High Temperature Deformation Mechanism of Aluminum

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 Tc from pipe to bulk diffusion controlled regime was found to be about 735 K. Steady state creep rate εs, r and h were represented by εs=A(σ⁄E)nexp(−Qc⁄RT), r=B(σ⁄E)nrexp(−Qr⁄RT) and h=C(σ⁄E)nhexp(−Qh⁄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, nr, nh, Qc, Qr and Qh 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 Tc.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 εs=r⁄h, εs is represented by εs=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 Tc within a factor of 3. The difference between Qc and Qr was briefly discussed.