Plastic Deformation Of Crystals Research Articles

Relaxation model for dynamic plastic deformation of materials

Mechanical model for plastic relaxation of stresses in metals has been proposed. Our approach based on a characteristic plastic relaxation time, which is assumed as a material parameter. In a number of tasks values of its parameter become similarly high and it leads to dynamic effect appearing even at low strain rates deformation condition. The phenomenon of yield drop is one of these manifestations. We also discuss a physically based assumption (in the frames of dislocation theory) for analytical form of relaxation time. We find that there are two different characteristic relaxation time parameters, which is playing a main role for different stages of plastic deformation in low defect crystals.

Early Variscan (Visean) granites in the core of central Pyrenean gneiss domes: implications from laser ablation U-Pb and Th-Pb studies

Laser ablation U-Pb and Th-Pb single zircon and monazite geochronology was applied to seven samples from two Variscan gneiss domes in the central Pyrenean Axial Zone. Detrital zircon ages obtained from a paragneiss in the western Aston dome range from Neoarchean (ca. 2.57 Ga) to Late Neoproterozoic (ca. 560 Ma), suggesting that late Ediacaran sediments were deposited along the northern margin of Gondwana. Cadomian magmatism is recorded for the first time in the central Pyrenees from a biotite metagranite (ca. 545 Ma) in the central Aston dome. A second sample of an orthogneiss from the Aston dome yields a Middle Ordovician protolith age (ca. 467 Ma), similar in age to other dated orthogneisses of the Axial Zone. Three samples from granite stocks situated in the core of the Aston and Bossòst domes have crystallization ages of ca. 339–337 Ma, providing the first evidence of Visean igneous activity in the Axial Zone. In contrast to the larger and significantly younger (ca. 312–305 Ma) granite plutons along the margins of the gneiss domes, Visean magmatism caused LP-HT contact metamorphism (M2). Migmatization throughout the Aston dome, though less well-constrained by overgrowth ages on zircon (ca. 331 Ma) and monazite ages (ca. 340–320 Ma), is likely to be related to Visean intrusions. The main S2 schistosity in the Aston and Bossòst domes is coeval with the Visean intrusions. Inclusion trails in porphyroblasts nucleating in the contact aureoles document the continuous development of S2 related to local extension. Crystal-plastic deformation of the granites indicates that the D2 deformation event continued after their crystallization. Visean magmatism played a crucial role in early stages of dome formation by thermally weakening the metasedimentary mantle, and to a certain degree the orthogneiss as well, enhancing the development of orogen-parallel shear zones in an overall N–S compressive setting.

Spatially resolved analysis of short-range structure perturbations in a plastically bent molecular crystal.

The exceptional mechanical flexibility observed with certain organic crystals defies the common perception of single crystals as brittle objects. Here, we describe the morphostructural consequences of plastic deformation in crystals of hexachlorobenzene that can be bent mechanically at multiple locations to 360° with retention of macroscopic integrity. This extraordinary plasticity proceeds by segregation of the bent section into flexible layers that slide on top of each other, thereby generating domains with slightly different lattice orientations. Microscopic, spectroscopic and diffraction analyses of the bent crystal showed that the preservation of crystal integrity when stress is applied on the (001) face requires sliding of layers by breaking and re-formation of halogen-halogen interactions. Application of stress on the (100) face, in the direction where π···π interactions dominate the packing, leads to immediate crystal disintegration. Within a broader perspective, this study highlights the yet unrecognized extraordinary malleability of molecular crystals with strongly anisotropic supramolecular interactions.

Localization of submicron inclusion re-equilibration at healed fractures in host garnet

Microstructures in Permian inclusion-bearing metapegmatite garnets from the Koralpe (Eastern Alps, Austria) reveal re-equilibration by coarsening of abundant submicron-sized inclusions (1 μm–2 nm diameter) at the site of healed brittle cracks. The microstructures developed during Cretaceous eclogite-facies deformation and the related overprinting of the host–inclusion system. Trails of coarsened inclusions (1–10 μm diameter) crosscut the garnet, defining traces of former fractures with occasional en-echelon overlaps. Trails are flanked by 10- to 100-μm-wide ‘bleaching zones’ characterized by the absence of ≤1-μm-sized inclusions in optical and SE images. FEG-microprobe data show that trails and bleaching zones can form isochemically, although some trails exhibit non-isochemical coarsening. Cross-correlation-based EBSD analysis reveals garnet lattice rotation of up to 0.45°, spatially correlated with bleaching zones. The garnet lattice in the center of trails is misoriented around different axes with respect to the lattice either side of the trail. Elevated dislocation density within bleaching zones is confirmed by TEM observations. Dislocations represent a plastic wake formed by crystal plastic deformation at the crack tip. Fracture enhanced diffusion rates in the lattice adjacent to crack planes by introducing dislocations, priming these areas to behave differently to the bulk of the garnet during Cretaceous metamorphism and facilitating localized coarsening of inclusions. Diffusion within the bleaching zone was enhanced by a minimum factor of 102. The partially closed host–inclusion system records the influence of deformation mechanisms on re-equilibration and contributes to understanding of the interaction between deformation and chemical reaction during metamorphism.

Upper–greenschist facies intragrain deformation of albite in mylonitic meta–pegmatite and the influence of crystallographic anisotropy on microstructure formation

We studied the deformation behaviour of albite from Permian meta–pegmatite in Cretaceous upper–greenschist facies shear zones from the Austroalpine Matsch Unit in the Eastern Alps (Italy). Sodium–feldspars from these rocks provide excellent natural examples for studying mechanisms of intragranular deformation under mid–crustal conditions in grains with different angular relations between their (010) planes and the kinematic frame. The studied rocks were deformed at c. 500 °C in localized shear zones with well characterized top–W shear kinematics supposedly during the Cretaceous upper–greenschist facies tectonometamorphic event. Microstructural and chemical data suggest that crystallographic anisotropies in albite exert a strong control on microstructure formation and that albite primarily deformed by a combination of brittle fracturing, dissolution–precipitation and incipient crystal plasticity as a function of the orientation of the crystallographic anisotropy relative to the supposed shortening direction. Dissolution along discontinuities forming stylolites perpendicular to the shortening direction is associated with the precipitation of fine–grained albite with some compositional variability (Ab96–98 and Ab89–91) in cracks. New albite precipitates form aggregates with straight segments of high angle grain boundaries, nearly 120° dihedral angles and only a poor or no orientation relation to the hosting clast. Intragranular kinking is related to continuous lattice rotation of up to 15° by a misorientation axis close to albite [100] and the formation of subgrain boundaries with maximum misorientations of 7°. Synthetic microshear zones supposedly nucleated on pre–existing cracks, and are associated with formation of subgrain boundaries in shortening quadrants and cracks together with precipitates of potassium feldspar in extensional quadrants adjacent to the microshear zone. New microstructural and textural data from mylonitic Permian meta–pegmatites document various closely linked crystal plastic and brittle deformation mechanisms and highlight the role of crystallographic anisotropies and their orientation with respect to the kinematic frame in microstructure formation.

Research of Stress Field Distribution in FCC-Single Crystal Samples in Compression

.Theoretical distribution of a stress field in a sample in the form of a rectangular parallelepiped in compression for an isotropic material was calculated using the method of finite elements. Calculations showed that the highest stress is observed at the top and front edges of a sample. There are areas of the tension stress on vertical edges in the area of sample tops. Shift anisotropy was determined by imposing of the distributed tension field in a sample on FCC crystal of a certain crystallographic orientation.Change of shift symmetry in single crystals for different crystallographic orientations of a compression axis was considered. It was established that a shift fragmentation in the parallel octahedral planes in the conditions of plastic deformation determines the process of low-symmetric shift deformation and maintenance of higher single crystal pseudo-symmetry. Connection of the obtained results and test data on heterogeneity of plastic deformation of nickel and aluminum single crystals is discussed.

Dislocation glide-controlled room-temperature plasticity in 6H-SiC single crystals

In situ transmission electron microscopy observations of uniaxial compression of sub-300nm diameter, cylindrical, single-crystalline 6H-SiC pillars oriented along 〈0001〉 and at 45° with respect to 〈0001〉 reveal that plastic slip occurs at room-temperature on the basal {0001} planes at stresses above 7.8GPa. Using a combination of aberration-corrected electron microscopy, molecular dynamics simulations and density functional theory calculations, we attribute the observed phenomenon to basal slip on the shuffle set along 〈11¯00〉. By comparing the experimentally measured yield stresses with the calculated values required for dislocation nucleation, we suggest that room-temperature plastic deformation in 6H-SiC crystals is controlled by glide rather than nucleation of dislocations.

High temperature indentation creep tests on anhydrite – a promising first look

Abstract. Indentation creep tests are established in materials engineering, providing information on rheology, deformation mechanisms, and related microstructures of materials. Here we explore the potential of this method on natural, polycrystalline anhydrite. The tests are run at atmospheric pressure, temperatures between 700 and 920 °C, and reference stresses between 7 and 30 MPa. An activation energy Q of 338 kJ mol−1 and a stress exponent n of 3.9 are derived. Deformation is localized into shear zones bounding a less deformed approximately conical plug underneath the indenter. Shear zone microstructures reveal inhomogeneous crystal–plastic deformation, subgrains, and extensive strain-induced grain boundary migration, while mechanical twinning appears not to be activated. Microstructure and mechanical data are consistent with deformation by dislocation creep.

In situ TEM observations of plastic deformation in quartz crystals

With in situ nanocompression experiments in a transmission electron microscope, we investigated plastic deformation in natural quartz crystals and observed both dislocation plasticity as well as mechanical twinning. Through this experimental method, we are able to provide direct evidence of Dauphine twin nucleation and could measure the intrinsic twinning stress. The twinning phenomena appear to include a memory effect, where the same twin can reappear upon successive loading and unloading events. The data provide insight into this twin generation mechanism and can be used as a benchmark for the use of twins in quartz for paleopiezometry. Together, the observation of room-temperature dislocation plasticity and reversible twinning adds new insight into the extensive field of quartz plasticity and demonstrates the usefulness of small-scale testing techniques for mineral physics.

Crackling Noise in Disordered Materials

Recent experimental and theoretical progress on the study of crackling noise in the plastic deformation of crystals, ferroelastics, and porous materials is reviewed. We specifically point out opportunities and potential pitfalls in this approach to the study of the nonequilibrium dynamics of disordered materials. Direct optical observation of domain boundary movement under stress and experimental results from acoustic emission and heat-flux measurements lead to power-law scaling of the jerk distribution with energy exponents between 1.3 and 2.3. The collapse of porous materials under stress leads to exceptionally large intervals of power-law scaling (seven decades). Applications in geology and materials sciences are discussed.

Origins of co-existing diverse magmas in a felsic pluton: the Lysterfield Granodiorite, Australia

Numerous batches of initially heterogeneous magma aggregated to form the I-type Lysterfield Granodiorite, with four geochemically distinct series of granodioritic rocks, emplaced in the shallow crust. The compositional heterogeneities originated through variations in the stoichiometries of melting reactions in the protolith terrane and variable degrees of peritectic assemblage entrainment. The high-K series contains igneous, microgranular enclaves that most probably formed through deep-level hybridisation of enriched mantle magmas with crustal melts. In the model presented here, this heterogeneous collection of magmas ascended to form a thin, sheet-like intrusion, quenched against cold wall rocks. Later, laccolithic inflation, through ingress of voluminous more felsic magmas, arched the pluton roof and fragmented the initial sheet, pieces of which fell back into the Granodiorite to become enclaves, some of which were further hybridised by plastic deformation and mechanical incorporation of host-derived crystals. This may be a common mechanism for the formation of such enclave suites.

Calorimetric features of release of plastic deformation induced internal stresses, and approach to equilibrium state on annealing of crystals and glasses

• Stress release in a glass occurs at a faster rate than structural relaxation. • Plastically-deformed glass would show two exothermic minima, and no glass transition. • Enthalpy matching procedure would yield an inaccurate fictive temperature. • Complex heat capacity may distinguish plastically-deformed from quench-formed glass. Plastic deformation of crystals and glasses produces internal strains (stresses), which change their energy and other thermodynamic properties. On annealing, these stresses decrease at a rate faster than the structure relaxes toward the equilibrium state. Mechanism of such relaxations in crystals differs from that in glasses and it also differs for glasses of different types. In all cases, the energy related properties decrease with time isothermally and on heating, resembling the structure relaxation of a stress-free glass. We consider these features and argue that kinetics of enthalpy loss with time yields the rate constants of the stress release and of the structure change, and not the viscosity determining α-relaxation time. Since thermal cycling does not recover the enthalpy from internal stresses, a glass with stresses has neither a glass-softening temperature, T g , nor a fictive temperature, T f . Plastic deformation would not rejuvenate a physically aged glass to the properties of its un-aged state. The Prigogine–Defay ratio can be extended to all T f s, and used to investigate the effect of distribution of relaxation times on its value, but it can not be defined for an internally stressed glass. After discussing the effects of annealing on the heat capacity and DSC scans, we conclude that on slow heating, glass with deformation-induced stresses would show two exothermic minima, and normal glass would show only one such minimum. Temperature-modulated scanning calorimetry would also distinguish an internally stressed glass from an equally high-enthalpy, stress-free glass. Enthalpy matching procedure would yield inaccurate T f value of a plastically deformed glass, but not of a hyperquenched glass. Phenomenological analysis given here has consequences for interpretation of the calorimetric changes observed on heating the shear-amorphized, as well as pressure-collapsed states of crystals.

Preservation of detrital shocked minerals derived from the 1.85 Ga Sudbury impact structure in modern alluvium and Holocene glacial deposits

Detrital shocked minerals can provide valuable residual records of eroded impact structures. Recent studies have reported shocked minerals in modern alluvium in a subtropical climate from the deeply eroded 2.02 Ga Vredefort Dome impact basin in South Africa. To evaluate the detrital shocked mineral record at a large impact structure in a temperate setting with a Holocene glacial erosional history, we investigated ∼4000 detrital zircons and ∼20,000 quartz grains at the less-eroded 1.85 Ga Sudbury Basin in Ontario, Canada, for the presence of shocked sand grains. Modern alluvium from rivers within and outside the basin, and Holocene glaciofluvial sands (eskers and outwash deltas) across the basin were investigated for shocked minerals. Shocked zircon and/or quartz were found in all modern rivers and most Holocene glacial deposits within, but not outside, the basin. Petrography and scanning electron microscopy (SEM; backscattered electron [BSE]; cathodoluminescence [CL]) imaging and analysis (energy-dispersive X-ray spectroscopy [EDS], electron backscatter diffraction [EBSD]) were used to document shock microstructures. Of the total detrital zircons surveyed, ∼3% (118/3978) were identified as shocked; Holocene samples contained higher average percentages of shocked zircon (63/1361, or 4.6%, with a high of 29%) compared to modern alluvium (55/2617, or 2.1%, with a high of 6%). EBSD analysis revealed a range of shock microstructures, including planar fractures, deformation microtwins, and crystal plastic deformation. At Sudbury, detrital shocked quartz is rare compared to zircon; only 15 grains (∼0.08%) were identified, all with decorated planar deformation features (PDFs). These results demonstrate that a detrital shocked mineral record exists at a large impact basin that is in a youthful stage of erosion, despite its age. In addition to modern alluvium, our results also identify glaciofluvial eskers and deltas as reservoirs for detrital shocked minerals; glacial episodes thus enhance the dispersal and preservation of shocked detritus in sedimentary systems. Despite physical differences, the observation that the two largest Precambrian impact basins continue to contribute detrital shocked minerals 2 b.y. after impact suggests that a shocked mineral record of impacts on early Earth should reside in Precambrian siliciclastic rocks.

Shock-induced plasticity and the Hugoniot elastic limit in copper nano films and rods

Shock deformation of copper nano-films and nano-rods is examined with Molecular Dynamics (MD) simulations. The influence of the small system size on the onset of plasticity, its origin resulting from the nucleation of dislocation loops, and its reversible nature are determined. While simulations of large systems with periodic boundary conditions indicate that tremendous axial stresses are needed to induce plastic deformation in perfect copper crystals, the present results suggest that the stress levels needed to initiate irreversible plasticity in nano-rods are more than one order of magnitude smaller than what has been reported for bulk single crystals. MD studies of nano-films show that shock waves are purely elastic up until the Hugoniot elastic limit of PHEL ≈ 30–40 GPa, at which point Shockley partial dislocations are internally nucleated at the shock front. However, our recent experiments on shocked nano-rods show that plasticity is evident at much lower axial stress levels, on the order of 1–2 GPa. The present MD simulations of shocked nano-rods show that Shockley partial dislocations prefer to nucleate at lower stresses from the rod surface, at PHEL ≈ 1–2 GPa, consistent with our concurrent experimental observations, leading to surface step formation and mechanical damage. Nucleated dislocations are found to be Shockley partials in the [100] and [111] oriented nano-rods, with the additional presence of perfect dislocations in the latter. MD simulations of rarefaction shock waves in nano-films indicate that they can be spalled via a mechanism of nano-void nucleation, growth and coalescence at the spall plane. The origin of these nano-voids is shown to be at the intersection of stacking faults on conjugate slip {111} planes. Spallation by void nucleation and coalescence is found not to be achievable in nano-rods. Rarefaction shocks with high stresses were found to either severely deform or melt the nano-rod before it can be spalled.

A database of plagioclase crystal preferred orientations (CPO) and microstructures – implications for CPO origin, strength, symmetry and seismic anisotropy in gabbroic rocks

Abstract. This study presents a unique database of 172 plagioclase Crystallographic Preferred Orientations (CPO) of variously deformed gabbroic rocks. The CPO characteristics as a function of the deformation regime (magmatic or crystal-plastic) are outlined and discussed. The studied samples are dominantly from slow- and fast-spread present-day ocean crust, as well as from the Oman ophiolite. Plagioclase is the dominant mineral phase in the studied samples. Plagioclase CPOs are grouped into three main categories: Axial-B, a strong point alignment of (010) with a girdle distribution of [100]; Axial-A, a strong point maximum concentration of [100] with parallel girdle distributions of (010) and (001); and P-type, point maxima of [100], (010), and (001). A majority of CPO patterns are Axial-B and P-type, in samples showing either magmatic or crystal-plastic deformation textures. Axial-A CPOs are less common; they represent 21% of the samples deformed by crystal-plastic flow. Although fabric strength (ODF J index) does not show any consistent variation as a function of the CPO patterns, there is a significant difference in the relationship between the ODF and pole figures J indices; the magmatic type microstructures have high (010) pole figures J indices, which increase linearly with ODF J index, whereas the high [100] pole figures J indices of plastically deformed samples vary in a more scattered manner with ODF J index. The multistage nature of plastic deformation superposed on a magmatic structure compared with magmatic flow, and the large number of possible slip-systems in plagioclase probably account for these differences. Calculated seismic properties (P wave and S wave velocities and anisotropies) of plagioclase aggregates show that anisotropy (up to 12% for P wave and 14% for S wave) tends to increase as a function of ODF J index. In comparison with the olivine 1998 CPO database, the magnitude of P wave anisotropy for a given J index is much less than olivine, whereas it is similar for S wave anisotropy. Despite a large variation of fabric patterns and geodynamic setting, seismic properties of plagioclase-rich rocks have similar magnitudes of anisotropy. There is a small difference in the aggregate elastic symmetry, with magmatic microstructures having higher orthorhombic and hexagonal components, whereas plastic deformation microstructures have a slightly higher monoclinic component, possibly correlated with predominant monoclinic simple shear flow in plastically deformed samples. Overall, plots for CPO strength (ODF J index), pole figure strength, CPO symmetry and seismic anisotropy show significant scattering. This could be related to sampling statistics, although our database is a factor of ten higher than the olivine database of 1998, or it could be related to the low symmetry (triclinic) structure of plagioclase resulting in the addition of degrees of freedom in the processes creating the CPOs.

The effect of nanoscratching direction on the plastic deformation and surface morphology of InP crystals

The microstructure of (001) InP crystals scratched with a sharp diamond tip depends strongly on the scratching direction. The scratch surface is found to conform to the radius of curvature of the tip (∼60 nm) by the formation of atomic crystal steps produced by dislocation glide along {111} planes. ⟨110⟩ scratches lead to coherent local crystal lattice movement and rotation causing deep dislocation propagation into the crystal and irregular pileups at the sides of the scratch surface. ⟨100⟩ scratches lead to incoherent lattice movement causing dislocation locking that inhibits their propagation and results in regular pileups.

Syn- to late-orogenic quartz veins marking a retrograde deformation path in a slate belt: Examples from the High-Ardenne slate belt (Belgium)

An integrated structural, petrographic and microthermometric methodology has been applied on syn- to late-orogenic quartz veins from the High-Ardenne slate belt (Belgium) to define their relative timing. The quartz precipitates, which do not show any mutual cross-cutting relationships, represent the meso-scale brittle accommodation during fold initiation, amplification and locking. Crystal-plastic deformation structures and P–T trapping conditions indicate that the different processes accommodating folding occurred in a progressive manner along a retrograde deformation path, associated with the gradual exhumation of the slate belt from ca. 7.5 to 6 km depth. Successive veining occurred from peak metamorphic conditions (ca. 300 °C and 190 MPa), measured in extrados veins, to subsequent lower P–T conditions in the periphery of a lenticular vein (ca. 275 °C and 180 MPa), late-orogenic saddle reef filling (ca. 245 °C and 160 MPa), and the core of a lenticular vein (ca. 220 °C and 150 MPa). Open-cavity growth and fluid-assisted brecciation indicate that veining occurred under overpressured fluid conditions. It is therefore fair to assume that the High-Ardenne slate belt acted – at least episodically – as a mid-crustal overpressured fluid reservoir, not only in the earliest and latest stages, but also during the main stage of orogeny.

Dislocation dynamics during plastic deformations of complex plasma crystals

The internal structures of most periodic crystalline solids contain defects. This affects various important mechanical and thermal properties of crystals. Since it is very difficult and expensive to track the motion of individual atoms in real solids, macroscopic model systems, such as complex plasmas, are often used. Complex plasmas consist of micrometer-sized grains immersed into an ion-electron plasma. They exist in solidlike, liquidlike, and gaseouslike states and exhibit a range of nonlinear and dynamic effects, most of which have direct analogies in solids and liquids. Slabs of a monolayer hexagonal complex plasma were subjected to a cycle of uniaxial compression and decompression of large amplitudes to achieve plastic deformations, both in experiments and simulations. During the cycle, the internal structure of the lattice exhibited significant rearrangements. Dislocations (point defects) were generated and displaced in the stressed lattice. They tended to glide parallel to their Burgers vectors under load. It was found that the deformation cycle was macroscopically reversible but irreversible at the particle scale.

The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep

Abstract. We present microfabrics in high-pressure, metamorphic, partly serpentinized peridotite mylonites from the Voltri Massif, in which porphyroclasts and matrix record independent deformation events. The microfabrics are analysed using polarization microscopy and electron microscopy (SEM/EBSD, EMP). The mylonites contain diopside and olivine porphyroclasts originating from the mantle protolith embedded in a fine-grained matrix consisting mainly of antigorite and minor olivine and pyroxene. The porphyroclasts record brittle and crystal-plastic deformation of the peridotite at upper-mantle conditions and differential stresses of a few hundred MPa. After the peridotites became serpentinized, deformation occurred mainly by dissolution–precipitation creep resulting in a pronounced foliation of the antigorite matrix, crenulation cleavages and newly precipitated olivine and pyroxene from the pore fluid at sites of dilation, i.e. in strain shadows next to porphyroclasts and folded fine-grained antigorite layers. Antigorite reveals a pronounced associated shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) with the basal (001) cleavage plane oriented in the foliation plane. In monomineralic antigorite aggregates at sites of stress concentration around porphyroclasts, a characteristically reduced grain size and deflecting CPO as well as sutured grain boundaries indicate also some contribution of crystal-plastic deformation and grain-boundary migration of antigorite. In contrast, the absence of any intragranular deformation features in newly precipitated olivine in strain shadows reveals that stresses were not sufficiently high to allow for significant dislocation creep of olivine at conditions at which antigorite is stable. The porphyroclast microstructures are not associated with the microstructures of the mylonitic matrix, but are inherited from an independent earlier deformation. The porphyroclasts record a high-stress deformation of the peridotite with dislocation creep of olivine in the upper mantle probably related to rifting processes, whereas the serpentinite matrix records dominantly dissolution–precipitation creep and low stresses during subduction and exhumation.

Role of superposition of dislocation avalanches in the statistics of acoustic emission during plastic deformation

Various dynamical systems with many degrees of freedom display avalanche dynamics, which is characterized by scale invariance reflected in power-law statistics. The superposition of avalanche processes in real systems driven at a finite velocity may influence the experimental determination of the underlying power law. The present paper reports results of an investigation of this effect using the example of acoustic emission (AE) accompanying plastic deformation of crystals. Indeed, recent studies of AE did not only prove that the dynamics of crystal defects obeys power-law statistics, but also led to a hypothesis of universality of the scaling law. We examine the sensitivity of the apparent statistics of AE to the parameters applied to individualize AE events. Two different alloys, MgZr and AlMg, both displaying strong AE but characterized by different plasticity mechanisms, are investigated. It is shown that the power-law indices display a good robustness in wide ranges of parameters even in the conditions leading to very strong superposition of AE events, although some deviations from the persistent values are also detected. The totality of the results confirms the scale-invariant character of deformation processes on the scale relevant to AE, but uncovers essential differences between the power-law exponents found for two kinds of alloys.