A new analysis of yielding and work hardening in AA1100 and AA5754 at low temperatures

摘要

Microstructural evolution determined by mechanical testing is usually modelled from perceived mechanisms of work hardening dur ing dislocation passage and storage in complex arrays. The predicted stress-strain behaviour is then compared with the measured one. Recently a new functional constitutive relation was derived by Saimoto and Van Houtte based on Taylor slip analysis that is optimally fitted to replicate the measured data. The curve fitting parameters arise from the incremental strain Δγ producing a mean slip distance λ and the retained dislocation density ρ assessed from the flow stress τ. A method has been derived to precisely determine λ from the mea sured data. Moreover, the yield stress determined from the optimum curve fitting is derived from strains beyond the yield point and, hence, has the capacity to separate the solid solution and grain size effects from that of the yield phenomenon. Typically two fit curves are required, the intersection of which defines τ_3, γ_3 and λ_3, akin to the stage II to stage III transition in single crystals. A tabulation of these parameters permits reproduction of the mechanical curves. Furthermore, a plot of flow stress x vs.1/λ results in a description of work hardening evolution onto which the inverse cell size 1/d_C can be superimposed. Using the results from standard materials testing practice the temperature dependence and evolution of the pertinent parameters show that point defect drag effects are prominent below 273 K for AA1100 but not for AA5754.