Change In Deformation Mechanism Research Articles

The effect of large plastic deformation on elevated temperature mechanical behavior of dynamic strain aging Al-Mg alloys

The tensile behavior at elevated temperature of heavily plastically deformed Al-Mg alloys with an Mg content of 1 to 5% was investigated. Large plastic deformation was imposed by confined channel die pressing at room temperature up to 18 passes. During heating to test temperature the specimens were observed to undergo recovery and partly recrystallization. The strain rate sensitivity was found to increase with increasing test temperature, and the previously reported asymmetry of the stress differential upon instantaneous up and down strain rate changes disappeared at elevated temperatures. The transition temperature from the low temperature to the high temperature behavior depended on the amount of pre-deformation. The observed phenomena were attributed to a change in deformation mechanism which occurred the earlier, i.e. at lower temperature, the higher the dislocation density introduced by pre-deformation.

Effects of Al and Ti additions on precipitation behavior and mechanical properties of Co35Cr25Fe40-xNix TRIP high entropy alloys

Precipitation behavior and mechanical properties of Ti and Al containing transformation-induced plasticity (TRIP) high entropy alloys (HEAs) are investigated. Plastic deformation mechanism during uniaxial tensile tests are systematically studied by electron backscatter diffraction and transmission electron microscope. The results indicate the thermally induced HCP phase is absent with alloying additions of Ti and Al elements in dual phase TRIP Co35Cr25Fe40-xNix (x = 5, 14) alloy. With the decrease of Ni content, the volume fraction of Heusler phase is high in a wider temperature range, inversely, L12 phase are not formed when the Ni content is 5 at.%. Deformation induced HCP phase are also not observed owing to the additions of Ti and Al, replaced by deformation twinning or stacking faults for different Ni content, which indicate the change of plastic deformation mechanism. The variation of hardness and yield strength of (Co35Cr25Fe40-xNix)94Al3Ti3 are discussed. The growth of Heusler particles is the main reason for the drop of ductility with the increase of aging time, resulting from the increasing difficulty for the dislocation movements across the grain boundaries. This work is beneficial to comprehensively understand the precipitation-hardened HEAs.

Microstructure and mechanical properties of Mg-5Li-1Al sheets processed by cross accumulative roll bonding

Mg-5Li-1Al alloy was processed by cross accumulative roll bonding (CARB). The processed Mg-5Li-1Al sheet possesses ultra-fine grains, and its strength increases both in the rolling direction (RD) and transverse direction (TD) with the elongation remaining 16˜18%. The proper CARB processing is helpful to improve the isotropy of mechanical properties of Mg-5Li-1Al sheet. At the same time, the CARB process has a beneficial effect on improving the plasticity of Mg-5Li-1Al sheet. With the change of the direction between the CARB passes, the basal texture of Mg-5Li-1Al sheet is weakened, and the non-basal texture is strengthened. Both prismatic slip and pyramidal slip are initiated. The dominant deformation mechanism changes from twin deformation to slip deformation, and finally shear deformation. The grain refinement mechanism changes from twin dynamic recrystallization to continuous dynamic recrystallization, and finally rotational dynamic recrystallization.

Void configuration-induced change in microstructure and deformation mechanisms of nano-porous materials

Void configuration plays an essential role in the deformation behavior of ductile metals; its effects on intervoid interference has not been investigated systematically until now. In the present study, molecular dynamics simulation was employed to study the void configuration-induced change in mechanical properties and deformation mechanisms during tensile loading. The results show that void configuration has a significant influence on the yield stress and yield strain, while its effect on the elastic modulus is about 3.14 ± 0.23%. The deformation mechanisms of porous materials with various void configurations at micro- and nanoscale are proposed: (i) local plastic deformation and (ii) homogeneous plastic deformation. Further analysis indicates that the difference between the above two deformation mechanisms is mainly caused by the competition and synergy between the stacking faults and dislocation. Local plastic deformation is mainly controlled by stacking faults. Homogeneous plastic deformation is dominated by dislocation motion with only a small amount of stacking shear motion, which gives material a superior plastic elongation. The result is of great value for improving the plasticity limit of nanostructures.

Change of Deformation Mechanisms Leading to High Strength and Large Ductility in Mg-Zn-Zr-Ca Alloy with Fully Recrystallized Ultrafine Grained Microstructures

Recently, we have found that fully recrystallized ultrafine-grained (UFG) microstructures could be realized in a commercial precipitation-hardened Magnesium (Mg) alloy. The UFG specimens exhibited high strength and large ductility under tensile test, but underlying mechanisms for good mechanical properties remained unclear. In this study, we have carried out systematic observations of deformation microstructures for revealing the influence of grain size on the change of dominant deformation modes. We found that plastic deformation of conventionally coarse-grained specimen was predominated by {0001} <11–20> slip and {10–12} <10–11> twinning, and the quick decrease of work-hardening rate was mainly due to the early saturation of deformation twins. For the UFG specimens, {10–12} <10–11> twinning was dramatically suppressed, while non-basal slip systems containing <c> component of Burgers vector were activated, which contributed significantly to the enhanced work-hardening rate leading to high strength and large ductility. It was clarified by this study that limited ductility of hexagonal Mg alloys could be overcome by activating unusual slip systems (<c + a> dislocations) in fully recrystallized UFG microstructures.

The Influence of Warm Rolling on Microstructure and Deformation Behavior of High Manganese Steels

In this work, a Fe23Mn0.3C1Al high manganese twinning-induced plasticity (TWIP) steel is subjected to varying warm rolling procedures in order to increase the yield strength and maintain a notable ductility. A comprehensive material characterization allows for the understanding of the activated deformation mechanisms and their impact on the resulting microstructure, texture, and mechanical properties. The results show a significant enhancement of the yield strength compared to a fully recrystallized Fe23Mn0.3C1Al steel. This behavior is mainly dominated by the change of the active deformation mechanisms during rolling. Deformation twinning is very pronounced at lower temperatures, whereas this mechanism is suppressed at 500 °C and a thickness reduction of up to 50%. The mechanical properties can be tailored by adjusting rolling temperature and thickness reduction to desired applications.

Strain rate sensitivity and activation volume of AISI 321 stainless steel under dynamic impact loading: Grain size effect

The rapid change in strain rate sensitivity (SRS) of metals at a critical true strain rate (ε̇Tc) is commonly attributed to change in rate-controlling deformation mechanism from thermally-activated process to viscous drag. In this study, metastable AISI 321 stainless steel with grain sizes of 0.24 μm (ultrafine [G1]), 3 μm (fine [G2]) and 37 μm (coarse [G3]) was deformed at high strain-rate. A change in SRS with increasing ε̇T is observed only in G2 and G3 and it occurs at ε̇Tc of ~5900 and 6800s−1, respectively; suggesting that grain size have an effect on the switch of rate-controlling deformation mechanisms. While the SRS of G1 is 0.101 and unchanged, those of G2 and G3 change from 0.094 to 0.326 and 0.091 to 0.634, respectively, once ε̇Tc is reached. Similarly, the estimated activation volume in G1 specimen is ~1.57b3, while those of G2 and G3 specimens change from ~2.95b3 to ~0.65b3 and from ~4.10b3 to ~0.45b3, respectively. Analysis of the deformed specimens using XRD, EBSD and TEM techniques revealed that the complexities in the activated deformation mechanisms in G2 and G3 specimens led to a change in ε̇Tc. Constant SRS in G1 specimen indicated no change in the operational deformation mechanisms.

A micromechanical constitutive model for unusual temperature-dependent deformation of Mg–NiTi composites

Based on the mean-field approach, a micromechanical constitutive model is established to describe the unusual temperature-dependent deformation of Mg–NiTi composites. Firstly, the constitutive equations of two phases are proposed: for NiTi shape memory alloy (SMA), a simplified Hartl–Lagoudas model (Hartl and Lagoudas, 2009) which considering the martensite transformation and plastic deformation is adopted; while, for magnesium (Mg), an elastic-plastic model including a new nonlinear plastic hardening law is employed. The dependence of elastic moduli and yield surfaces of two phases at ambient temperature is addressed. Then, a non-isothermal incremental Mori–Tanaka homogenization method is employed and further extended to describe the interaction between two phases and calculate the macroscopic overall stress-strain response of Mg–NiTi composites. Finally, comparisons between the simulated results and the corresponding experimental ones show that the temperature-dependent deformation of the composites with different volume fractions of NiTi SMA phase can be well captured by the proposed model. Predicted results demonstrate that the unusual temperature-dependent deformation of Mg–NiTi composites originates from the change in the inelastic deformation mechanism of NiTi SMA with the variation of temperature.

Deformation band formation as a function of progressive burial: Depth calibration and mechanism change in the Pannonian Basin (Hungary)

Abstract Deformation bands (DB) are ubiquitous structural elements that can be found in Miocene pre- and syn-rift sediments of the extensional Pannonian Basin, central Europe. Across the field sites we see examples of disaggregation to cataclastic DB sets and evidence of frequent reactivation by discrete faulting. Thin section analysis, cross-cutting relationships and well-defined time constraints of analysed brittle structures demonstrate that with increased burial depth, DB deformation mechanisms progressed from granular flow to cataclasis. The DB sets were classified into 10 deformation phases based on formerly published independent fault-slip analysis and structural mapping. Subsidence curves were constructed for each stratigraphic level involved in deformation and were used along with the intersection of deformation episodes to calculate the depth intervals of DB generations and subsequent structural elements. The DB formation depths obtained were transferred to the depth range of the related mechanisms. This combined methodology permits a more quantitative approach to determine the changes in the deformation mechanisms with depth. Our results show that granular flow (disaggregation bands) dominates down to 100–150 m as the earliest deformation structure, followed by weak then moderate cataclasis. The transition between weak and moderate cataclasis is at approximately 300 ± 100 m for host rock rich in feldspar or fragile tuffitic components and from around 900 ± 100 m in quartz-rich sediments. In addition, deformation by frictional sliding concentrates on discrete fault planes at the margin of cataclastic bands or on new fracture planes from ∼500 ± 100 m in volcanoclastic or feldspar rich host rocks, and 1000 ± 100 m in quartz-rich host rocks. We suggest that burial-induced diagenetic processes is dependent on subsidence history, and partly in connection with regional fluid migration path, control the transition from moderate or advanced cataclasis to discrete fault slip. All these changes affect the pore structure and porosity that contribute to rheological changes, and hence change in deformation mechanism of coeval fracturing events.

Enhancement of Superelastic Property in Ti-Zr-Ni-Cu Alloy by Using Glass Alloy Precursor with High Glass Forming Ability

The glass forming ability (GFA), crystallization behavior and subsequent superelastic property after crystallization in Ti50-xZrxNi35Cu15 (x= 5, 10, 15, 20 at. %) alloys have been investigated with an aim to develop glassy alloy precursor for shape memory alloy having both high glass forming ability and superior superelastic property. The addition of Zr is effective in improving the GFA exhibiting a critical thickness larger than 100 by increasing both liquid stability and resistance to crystallization. The glassy Ti50-xZrxNi35Cu15 alloys are polymorphically crystallized into B2 structure With increasing Zr content, the grain size after crystallization decreases significantly by accelerated nucleation process. Thus, the stability of austenite increases, resulting in the decrease of Ms. The critical stress for slip deformation gradually increases with alloying Zr up to 15 at.%, improving the superelastic recovery. The Ti35Zr15Ni35Cu15 alloy exhibits the highest remnant depth ratio value of 7.2 %. By contrast, the superelasticity is greatly deteriorated in the Ti35Zr20Ni35Cu15 nanocrystalline alloy due to the change of deformation mechanism following inverse Hall-Petch relation. Considering both glass forming ability and sueprelastic property, the Ti35Zr15Ni35Cu15 alloy is confirmed to be the most suitable alloy satisfying wide supercooled liquid region (~40 K), large maximum thickness for amorphous alloy formation (~100 m) and excellent superelasticity after crystallization.

Molecular dynamics simulation of mechanical properties of nanocrystalline platinum: Grain-size and temperature effects

A series of molecular dynamics simulations has been carried out to study the mechanical properties of nanocrystalline platinum. The effects of average grain size and temperature on mechanical behaviors are discussed. The simulated uniaxial tensile results indicate the presence of a critical average grain size about 14.1 nm, for which there is an inversion of the conventional Hall-Petch relation at temperature of 300 K. The transition can be explained by a change of dominant deformation mechanism from dislocation motion for average grain size above 14.1 nm to grain boundary sliding for smaller grain size. The Young's modulus shows a linear relationship with the reciprocal of grain size, and the modulus of the grain boundary is about 42% of that of the grain core at 300 K. The parameters of mechanical properties, including Young's modulus, ultimate strength, yield stress and flow stress, decrease with the increase of temperature. It is noteworthy that the critical average grain size for the inversion of the Hall-Petch relation is sensitive to temperature and the Young's modulus has an approximate linear relation with the temperature. The results will accelerate its functional applications of nanocrystalline materials.

Effect of Cyclic Expansion-Extrusion Process on Microstructure, Deformation and Dynamic Recrystallization Mechanisms, and Texture Evolution of AZ80 Magnesium Alloy

The microstructure, deformation mechanisms, dynamic recrystallization (DRX) behavior, and texture evolution of AZ80 magnesium alloy were investigated by three-pass cyclic expansion-extrusion (CEE) tests. Optical microscopy (OM), electron back-scattered diffraction (EBSD), and X-ray diffraction (XRD) were employed to study microstructure, grain orientation, DRX mechanism, and texture evolution. The results show that the grain sizes decrease continuously with the increase of CEE pass. The grain refinement effect of the first pass is the most remarkable, and there appear a large number of twins. After three-pass CEE, a well-distributed structure with fine equiaxed grains is obtained. With the increase of CEE pass, the deformation mechanism changes from twinning to slipping and the DRX mechanism changes mainly from twinning-induced dynamic recrystallization (TDRX) to rotation dynamic recrystallization (RDRX) and then to continuous dynamic recrystallization (CDRX). The grain misorientation between the new grains and matrix grains deceases gradually, and a relatively small angle misorientation is obtained after three-pass CEE. Grain misorientations of the first two passes are attributed to TDRX and RDRX behaviors, respectively. The grain refinement changes the deformation and DRX mechanisms of CEE process, which leads the (0002) basal texture intensity first decrease and then increase suddenly. Eventually, the extremely strong basal texture is formed after three-pass CEE.

Can zinc alloys be strengthened by grain refinement? A critical evaluation of the processing of low-alloyed binary zinc alloys using ECAP

In this work, the effect of equal channel angular pressing (ECAP) on the microstructure and mechanical properties of zinc and zinc alloys with Ag, Cu, and Mn additions (0.5 at%) was investigated. Four passes of ECAP Route BC was performed at room temperature for each material. Properties of investigated materials after ECAP were compared to their coarse grained counterparts obtained via indirect hot extrusion at 300 °C. Highest strengthening effect was observed for alloy containing the Mn addition. Grain refinement in materials after ECAP was obtained, mean grain diameter is equal to 20 µm in the case of pure zinc, and less than 3.2 µm for alloys. Strain rate dependent plasticity increase was observed for all fine grained materials, with maximum elongation of 510% measured for Zn-Cu alloy after ECAP. Grain refinement did not result in increased yield and ultimate tensile strength of alloys after ECAP. In all investigated materials tensile properties after ECAP were 20 ~ 60% lower than in hot extruded samples. Based on the tensile properties, microstructure and texture analysis, the changes in the main deformation mechanisms were considered. It was presented that the crystallographic texture and grain size are the main factors affecting twinning, slip and non-slip deformation mechanisms resulting in large differences in observed mechanical properties.

Optimizing mechanical properties of Fe26.7Co26.7Ni26.7Si8.9B11 high entropy alloy by inducing hypoeutectic to quasi-duplex microstructural transition

High-entropy alloys (HEAs) have inspired considerable interest due to their attractive physical and mechanical properties. In this work, the microstructural evolution induced by different heat treatments on rapidly solidified hypoeutectic precursors of a Fe26.7Co26.7Ni26.7Si8.9B11 HEA is investigated and correlated with the corresponding mechanical properties. The microstructures of the rapidly solidified precursors are composed of primary fcc solid solution dendrites embedded in a eutectic matrix. When the samples are annealed at different temperatures after furnace cooling or quenching, respectively, the eutectic structure gradually decomposes into fcc, tetragonal (Fe,Co)2B, and hexagonal Ni31Si12 crystals with increasing annealing temperature, leading to a gradual increase of the content of the fcc crystals and both their aggregation and coarsening. Then the dominant structural framework gradually transforms from eutectic structures to fcc dendrites and ultimately the (Fe,Co)2B crystals become isolated as dominant reinforcement particles distributed in the interdendritic regions. This gradual microstructural transition from hypoeutectic to quasi-duplex structures leads to the change of the dominant deformation mechanism from crack-controlled to dislocation-dominated deformation, which allows to control both ductility and strength in a wide range. Hence, this study provides some guideline for how to tune the microstructure and mechanical properties of HEAs.

Change in dominant deformation mechanism of Mg alloy via grain boundary control

The impact of grain boundary structures on mechanical properties and its deformation behavior was investigated on a wrought processed Mg-0.3 at%Mn binary alloy. The Mg-Mn alloy, which had an average grain size of 1.2 µm and a high density of low-angle grain boundaries, was fabricated via caliber rolling. This alloy exhibited yield strength of more than 300 MPa and had low strain rate dependence in both tension and compression tests at room temperature. The major deformation mechanism was dislocation slip for tension and deformation twinning for compression, respectively, as is well established for conventional wrought processed magnesium alloys. However, an extruded Mg-Mn alloy having an average grain size of 1.8 µm is reported to exhibit a good elongation-to-failure and large strain rate dependence. While the same cast billet was used, the result obtained from caliber rolled alloy is inconsistent with that from the extruded alloy. This is due to the difference in grain boundary structures, i.e., the existence of low-angle grain boundaries in the caliber rolled alloy. Low-angle grain boundaries, which do not have free volume, are difficult for segregation of alloying elements. The mechanical properties and deformation mechanism can be clearly controlled by grain boundary structures through wrought process.

Effect of strain rate on microstructures and mechanical properties of Fe–18Cr–8Ni steel

ABSTRACTThe effect of strain rate on deformation microstructures and mechanical properties of Fe–18Cr–8Ni austenitic stainless steel was investigated at strain rates of from 10−3 to 100 s−1. The results indicated that the deformation mechanism of steel changes from transformation induced plasticity (TRIP) to TRIP + twinning induced plasticity (TWIP) effect when the strain rate is increased from 10−3 to 100 s−1. The yield strength of steel increases gradually with strain rate increased, while the tensile strength and elongation first decreases and then increases slowly. The changes in tensile strength and elongation are due to the change of deformation mechanism with the strain rate increased.

Pressure-induced amorphization in plagioclase feldspars: A time-resolved powder diffraction study during rapid compression

The pressure-induced amorphization of the two endmembers of the plagioclase ((Na1−xCax)Al1+xSi3−xO8) solid-solution, anorthite (CaAl2Si2O8) and albite (NaAlSi3O8), has been studied as a function of compression rate by means of time-resolved powder diffraction. Anorthite and albite were compressed in a diamond anvil cell to 80 GPa at multiple rates from 0.05 GPa/s to 80 GPa/s. The amorphization pressure decreases with increasing compression rate. This negative strain rate sensitivity indicates a change in deformation mechanism in the plagioclase solid-solution endmembers from brittle to ductile with increasing compression rate. The presented data support the previously proposed shear deformation mechanism for the amorphization of plagioclase. Furthermore, amorphization progresses over a wide pressure range suggesting heterogeneous amorphization, similar to observations based on recovered material from shock-compression experiments of plagioclase. Our experiments support the contention that amorphization pressures for plagioclase may occur at lower pressures than usually considered.

Strain rate dependence on the evolution of microstructure and deformation mechanism during nanoscale deformation in low carbon-high Mn TWIP steel

We elucidate here the strain rate dependence on the deformation behavior and accompanying deformation mechanism in Fe-30Mn-0.3C TWIP steel via nanoscale deformation experiments and post-mortem microscopy of the deformed region. The nanoindentaion hardness increased with increased strain rate from 0.01 s-1 to 1 s-1, and exhibited a positive strain rate sensitivity of 0.095 with an activation volume of 18b3. At a low strain rate, dislocations dominated the deformation behavior with a high density of 2.7 × 1016 m-2. With increased strain rate, the dislocations decreased and the stacking faults and nanotwins gradually increased. However, nanotwins with secondary twins were the dominant deformation process at high strain rate of 1 s-1. The deformation behavior was significantly impacted by the interplay between strain rate, stacking fault energy and deformation mechanisms. A critical theoretical analysis suggested that the strain rate influenced the critical shear stress for twinning and dislocation slip, resulting in the change in deformation mechanism from dislocation slip to twinning.

Effect of Increased Temperatures on the Deformation and Strength Characteristics of a GFRP Based on a Fabric of Volumetric Weave

The aim of this research was to experimentally study the property degradation of a glass-fiber plastic with increasing temperature and the features of its mechanical behavior. A glass-fiber plastic of aerospace design based on a fabric of volumetric weave from a K11C6170-BA silica thread and an EDT-10P epoxy binder was investigated. The results of tensile tests performed using a technique harmonized with domestic and foreign methods for testing GFRP specimens at temperatures of 22, 100, 175, 250, and 300oC are presented. Their deformation diagrams are obtained, and values of the basic mechanical characteristics are determined. Relations describing the degradation of the elasticity modulus, the tensile strength, Poisson ratio, and the strains corresponding to the strength limit with increasing temperature are constructed and analyzed. Features of the mechanical behavior of the composite material are discussed; in particular, the nonlinear change in strength and qualitative changes in the types of deformation diagrams with growing temperature, which are associated with changes in deformation and fracture mechanisms. An analysis of degradation of glass-fiber plastic structures after the tests at normal and increased temperatures is performed based on data found with the use of a stereomicroscope

Soft MAX phases with boron substitution: A computational prediction

With a goal to improve upon the mechanical properties of the MAX phase, materials of high technological interest, we explore boron substitution in these compounds. Employing first-principles density functional theory (DFT) calculations, combined with continuum modeling to access the core structure of dislocations, we investigate the effect of boron-substitution on plastic deformation properties of a typical MAX phase compound, ${\mathrm{V}}_{2}\mathrm{AlC}$. Our $T=0$ K results show that, due to the differential nature of chemical bonding between V and B compared to that between V and C, both V-Al and V-B basal slip planes get activated in boron-substituted compounds, compared to only V-Al basal slip in the parent compound. This, in turn, makes the boron compounds significantly more ductile compared to their carbide counterpart ${\mathrm{V}}_{2}\mathrm{AlC}$. The computation of temperature-dependent free energies of stable and unstable stacking faults further reveals the important and interesting role of thermal fluctuations in the deformation behavior of the boride compounds at elevated temperature. This suggests a change in the microscopic mechanism of plastic deformation upon varying temperature condition in the proposed boron compounds. Our study should motivate future exploration of boron-based MAX compounds.