Mechanical resonance of the austenite/martensite interface and the pinning of the martensitic microstructures by dislocations inCu74.08Al23.13Be2.79<

Abstract

undergoes a martensitic phase transition at 246 and 232 K under heating and cooling, respectively. The phase fronts between the austenite and martensite regions of the sample are weakly mobile with a power-law resonance under external stress fields. Surprisingly, the martensite phase is elastically much harder than the austenite phase showing that interfaces between various crystallographic variants are strongly pinned and cannot be moved by external stress while the phase boundary between the austenite and martensite regions in the sample remains mobile. This unusual behavior was studied by dynamical mechanical analysis ͑DMA͒ and resonant ultrasound spectroscopy. The remnant strain, storage modulus, and internal friction were recorded simultaneously for different applied forces in DMA. With increasing forces, the remnant strain increases monotonously while the internal friction peak height shows a minimum at 300 mN. Transmission electron microscopy shows that the pinning is generated by dislocations which are inherited from the austenite phase.

Avalanche criticalities and elastic and calorimetric anomalies of the transition from cubic Cu-Al-Ni to a mixture of18Rand2Hstructures

“…The details of the softening are unusual for martensites, however, where the shear modulus remains temperature independent once the martensite is formed. This archetypal behavior was reported, e.g., for Cu 74.08 Al 23.13 Be 2.79 [36]. The microstructure of the martensite phases in Cu-AlNi still changes at low temperatures so that the athermal behavior is never established for these phase transitions.…”

Pinning of the martensitic microstructures by dislocations in Cu74.08Al23.13Be2.79

“…This indicates that these dislocations in the martensite and inherited from the austenite, have a clear impact on stacking defects of the 18R martensite structure. By tilting away the sample from the [-2-10] zone axis it could be concluded that the type A dislocations in the martensite correspond to a couple of partial dislocations (PD) bounding stacking faults (SF) with a dissociation of around 15 nm [3]. Stacking faults on {12-8} type planes were observed before in 18R martensite of Cu-Al-Ni alloys [4,5].…”

“…In order to investigate an uncommon increase in hardness upon transforming to martensite, the material was investigated by conventional two-beam and in-situ cooling transmission electron microscopy (TEM). More detail on the experiments and used instruments will be given in a forthcoming paper [3].…”

“…Instead of softening in the martensite phase the material becomes effectively harder. This was concluded from measurements from dynamical mechanical analysis (DMA) at low frequencies and resonant ultrasonic spectroscopy (RUS) at frequencies in the kHz region [3]. Fig.…”

Breakdown of Shape Memory Effect in Bent Cu–Al–Ni Nanopillars: When Twin Boundaries Become Stacking Faults

“…54 In summary, the finite size scaling of the SME is a consequence of the mixture of these two features: twinning is reversible under heating to the austenite phase but disordered stacking faults are not and even prevent the phase transformation altogether. In the intermediate regime, stacking faults may prevent twins from moving 55 and partially stabilize the 2H structure. The experimentally determined size dependence of the SME is shown in Figure 5b.…”