On the Stress- and Temperature-Dependence of Creep of Nimonic 80A

Abstract

The dependence of the steady creep rate, ε _s , on stress, σ, and temperature, T, for Nimonic 80A at 1023K 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:mo>)</mml:mo> </mml:mrow> </mml:math> </disp-formula> The values of the stress exponent, n (∼ 8.3) and the activation energy for creep, Q_c , (≃460 kJ/mol) are considerably higher than those for the nickel-20% chromium matrix material. These anomalously high values of n and Q_c are accounted for in terms of the stress- and temperature-dependence of the friction stress, σ₀, which is determined by a technique involving consecutive small stress reductions during creep. The stress- and temperature-dependence of ε _s for Nimonic 80A can then be represented 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:mrow> <mml:mo>(</mml:mo> <mml:mi>σ</mml:mi> <mml:mo>−</mml:mo> <mml:msub> <mml:mi>σ</mml:mi> <mml:mn>0</mml:mn> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> <mml:mn>4</mml:mn> </mml:msup> <mml:mtext> exp</mml:mtext> <mml:mo>(</mml:mo> <mml:msubsup> <mml:mi>Q</mml:mi> <mml:mi>c</mml:mi> <mml:mo>*</mml:mo> </mml:msubsup> <mml:mo>/</mml:mo> <mml:mi>R</mml:mi> <mml:mi>T</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> </disp-formula> where Q_c *, determined from creep rates recorded at different temperatures at the same value of (σ – σ₀), is 305 kJ/mol, which is close to that for creep and diffusion in the matrix material. This suggests that creep of two-phase alloys is controlled by processes occurring in the matrix.