Transformation temperatures, elastic and anelastic properties of Cu-Al-Ni crystals subjected to impact loading

Abstract

Experimental investigations of the influence of impact loading on properties of Cu-AI-Ni single crystals have been performed. Crystals in the p\ phase were impacted with pulses of a uniaxial plane strain wave with a duration of about 2X10" 6 s. A normal component of stress in the direction of the pulse propagation ranged from 0.5 to 5.4 GPa. For as-quenched and impacted crystals martensitic transformation temperatures were determined, the Young's modulus (YM), strain amplitudeindependent and strain amplitude-dependent internal friction were measured at a frequency of about 100 kHz over the temperature range of 300-90 K for strain amplitudes of IO" 7 -2xlO" 4 . Experimental results indicate that Pi->Yi' martensitic transformation (MT) and plastic deformation of the martensite are induced by the impact. The impact loading generates a «structure memory effect»: the YM in the austenite is not sensitive to the impact, but the consequent temperature-induced MT reveals a dramatic influence of the impact on the YM of the y,' martensite. The conclusion is drawn that the observed effect is fundamentally similar to the two-way memory effect, but is extremely sensitive to the impact stress. The origin of this phenomenon is attributed to the internal stresses, created by impact, which control the nucleation of preferentially oriented anisotropic martensitic variants during temperature-induced MT in impacted crystals. No influence of the impact loading on the transformation temperatures was detected for pi->Yi' MT, in contrast to elastic properties of the martensitic phase, indicating that structural changes due to the high-velocity impact do not affect appreciably the thermoelastic equilibrium and hysteretic motion of parent -martensite boundaries during the MT.