The Effect of Particle Size and Spacing on Creep of Two-Phase Copper–Cobalt Alloys

Abstract

The effects of variations in particle diameter and spacing on the steady creep rate (έ s ) of three copper–cobalt alloys containing 0·88, 2·48, and 4·04%Co have been studied over a range of stresses at 712 K. At constant particle size and spacing, the stress (σ) and temperature-(T) dependence of έ s for each alloy can be described as <disp-formula> <mml:math> <mml:mrow> <mml:msub> <mml:mrow> <mml:mover> <mml:mi>ɛ</mml:mi> <mml:mi>˙</mml:mi> </mml:mover> </mml:mrow> <mml:mi>S</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mi>A</mml:mi> <mml:msup> <mml:mi>σ</mml:mi> <mml:mi>n</mml:mi> </mml:msup> <mml:mtext> exp</mml:mtext> <mml:mo>−</mml:mo> <mml:msub> <mml:mi>Q</mml:mi> <mml:mi>c</mml:mi> </mml:msub> <mml:mo>/</mml:mo> <mml:mi>R</mml:mi> <mml:mi>T</mml:mi> </mml:mrow> </mml:math> </disp-formula>A change in stress exponent, n, and activation energy for creep, Q c , occurs at a stress comparable with the macroscopic yield stress. It is proposed that with stresses < YS, when n ≃ 4·5 and Q c ≃ 140 kJ/mol, only limited deformation of the grains takes place and creep is attributable primarily to grain-boundary sliding and accommodating deformation. In the high-stress regime, when n ≃ 12 and Qc ≃ 210 kJ/mol, deformation occurs both in the grains and in the grain-boundary regions. For each alloy tested at σ > YS, έ s decreases with increasing particle diameter until a minimum value is reached and then increases rapidly with further increase in particle size. The decrease in έ s appears to be a result of the increase in work done by the dislocations in cutting particles as their size increases, until at maximum creep-resistance the dislocations are forced to bow between the particles. The creep rate increases with further increase in particle size and spacing as dislocation bowing becomes progressively easier. Under all conditions, the effect of particles on the creep rate is shown to be largely a consequence of their effect on the rate of recovery.