Mobility of the β₁-γ ₁ ^′ martensitic interface in Cu-Al-Ni: Part I. Experimental measurements

The mobility of the β ₁-γ ₁ ^′ martensitic interface in thermoelastic β-Cu-Al-Ni alloys has been investigated using stress-assisted single interface transformation over the temperature range 180 to 410 K. The imposed interfacial velocities varied between 10^-6 and 10^-2 ms^-1. The kinetic behavior is found to be consistent with thermally-activated interfacial motion, although an anomalous temperature dependence is observed below 210 K. This low temperature anomaly is attributed to the effect of the experimentally verified elastic softening. At higher temperatures, measured activation energies of 1.5 × 10^-20 to 3.5 × 10^-20 J and activation volumes of 10³ to 10⁴ atomic volumes are rationalized in terms of the rate-controlling interaction of the moving interface with fine-scale lattice displacements, the tweed structure, observed by transmission electron microscopy. Coarse particles of γ- and 2H-phase, detected by the TEM, are found to give rise to noncontact particle/interface interactions which are too long-range to be surmounted by thermal activation. Consequently, they produce athermal friction stresses opposing interface motion.