Stress-strain curves in alpha Ti alloys—Some problems and results

Abstract

Stress-strain curves for alloys in the Ti−(0–15)Al system do not exactly obey a power law. As Al content increases, the uniform strain to ultimate load first decreases, then increases. Uniform strain and the strain hardening exponent correlate well experimentally in accord with mathematical requirements. This fact however does not prove that a power law best represents the data. The same is true of straight lines on plots of logdσ/d∈ vs log ∈ when the strain hardening exponent slightly exceeds unity, as it does for The Ti−(12, 15)Al alloys at large plastic strains. For these alloys, true stress may be described equally well by a relation of the form σ = σ" exp ∈ at large plastic strains. These observations support the view of Cass that new slip bands nucleate while old ones broaden via a cross slip mechanism in such a way that the mobile dislocations are those essentially that are moving into virgin crystal. It is proposed that the “doublem” behavior observed in the Ti−(12–15)Al alloys is a manifestation of slip band nucleation in the low plastic strain region followed by slip band growth in the high plastic strain region.