries in material strained at 6%. The density of these dislocation tangles decreased with annealing time at 800°C and was reduced considerably after 20min. Experimental results for the variation of twin density with grain size showed that the twin density decreased with increase in grain size. However, there was a tendency for the twin density to decrease more slowly with increase in grain size in the more highly strained samples. The experimental data for twin density were compared with values calculated from the equation suggested by Pande et al. . With appropriate choice of the two variable parameters in the equation, a good “average” fit for all the data was obtained. However, the effect of plastic strain on the twin density could not be accommodated in the model. The experimental twin density–grain size relationship was also compared to values calculated from the formulation developed by Gleiter modified to accommodate the effects of prior plastic train on the twin densities of annealed samples. Calculations from the modified model followed the observed trend that twin densities for the more highly deformed samples decreased more slowly with grain size than those for lesser strained samples. Good quantitative agreement between the experimental values of twin density and those calculated from the modified Gleiter formulation was achieved. [Copyright &y& Elsevier]