The response of polycrystalline nitinol with solely austenite structure was studied in three series of planar impact tests characterized by loading of the nitinol samples of 0.5-10 mm thickness by 1 mm thick aluminum impactor accelerated up to velocities of about 387, 429, and 567 m/s. In all the tests, the velocities of the free surfaces of the samples were monitored by a laser velocity interferometer. It was found that in all three test series, the amplitude of elastic precursor wave, being initially greater than 4 GPa, rapidly decays with the propagation distance down to similar to 2.5 GPa, below which the decay is hindered by atomic clusters of the nanometer size. Based on the part of the velocity histories indicating the shock-induced austenite-martensite transformation, the initial, of about 2.5 x 10(3) s(-1), and the maximum, up to 1 x 10(5) s(-1), rates of the transformation were determined. As well, the impact stress slightly greater than 4 GPa was determined as that required for the onset of the B2 -> B19 ' transformation under shock loading. The unloading parts of the same velocity histories allowed a rough estimate of the fraction of the shock-transformed martensite and the elucidation of the virtually complete reversibility of the transformation. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.

The present research combines Transmission Electron Microscopy and planar impact testing to investigate the geometry, concentration, and individual strengths of Ni3Al precipitates in Ni-Al and Ni-Al-Fe alloys. The study focuses on homogenized and aged samples of a binary (88.5% Ni, 11.5% Al) and a ternary (79.3% Ni, 10.2% Al, 10.5% Fe) systems. A detailed analysis of experimental results reveals significant difference between structures of the Ni3Al precipitates in these alloys. In the binary Ni-Al alloy, precipitates are predominantly small and dense spheres with an average diameter of 4 nm. Conversely, the ternary Ni-Al-Fe alloy exhibits larger and loose spherical precipitates, averaging 50 nm in diameter. A key finding of this study is the superior strength of smaller precipitates, which are approximately 40% stronger than their larger counterparts. The latter are identified as regions of Ni-based solid solution densely packed with minuscule, 2 nm-diameter, islands of the ordered Ni3Al phase. These insights contribute to a deeper understanding of the microstructural factors influencing the mechanical properties of Ni-Al based alloys.

The response of plane-parallel 2 mm thick samples of 47.3Ni-52.7Ti alloy was studied in two series of planar impact tests at temperatures between 300 and 473 K and between 473 and 318 K (heating to 473 K followed by cooling). In two additional series, the samples of 0.4–4 mm thickness were tested at 300 and 338 K (after preheating up to 473 K). In all the tests, the samples were loaded by 1 mm thick copper impactors having velocities equal to 314 ± 2 m/s. The velocity of the rear sample surface was continuously monitored by a laser Doppler velocimeter. It was shown that substantial, by an order of magnitude, variation of Hugoniot elastic limit σHEL and compressive strength Y of the nitinol with temperature are caused by the martensite–austenite transformation and its reversal. The variation of the dynamic tensile (spall) strength σsp of the nitinol along the heating–cooling path was found similar to that of σHEL although the difference between σsp values of austenite and martensite, ∼20%, is much more modest than in the case of σHEL. The test series performed at constant temperatures with samples of different thicknesses allows one to conclude that the plastic deformation in shocked austenite is presumably realized by dislocation motion and multiplication controlled by phonon viscosity. In the shocked martensite, the plastic deformation mechanism at a stress lower than ∼0.3 GPa is likely a thermally activated combination of deformation twinning and slip of kinking dislocations.

The effect of modest, 0.6% and 5.5%, pre-straining on the impact response of 2 mm thick samples of annealed polycrystalline vanadium of commercial purity was studied in a series of planar impact tests. The loading of the samples by 0.5 mm thick copper impactors having velocities varying between 300 and 610 m/s was accompanied by continuous laser Doppler velocimetry of their rear surface. Based on the recorded velocity histories, the dynamic compressive σY and tensile (spall) σsp strengths and the strength σYsc of vanadium in the shock-compressed state were determined. Adjacent to the impact surface part of the cross sections of the softly recovered samples, the number of twins Ntw per unit area was counted. It was found that the main parameter governing both the strength σY of pristine (in the shock sense) material and that in the shock-compressed state, σYsc, was the initial dislocation density η0. Moreover, the dislocation surplus caused by pre-straining was responsible for complete suppressing of twinning in the 0.6% and 5.5% pre-strained samples. In undeformed vanadium, the twinning was partially suppressed by the presence of impurity atoms which, however, did not affect the twinning stress, which was equal to approximately 0.7 GPa.

The temperature dependencies of twinning stress tau(tw) in pure polycrystalline BCC vanadium and tantalum were established using shock wave approach based on two types of shock wave experiments. In the tests of the first type, relatively thick samples were softly recovered after planar impact loading and underwent metallographic examination for determination of twin density N-tw(h) (number of twins per unit area) variation as a function of distance h from the impacted sample surface. In the impact tests of the second type, variation of the shear stress with h, tau(h), was studied with the samples of the same metal of different thickness. The latter, being combined with N-tw(h), allows determination of N-tw(tau) and of the shear stress tau(tw) at which N-tw becomes equal to zero. Varying initial sample temperature T made it possible to establish temperature dependencies tau(tw)(T) over 300-700 and 300-800 K temperature ranges for vanadium and tantalum, respectively. The relation of the obtained results with possible scheme of twins nucleation and growth is discussed.

Based on three series of planar impact tests performed on beryllium copper samples, having thickness 0.25-6 mm, individual contributions of beryllium clusters (in the as quenched supersaturated solid solution), Guinier-Preston zones (GP, formed following ageing at 300 degrees C for 26 min) and metastable gamma '' precipitates (crystallizing after 400 min ageing at 300 degrees C) to the strength of alloy were quantified along with corresponding activation volumes. These data being coupled with Transmission Electron Microscopy evaluation of geometry and statistics of GP and gamma '' precipitates, made determination of the resistance tau(p) to dislocation motion (Peierls stress) of the lattices of these particles possible. The great difference between values measured in these two cases, namely 3.7 GPa for GP and 1.4 GPa for the gamma '' precipitates may be considered as an additional confirmation of the fact that in these two cases the strengthening occurs due to different phases.

Impact response of <111> oriented germanium single crystals and polycrystalline samples obtained by high-pressure spark plasma sintering of pure germanium powder was studied in two series of planar impact tests performed at 300 and 1143 K with samples of different thicknesses and in a series of tests with 2 mm single crystals preheated up to the temperatures 300–1143 K. In all the tests, the samples were shock-loaded by tungsten impactors having velocity 980 ± 40 m/s, while the velocity of the interface between the germanium sample and the fused silica window was continuously monitored by velocity interferometer. Under compression, the cubic diamond (cd) germanium transforms into its high-pressure (β-Sn or liquid) modification. The stress corresponding to the upper bound of the existence of impact loaded cd germanium was found to depart upward from that obtained in the static experiments. At temperatures greater than 900 K, this departure increases due to the initiation of melting in the shock-loaded material. Part of the velocity histories recorded with either single or polycrystalline samples was characterized by a four-wave (instead of the expected three-wave) structure. This “surplus” wave seems to be caused by a short-term existence of an intermediate (nonequilibrium) germanium phase which, however, does not affect the principal germanium Hugoniot.

Evolution of precipitates in aluminum 6061 alloy, quenched after 2 h hold at 550ºC (super-saturated solid solution state – SSSS) and aged at 145ºC (for up to 960 min), was studied by routine, ex-situ, transmission electron microscopy (TEM). In parallel, initial stages of the precipitation hardening process (after few-minute ageing) were studied by in-situ TEM. It was An appearance of short-living (~8 min) loose, disordered, spherical ~6 nm in diameter precipitates was captured. These precipitates rearranged after relatively long interval of time (~100 min, corresponding to 240 min ageing in the ex-situ tests) into long-range ordered rod-like Guinier–Preston (GP) zones (presuming GPI). The dimensions of the latter, 20 nm×2.1 nm, was determined based on the ex-situ TEM images.-Longer, 960 min, ageing results in GPI → GPII transformation accompanied by an increase of dimensions and strength of the rod-like precipitates. Determined geometrical parameters are in a reasonable agreement with corresponding parameters assessed previously using shock-wave technique. This fact implies that shock-wave technique can be considered as a useful tool for studying dislocation/defect interactions in a wide variety of strengthened alloys.

The evolution of shock compressive pulses and dynamic tensile (spall) strength of pristine and pre-strained (0.6% and 5.4% compression) samples of pure [100]-oriented molybdenum single crystals were studied in a series of planar impact tests accompanied by continuous monitoring of the free surface velocity of the samples by an optic velocimeter. The impact loading of Mo samples of different thicknesses was produced by copper impactors accelerated in the smooth bore gun up to a velocity of about 350 m/s. Analyzing the recorded waveforms showed that pre-straining results in a substantial decrease of the molybdenum Hugoniot elastic limit while the dynamic tensile (spall) strength increases with pre-straining. The spall fracture of all tested (and spalled) samples was found to be brittle and characterized by a weak dependence of spall strength on the tensile strain rate. The obtained results are discussed in the terms of generally accepted theories of elastic precursor decay in ductile and spall fracture in brittle solids.

Impact response of a tungsten heavy alloy (WHA) prepared by liquid phase sintering of tungsten powder (∼80 vol. %) with an Ni–Co–Fe (3.50–1.25–1.0 weight ratio) binder was studied over a 23–1100 °C temperature range in a series of planar impact tests accompanied by continuous monitoring of the velocity of the WHA sample rear surface. The temperature dependence of the proof stress Y0.1(T) was found based on the 1D numerical simulations of the performed impact tests using a modified Steinberg–Cohran–Guinan constitutive model, and the temperature dependencies of the density ρ0(T) and longitudinal cl(T) and bulk cb(T) speeds of sound were found using rule of mixtures. The bulk speed of sound cb(T) was also used in determination of the temperature dependence of the spall strength σsp(T) of the alloy based on the experimentally recorded velocity pull-backs Δupb. The strong decrease of both Y0.1(T) and σsp(T) with temperature (Y0.1 decreases almost sixfold between 23 and 1100 °C) allows one to assume that the tensile (spall) fracture of the alloy is controlled by the strength of its matrix.

In the course of study of shock-induced twinning in commercially pure (99.8 wt%) polycrystalline vanadium, some unexpected metallurgical features were found. In all vanadium samples softly recovered after planar impact loading by copper impactors with velocities ranging from 262 to 610 m/s, the domain of twinned grains (located at the distance 100?900 ?m from an impacted sample surface) preceded by a relatively narrow strip, 60?100 ?m, densely filled by martensite lenticles of micron size. The distribution of shock-induced twins and the stress, required for their nucleation, were considered in the Part I of the present paper series. Part II of this series is focused on the Transmission Electron Microscopy study of the lenticles, formed in immediate proximity to the impacted surface. It was found that these lenticular particles are oblate ellipsoids of micron size filled with the stacks of 10?30 nm thick planar slabs, which possess tetragonal crystal structure. The slabs have alternating orientation of tetragonal axes c while the parameters of their unit cell are derivatives of cubic lattice parameter of vanadium, aV, namely a = b = 2aV and c = aV. Possible model, based on a sequence of glides, capable to generate such microstructure, and the cause for the disappearance of the lenticles beyond 100 ?m apart from the impacted surface are discussed.

The shock-induced twinning at room temperature was studied using 3 mm thick vanadium samples of commercial purity (99.8 wt%), softly recovered after planar impact loading by copper impactors with velocities ranging from 262 to 610 m/s. Microscopic (Light and Scanning Electron) examinations of the samples? crosssections revealed twins in a strip of vanadium grains, located 100?900 ?m apart from the impacted sample surface. Transmission electron microscopy characterization allowed concluding that these twins are produced by a/6 (111){211} glide in subsequent {211} planes. The number of twins, Ntw, per unit area varied from its maximum value, measured at the distance h = 0.2?0.3 mm from the impacted sample surface, to Ntw = 0 at h = 0.7?0.9 mm apart from the surface. Juxtaposition of the presently obtained Ntw(h) dependencies with previously reported spatial distribution of the shear stress, ?(h), in shock-loaded vanadium samples made it possible to determine twinning stress in vanadium as: ? (Ntw = 0) = ?tw = 0.68(?0.03) GPa.

A series of planar impact tests were performed aiming at measuring the compressibility of the paraffin–poly-methylmethacrylate (PMMA) mixture up to a pressure of 2.5 GPa. It was found that compressive waves in the mixture consist of the initial part, characterized by a virtually linear increase of parameters, followed by the part associated with their relaxation. The dependence of the maximum strain rate of compression of the mixture on the final shock pressure was found to be much weaker than observed in metals and liquids. This inconsistency was explained by the difference of the viscosity mechanisms in these media. Based on an assumption of additivity of volumes of the mixture constituents, the estimates of bulk PMMA compressibility were obtained.

The shear stresses required for dislocations passage of highly coherent NiAl-based precipitates, formed in commercial PH 13-8 Mo steel after homogenization, quenching and aging for different duration of time at 510 degrees C, were determined in a series of planar impact experiments using samples with different thickness. The experiments revealed two regimes of decay of the elastic precursor waves: a fast one, at the shear stress s greater than some threshold tau*, associated with the interaction of moving dislocations with lattice phonons, and a slow one, at tau < tau*, corresponding to the precipitates' cutting with the help of thermal fluctuations. Consequently, the stress tau* can be regarded as the stress which permits the passage of the precipitate by a dislocation without thermal support. Precipitates' geometry and size, determined based on the activation volume at thermally-activated regime, are in a reasonable agreement with precipitates dimensions estimated from High Resolution Transmission Electron Microscopy images. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The strengthening of aluminum alloy 6061 (AA6061) by different point defects was determined experimentally using a shock wave technique. Decay of the amplitude of elastic precursor wave τel with propagation distance h was studied in four groups of AA6061 samples, namely, that in the super-saturated solid solution state (SSSS), and those strengthened by atomic clusters (short-range order), by Guinier-Preston zones I and by Guinier-Preston zones II after, respectively 7.5, 240 and 960 min aging of the SSSS samples at 145 °C. The dependences τel(h) were found to be kinked at stress τ*, corresponding to the transition of the control of dislocation motion from phonon viscous drag at τel>τ* to thermally activated obstacle passage at τel<τ*. The values of strengthening by atomic clusters, GP-I, and GP-II zones were found equal to τsro=67, τGP-I=67, and τGP-II=124MPa, respectively. Activation volumes corresponding to the interaction of dislocations with the studied defects, estimated based on τel<τ* segments of the measured dependences τel(h), were found to be in reasonable agreement with existing concepts of dislocation/defect interactions.

The shock wave response of commercial poly(methyl methacrylate), PMMA, in the vicinity of its glass transition temperature (Tg = 110–120 °C) was studied in two series of impact experiments, the first of which was aimed at the variation of the PMMA spall strength with temperature. The purpose of the second series was the effect of temperature on the PMMA Hugoniot. It was found that the spall strength of PMMA experiences an abrupt twofold drop at ≈120 °C. The PMMA Hugoniot Us=C0+sup was found to be that of a virtually elastic solid with C0 close to the longitudinal speed of sound, cl, at room temperature and transformed above Tg into a viscous material with no shear strength and C0 close to the bulk speed of sound, cb.

Herein, we report on the response of the MAX phase, Ti3SiC2, to shock wave compression at strain rates above 10(4) s(-1). The shock response was determined by measuring the rear, free surface, and velocity of samples-subjected to impact by high-velocity projectiles launched by a gas-gun-using interferometry. The effects of temperature and sample thickness on the dynamic yield and dynamic tensile (spall) strengths were studied. The most important result of this work is the unique dual nature, at high strain rates, of the response of Ti3SiC2, in that it is reminiscent of both metals and ceramics. For low-energy impacts, the elastic response is reminiscent of ductile metals. However, for high-energy impacts, it performed like a hard ceramic with quite high work hardening rates. In other words, Ti3SiC2 behaves like nothing before it and thus must reflect its nanolayered structure. This work not only provides results on the dynamic mechanical properties of Ti3SiC2, but is a critical first step toward understanding the response of ripplocations in layered solids to high strain rates.