K. Sapozhnikov

Influence of martensite stabilization on the low-temperature non-linear anelasticity in Cu-Zn-Al shape memory alloys

Abstract The advanced acoustic technique has been used to investigate the mobility of partial dislocations/intervariant boundaries in the β 1 ′ martensite of a Cu-Zn-Al alloy subjected to the martensite stabilization and to the β-phase ageing, suppressing the stabilization effect. The non-linear anelasticity has been studied for frequencies of about 100 kHz and strain amplitudes 2×10 −7 –2×10 −4 over the temperature range 300–8 K. Measurements at low temperatures, below approximately 70 K, allowed us to eliminate anelastic effects associated with the motion of quenched-in defects, which are ‘frozen’ for these temperatures, and to assess the intrinsic mobility of partial dislocation/intervariant boundaries. The results obtained for stabilized samples are compared with those for β-phase aged samples, and with the previously reported data for the Cu-Al-Ni alloy, which is not prone to the stabilization at ambient temperatures. We suggest distinguishing mechanisms of stabilization according to their localization: a homogeneous and a heterogeneous component. Namely, short-range reordering occurring in the bulk of the crystal is responsible for the homogeneous component of the stabilization. The local rearrangement of the martensite structure in the vicinity of lattice defects (pinning of partial dislocations/intervariant boundaries by quenched-in defects and more intense than in the bulk localized re-ordering) is assumed to be responsible for the heterogeneous component of the stabilization process. The acoustic technique is shown to be able to distinguish and to study details of various effects associated with the heterogeneous and homogeneous changes in the structure of martensite, induced by the stabilization and different heat treatments.

Structural anelasticity of NiTi during two-stage martensitic transformation

Abstract The two-staged thermoelastic martensitic transformation (TMT) B2→R→B19′ in polycrystalline equiatomic NiTi has been studied by means of measurements of strain amplitude-independent and amplitude-dependent internal friction (ADIF), Young’s modulus and amplitude-dependent modulus defects. The internal friction measurements were performed at a frequency of about 100 kHz, rendering negligible the transient internal friction component and allowing one to investigate the structural internal friction, much less dependent on the external parameters such as the heating/cooling rate or the frequency of vibrations. Attention is focussed on the amplitude-dependent anelasticity. Based on the data obtained, the anelasticity is associated with the dislocations inside the martensitic variants, not with the interfaces or interface dislocations, as is traditionally done. The ADIF and anelastic strain in the R phase have been found to be an order of magnitude higher than in the B19′ martensitic phase. This observation is explained by a much higher density of the dislocations inside the variants of the R phase as compared with that of the B19′ phase.

Transient internal friction during thermal cycling of Cu–Al–Ni single crystals in β1′ martensitic phase

Pronounced transient internal friction, accompanied by shear modulus defect and reversible torsional deformation, has been revealed during thermal cycling of Cu–Al–Ni single crystals in the β1′ martensitic phase. These phenomena are associated with microplastic straining of the martensitic phase due to anisotropy of thermal expansion of the martensitic variants.

DETECTION OF SHOCK-WAVE-INDUCED INTERNAL STRESSES IN Cu-Al-Ni SHAPE MEMORY ALLOY BY MEANS OF ACOUSTIC TECHNIQUE

University of Antwerpen (RUCA), IMS, Middelheimlaan 1, B-2020, Antwerpen, Belgium(Received April 12, 2000)(Accepted in revised form June 21, 2000)Keywords: Copper alloy; Martensitic transformation; Impact shock-wave loading; Elastic properties;Internal frictionIntroductionA response of materials, exhibiting the thermoelastic martensitic transformation (TMT), on the appliedmechanical stress leads to the superelasticity, plasticity of the martensitic phase, shape memory effect,high damping capacity, depending on a variety of parameters such as alloy composition, temperature,magnitude of the applied stress, etc. [1,2]. A predictability of such response is of a great importancefrom the engineering standpoint. Since the TMT is a diffusionless phase transition, the determinationof a threshold time to induce the TMT is a challenging fundamental problem. Another aspect, relatedto both of the above mentioned issues is the performance of TMT materials under the ultimateconditions of high-energy impact loading by stress pulses with short duration. In the present work theacoustic technique has been used to detect the structural changes induced by shock-wave loading ofCu-Al-Ni crystals. The same acoustic technique provides us with an estimate of the upper time limit toinduce the TMT and plastic deformation of the martensitic phase.ExperimentalPlate-shaped samples with dimensions of about 1 3 3 3 30 mm

Influence of high-energy impact actions on the elastic and anelastic properties of martensitic Cu–Al–Ni crystals

Abstract The influence of high-energy impact shock-wave loading on the microplasticity and macroscopic performance of the Cu–Al–Ni crystals in the β 1 ′ martensitic phase has been studied. Elastic and anelastic properties of quenched and aged polyvariant single crystals before and after impact shock-wave loading were measured in the temperature range 80–300 K, at a frequency of about 100 kHz in the strain amplitude-independent and amplitude-dependent ranges by means of the composite oscillator technique, and in the MHz frequency range using the pulse–echo technique. High-velocity impact loading of the specimens was realised by plane shock-waves with stress pulses with a duration of ∼2·10 −6 s and stress amplitudes up to 5 GPa. A pronounced influence of impact shock-wave loading on the elastic and anelastic properties of the β 1 ′ martensite has been observed. A strongly marked softening of the material and an enhancement of damping properties are revealed up to the highest stress pulse amplitudes. This behaviour differs fundamentally from the one observed in ‘ordinary’ fcc metals. Changes of the defect structure induced by shock-wave loading, which may be responsible for the observed phenomena, have been discussed.

Towards understanding anelasticity of the β1′ martensitic phase of Cu–Al–Ni

Abstract The present work continues the series of experimental investigations undertaken in order to elucidate the mechanisms controlling elastic and anelastic properties of the β 1 ′ martensitic phase of Cu-based shape memory alloys. The paper reports an attempt to distinguish between ‘dislocation’ and ‘interface’ mechanisms of the internal friction in the β 1 ′ martensitic phase of Cu–Al–Ni single crystals. Two types of experiments have been performed. First, the ultrasonic strain amplitude-independent and amplitude-dependent internal friction (ADIF) of a monovariant specimen for temperatures 90–300 K is carefully re-examined. Second, in situ measurements of the ADIF and of the influence of ultrasonic oscillations on the plastic deformation (acoustoplastic effect) were carried out during quasistatic deformation of a quenched polyvariant specimen. Experimental results support a dislocation rather than an interface mechanism of anelasticity, at least at ultrasonic frequencies and moderate strain amplitudes.

On the Effect of Hydrogen on the Low-Temperature Elastic and Anelastic Properties of Ni-Ti-Based Alloys

Linear and non-linear internal friction and the effective Young’s modulus of a Ni50.8Ti49.2 alloy have been studied after different heat treatments, affecting hydrogen content, over wide ranges of temperatures (13–300 K) and strain amplitudes (10−7–10−4) at frequencies near 90 kHz. It has been shown that the contamination of the alloy by hydrogen strongly affects the internal friction and Young’s modulus of the martensitic phase. Presence of hydrogen gives rise to a non-relaxation internal friction maximum due to a competition of two different temperature-dependent processes. The temperature position and height of the maximum depend strongly on the hydrogen content. We conclude that many of the internal friction peaks, reported earlier for differently treated Ni-Ti-based alloys, had the same origin as the present maximum.

The effect of shock-wave loading on transformation temperatures, elastic and anelastic properties of β′ 1 Cu-Al-Ni martensitic single crystals

Abstract The effect of high-velocity impact loading on the structure, transformation temperatures, elastic and anelastic properties has been studied for a Cu-Al-Ni shape memory alloy in the β′1 martensitic phase. The impact loading of crystals has been performed by means of a light gas gun, producing compressive plane-strain wave pulses with a duration of about 2 × 10−6 s. The normal component of stress in the direction of the strain wave propagation ranged from 0.5 to 5 GPa and was used as a characteristic of the impact magnitude. The influence of the impact loading on the transformation temperatures and the structure of martenistic variants cannot be discerned in the present experimental results. In contrast with the macroscopic properties of crystals, the elastic and anelastic properties, studied at a frequency of about 100 kHz, are found to be strongly influenced by the impact loading. The difference between the effects of the impact on elastic and anelastic properties and on the macroscopic performance of crystals is interpreted on the assumption that these properties are related to different structural entities. Changes in the system of partial dislocations in the faulted martensite and variations in internal stresses are considered as basic contributors to the observed elastic and anelastic effects. The stability of the temperature range and hysteresis of the martensitic transformation are ascribed to the rather stable (under the present impact conditions) martensitic variant structure.

Microstructural Mechanisms of the Acoustoplastic Effect in Crystals

Results of comprehensive experimental investigations of the acoustoplastic effect and amplitude‐dependent anelasticity in crystals are summarised. Based on these investigations, conclusions have been drawn about basic microstructural mechanisms of the acoustoplastic effect.