Transformation superplasticity of iron and Fe/TiC metal matrix composites

Unreinforced iron was thermally cycled around the alpha/gamma phase field under an externally applied uniaxial tensile stress, resulting in strain increments which could be accumulated, upon repeated cycling, to a total strain of 450 pct without failure. In agreement with existing theory attributing transformation superplasticity to the biasing of the internal allotropic strains by the external stress, the measured strain increments were proportional to the applied stress at small stresses. However, for applied stresses higher than the nominal yield stress, strain increments increased nonlinearly with stress, as a result of strain hardening due to dissolved carbon and iron oxide dispersoids. Also, the effects of transient primary creep and ratchetting on the superplastic strain increment values were examined. Finally, partial cycling within the alpha/gamma phase field indicated an asymmetry in the superplastic strain behavior with respect to the temperature cycling range, which is attributed to the different strengths of ferrite and austenite. Transformation superplasticity was demonstrated in iron-matrix composites containing 10 and 20 vol pct TiC particles: strain increments proportional to the applied stress were measured, and a fracture strain of 230 pct was reached for the Fe/10TiC composite. However, the strain increments decreased with increasing TiC content, a result attributed to the slight dissolution of TiC particles within the matrix which raised the matrix yield stress by solid-solution strengthening and by reducing the transformation temperature range.