High Temperature Deformation Mechanism of Aluminum

Abstract

Recovery and work hardening of Al have been investigated at temperatures from 623 to 848 K and strain rates from 2×10⁻⁴ to 7×10⁻³ s⁻¹. Rates of recovery r (≡−∂σ⁄σt) and work hardening h (≡∂σ⁄∂ε) were obtained from stress relaxation analysis at steady states. The transition temperature T_c from pipe to bulk diffusion controlled regime was found to be about 735 K. Steady state creep rate \dotε_s, r and h were represented by \dotε_s=A(σ⁄E)ⁿexp(−Q_c⁄RT), r=B(σ⁄E)^n_rexp(−Q_r⁄RT) and h=C(σ⁄E)^n_hexp(−Q_h⁄RT), where A, B and C were constants, E the Young’s modulus, σ the applied stress, R the universal gas constant and T the temperature. n, n_r, n_h, Q_c, Q_r and Q_h were 4.0(4.6), 2.7(2.3), −1.5(−2.4), 140(110) kJ·mol⁻¹, 84(38) kJ·mol⁻¹ and −65(−73) kJ·mol⁻¹, respectively, where the values in the parenthises were those below T_c.
Based on a network growth model and an empirical dislocation density-strain relationship, r and h were roughly estimated. The model gives the correct order of magnitude ot them. Using \dotε_s=r⁄h, \dotε_s is represented by \dotε_s=4.5×10⁴(DGb⁄kT)(σ⁄G)⁴, where D is the coefficient of self-diffusion, G the shear modulus, b the Burgers vector and k the Boltzmann constant. The equation predicts the measured steady state creep rates above T_c within a factor of 3. The difference between Q_c and Q_r was briefly discussed.