R. De Batist

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.

Temperature- and amplitude-dependence of internal friction in Cu-Zn-Al alloys

AbstractInternal friction of Cu-Zn-AI alloys, which can transform into martensite either by cooling or by stressing, has been measured as a function of temperature and amplitude. The highest internal friction was observed around the transformation temperature. It was shown that the internal friction of the βphase alloys is low compared with the internal friction of the martensite. By plotting the results in a Granato-Lucke plot, it could be shown that at high amplitude the movement of the martensite plate boundaries is responsible for the high damping and that at low amplitudes only the dislocations contribute to the damping.

Sintering characterization of UO2 powders

The influence of a number of powder characteristics on the sintering behaviour of UO2 has been evaluated. A good correlation is found between the green and sintered densities (dg and ds) and the specific surface area as determined by Knudsen flow permeametry. This correlation, however, does not reveal the differences in behaviour which are borne out by the sintering index defined as: Is = (ds − dg)/(100 − dg). These differences are explained in terms of the absence or presence of open porosity within the powders, which can be measured by means of mercury porosimetry. Thermal treatments have a more pronounced effect on the sinterability of powders with open porosity than on those without.

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.

Low-temperature TSD measurements on γ-irradiated polyvinylidene fluoride

As a part of the fundamental study of the relaxational behaviour of the semi-crystalline polymer polyvinylidene fluoride (-(CH2-C2)n-, PVDF), thermally stimulated depolarization (TSD) measurements have been carried out on γ-irradiated thin films, poled in vacuo. Following irradiation, the total TSD relaxation strength below room temperature is depressed. Mathematical analysis of this region of the TSD spectrum reveals the reduction to be caused by the decrease of the β-(glass-transition) relaxation peak. One also observes the irradiation-induced creation of two relaxation peaks, both with higher activation energies than their neighbours. Tentative interpretations for the different components of the relaxation spectrum are proposed.

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.

Internal friction of iron-chromium alloys in the temperature range between 20 and 800 °c☆

Abstract Internal friction of a number of iron-based alloys is measured between room temperature and 800 °C. The exponentially rising damping observed above 500 °C corresponds to an activation energy which is close to the energy for self-diffusion in iron. The grain boundary relaxation peak is observed around 620 °C, in iron-chromium alloys, around 500 °C in iron with oxide additions. For chromium-free specimens, the activation energy is found to be 2.5 eV. In iron-chromium alloys, large recovery effects which are observed between 500° and 800 °C seem to be caused by processes governed by a wide spectrum of activation energies.

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

Determination of the yield strength of nuclear reactor pressure vessel steels by means of amplitude-dependent internal friction

Amplitude-dependent internal friction measurements allow the determination of a critical amplitude which corresponds to the onset of plastic behaviour and is related to the yield stress (YS) in torsion. The results compare well with the YS obtained from static tensile tests on three different pressure vessel steels and can be fitted with a two- or three-component model comprising long- and short-range dislocation-defect interactions and grain-boundary effects. The effect of thermal aging is to emphasize the presence of dragging contributions to the yield strength, while neutron-irradiation results in an athermal increase in the YS related to irradiation-induced clustering.

Pretransformational amplitude-dependent internal friction in CuAlNi single crystals undergoing martensitic transformation

In CuAlNi single crystals, amplitude-independent and amplitude-dependent internal friction due to phase transformation exhibits different temperature dependence in the transformation temperature range during direct transformation. An ADIF{sub pt} maximum is observed near the M{sub s} temperature, suggesting that at least part of the ADIF{sub pt}, is a pretransformational phenomenon, which may be associated with the nucleation of martensite at the dislocations in {beta}-phase. The hypothesis that both AIIF{sub pt} and ADIF{sub pt} may be separated into premartensitic and phase transition components, may allow one to explain rather complicated experimental observations in a coherent fashion.