S. Golyandin

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.

Influence of impurity content on the acoustoplastic effect, internal friction, and Young's modulus defect during deformation of Cu-Ni single crystals

Abstract Oscillatory stress amplitude dependences of the acoustoplastic effect, absorption of ultrasonic vibrations (frequency of about 100 kHz) causing this effect, and Youngs modulus defect were simulataneously measured in situ during quasistatic deformation of Cu-1·3–7·6 at.% Ni single crystals. The time dependences of the magnitude of the acoustoplastic effect at constant oscillatory stress amplitudes were also obtained. The amplitude-dependent internal friction and Youngs modulus defect diminish drastically with increasing Ni content. The dependence of the magnitude of the acoustoplastic effect on the Ni concentration is much less pronounced and is reversed with increase in oscillatory stress amplitude. Data on the kinetics of the acoustoplastic effect revealed ‘instant’ (time-independent) and ‘relaxational’ (time-dependent) components. Both components show a similar dependence on Ni concentration, thus indicating their common origin. It is concluded that, while the amplitude—dependent internal fri...

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.

STRAIN AMPLITUDE-DEPENDENT ANELASTICITY IN CU-NI SOLID SOLUTION DUE TO THERMALLY ACTIVATED AND ATHERMAL DISLOCATION-POINT OBSTACLE INTERACTIONS

Experimental investigations of the internal friction and the Young’s modulus defect in single crystals of Cu-(1.3–7.6) at. % Ni have been performed for 7–300 K over a wide range of oscillatory strain amplitudes. Extensive data have been obtained at a frequency of vibrations around 100 kHz and compared with the results obtained for the same crystals at a frequency of ∼1 kHz. The strain amplitude dependence of the anelastic strain amplitude and the average friction stress acting on a dislocation due to solute atoms are also analyzed. Several stages in the strain amplitude dependence of the internal friction and the Young’s modulus defect are revealed for all of the alloy compositions, at different temperatures and in different frequency ranges. For the 100 kHz frequency, low temperatures and low strain amplitudes (∼10−7–10−5), the amplitude-dependent internal friction and the Young’s modulus defect are essentially temperature independent, and are ascribed to a purely hysteretic internal friction component. ...

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

Mechanical damping in the martensitic phase of Cu-Al-Ni crystals

Amplitude and frequency dependences of intrinsic damping have been studied in copper-aluminum-nickel (Cu-Al-Ni) single crystals in the martensitic state after different heat treatments. The measurements of intrinsic damping were performed by resonant and forced oscillation techniques in a range from infra- to ultrasonic frequencies. The conclusion is drawn that during aging in β-phase (200°C) the behavior of the pinning obstacles in the martensitic state at room temperature changes from highly mobile and forming atmospheres around the mobile defects, presumably dislocations, to rather immobile and homogeneously distributed in the bulk of the crystal. Observed regularities of the amplitude-dependent intrinsic damping do not correspond to the breakaway model of amplitude-dependent internal friction both for quenched and aged specimens. In contrast to the generally accepted point of view, a notable frequency dependence of the amplitude-dependent damping is revealed, especially in the case of quenched specimens. Short-range pinning paints are considered as basic obstacles for dislocation motion in the quenched specimens. Two types of obstacles are supposed to form the amplitude-frequency spectra of damping for the aged specimens: purely hysteretic or athermal, due to the presence of γ-precipitates, and thermally activated due to the existence of short-range pinning points.

Acoustic study of martensitic-phase aging in copper-based shape memory alloys

An acoustic technique was applied to study aging of the β1′ martensitic phase in a number of copper-based shape memory alloys (Cu-Zn-Al, Cu-Al-Ni, Cu-Al-Be) characterized by various degrees of martensitic-phase stabilization. The nonlinear anelasticity of the martensitic phase was studied in wide ranges of temperature (7–300 K) and vibrational strain amplitude (2 × 10−7 −2 × 10−4) at vibrational-loading frequencies of ∼100 kHz. It was shown that aging effects of the martensitic phase can have homogeneous and heterogeneous components. The homogeneous component is associated with a change in the degree of atomic order in the crystal volume. The basic heterogeneous mechanisms of martensitic-phase aging are associated with the formation of atmospheres of point defects and local changes (which are greater than those in the crystal volume) in the degree of atomic order in the vicinity of partial dislocations and the boundaries between martensite variants. It is concluded that various stabilization properties of the alloys at hand result not only from the different diffusion properties of quenching point defects but also from the different influence of these defects on the degree of atomic order and the different features of their interaction with partial dislocations and intervariant boundaries.

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.

Temperature Dependence of the Internal Friction of Polycrystalline Indium

The temperature dependences of the internal friction and the elastic modulus of polycrystalline indium have been investigated in the temperature range 7–320 K at oscillatory loading frequencies of approximately 100 kHz. The effect of temperature on the amplitude dependence and the effect of high-amplitude loading at 7 K on the temperature and amplitude dependences of the internal friction of indium have been analyzed. It has been demonstrated that the thermocycling leads to microplastic deformation of indium due to the anisotropy of thermal expansion and the appearance of a “recrystallization” maximum in the spectrum of the amplitude-dependent internal friction. The conclusion has been drawn that the bulk diffusion of vacancies and impurities begins at temperatures of approximately 90 K and that, at lower temperatures, the diffusion occurs in the vicinity of dislocations. It has been revealed that the high-temperature internal friction background becomes noticeable after the dissolution of Cottrell atmospheres.