Intragranular Deformation Research Articles

Enhancing ambient temperature grain boundary plasticity by grain refinement in bulk magnesium

Room temperature ductility of magnesium is known to be dramatically enhanced by partial contribution of grain boundary sliding. However, the effect of grain size on grain boundary plasticity as well as accommodation process for this unique room temperature grain boundary sliding are unclear. In this study, tensile tests at ambient temperatures and detailed deformed microstructural observations are performed to investigate these points using extruded magnesium of average grain sizes between 1 μm and ∼50 μm. The results of tensile tests exhibit that elongation-to-failure in tension and deformation mechanism vary with grain sizes. Fine-grained magnesium (in orders of several micrometer) shows the elongation-to-failure in tension of ∼100% with activation energy of 80 kJ/mol, which is close to grain boundary diffusion of magnesium. This high ductility results from grain boundary sliding accommodated with dislocation slip by diffusion process. Accordingly, the ductility and activation energy reduces with grain size coarsening. Deformed microstructural observations using EBSD and TEM display the activation of non-basal dislocations at grain boundaries. Specific regions in geometrically necessary dislocation map of grain interior consisting of meso- and coarse-structures (>∼10 μm) show a high value, which suggests the presence of strains. Moreover, the formation of sub-grain boundaries is confirmed in grain interior of coarse-grained structure. This is attributed to dominant occurrence of intragranular deformation mode, associated with insufficient diffusivity, such as grain boundary diffusion.

High temperature deformation of polycrystalline C40 Mo(Si,Al)[formula omitted

Polycrystalline Mo(Si,Al)2 with C40 crystal structure was deformed in compression with a strain rate of 10−4 s−1 at 1300 °C. The specimens were deformed to a strain of 10%–15% and showed maximum stresses around 150 MPa prior to pronounced softening. No crack formation or significant increase in porosity could be observed. Post-test microstructure analysis revealed that the material was inhomogeneously deformed on both inter- and intragranular levels. Dynamic recrystallization occurred alongside low angle grain boundary formation in highly deformed grains. Furthermore, complex intragranular deformation fields suggest that slip systems other than 21̄1̄0[0001] may be active during deformation.

Effect of grain size on the yield stress and microscopic mechanism of a near-α titanium alloy during non-superplastic hot deformation

In this paper, the effect of grain size on the yield stress of a near-α TA15 titanium alloy during the non-superplastic hot deformation was modelled, and the microscopic mechanism was analyzed through the crystal plastic finite element simulation. The effect of grain size on the yield stress changes from the refinement strengthening to refinement softening with the increase in temperature. To model this transformative effect, the grain boundary region is separated to consider the deformation mechanism of grain boundary sliding, and a concise relationship was obtained with a max prediction error of 3.7%. The distribution of microscopic strain is determined by the critical grain size Dc, when the strength of grain boundary regions is equal to α phase. When the grain size is greater than Dc, there are two high strain regions in the α phase, which are the intragranular deformation band caused by the prismatic slipping and the deformation band near grain boundaries caused by the multiple slipping. The continuous and discontinuous dynamic recrystallization appears simultaneously. When the grain size is less than Dc, the strain in the α phase is concentrated near the grain boundary due to the stimulant grain boundary sliding, and the discontinuous dynamic recrystallization is promoted.

Studies on the surface characteristics of Ti60 alloy induced by turning combined with ball burnishing

Ball burnishing can enhance the service performance of parts and is more and more widely used in industrial production. To reveal the formation mechanism of the surface characteristics of ball burnishing of Ti60 alloy, the turning combining ball burnishing experiment was carried out, and the surface characteristics, such as surface morphology, roughness, residual stress, microhardness, and microstructure were investigated in detail. The results show that the surface roughness ranges from 63.9 nm to 110.3 nm and the height difference of the surface roughness profile is within 600 nm after burnishing. The stress concentration factors after burnishing change between 1.06 and 1.204. There is a positive correlation between the stress concentration factor and the surface roughness. The sinusoidal decay function is an effective method to predict the distribution of residual stress along the depth. After burnishing, the micro-hardness distribution shows a trend of first increasing and then decreasing. In addition, the second α phase (αs) and β phase are observed to be deflected obviously after burnishing. The kernel average misorientation (KAM) indicates that significant plastic deformation occurs in the surface layer after burnishing. The average values of local misorientation before and after burnishing are 0.52° and 0.69° respectively. The geometrically dislocation density increases from 7.02 × 109 mm−2 to 9.31 × 109 mm−2. The grain size is reduced from 13.1 μm to 10.5 μm, and the proportion of low-angle grain boundaries (LAGBs) has increased from 12.7% to 22.5%. Intragranular deformation increases significantly and texture exists after burnishing. The research methods and conclusions can provide technical and data support for better burnishing effect of Ti60 alloy.

Mechanical properties and microstructure evolution of aluminum alloy tubes with normal gradient grains under biaxial stress

Grain size gradient materials are a type of new structural material with the advantages of both coarse and fine grains. To study the effect of normal gradient grain on the mechanical properties and microstructure of aluminum alloy tubes during hydroforming, the normal gradient grain distribution of the outer fine and inner coarse grains was obtained using spinning and annealing methods, and the biaxial stress was determined using hydraulic bulging experiments. Gradient grain tubes with thicknesses of 300, 475, and 575 μm were obtained through spinning and annealing at different temperatures; the tensile strengths of the tubes were 79, 89, and 109 MPa; the maximum expansion rates were 18%, 17%, and 10%; and the work-hardening indices were 0.19, 0.20, and 0.17, respectively, under biaxial stress. With an increase in the refined grain area, the density of the low-angular grain boundaries (LAGBs) increased and the chance of stitching dislocation increased in the process of intragranular deformation, causing an increase in strength. However, the increase in the refined thickness weakened the compatible deformation due to a reduction in the number of large grains, leading to a decrease in plasticity. The obtained quantitative relationship between gradient grain and strength/ductility can be applied to guide production of aluminum alloy tubular parts in the aerospace and automotive industries.

Analysing the Interaction between Microscopic Deformation, Microstructure and Void Evolution of Near-α Titanium Alloys during Non-Superplastic Hot Deformation by an Integrated Crystal Plasticity Finite Element Model.

High-efficiency and low-cost hot forming technologies for titanium alloys have been developed for producing complex-shaped, thin-walled tubular components under non-superplastic forming conditions. Under these forming conditions, there exist complex and highly integrated material evolution processes including microscopic heterogeneous deformation, microstructure evolution and damage behaviour. This paper presents an integrated crystal plasticity finite element model of near-α titanium alloys during non-superplastic hot deformation conditions considering grain boundary sliding (GBS), dynamic recrystallisation (DRX), as well as void evolution. The polycrystalline model of a near-α TA15 titanium alloy was established, containing α phase, β phase and grain boundary (GB) regions, in which the GB region was a visualised representation of GBS. The quantitative strength ratio between the GB regions and α phase was calculated according to the Zener–Holloman parameter Z and grain size, which determined the microscopic deformation behaviour. There were found to be two high microscopic strain regions in the α phase: intragranular deformation bands through the most favourable slipping and near the GBs through multiple slipping, which promoted continuous and discontinuous DRX, respectively. With the decrease in parameter Z or grain size, the activated dislocations accommodating GBS were found to no longer pile up inside the grain, but instead travel across the grain interior. Finally, methods to improve the macroscopic plastic formability were proposed for the difficult-to-form titanium alloys experiencing non-superplastic hot deformation.

The role of grain boundary sliding and intragranular deformation mechanisms for a steady stage of superplastic flow for Al–Mg-based alloys

The superplastic forming technique, which realized extremely high plastic deformation, is used for producing complex shaped lightweight constructions. Physical models and experiments with surface microstructure evolution indicate that superplastic deformation predominantly occurs owing to grain boundary sliding (GBS) that accommodated by dislocation slip/creep and diffusional creep. Unusually weak GBS and increased contributions of accommodation mechanisms during the initial stage of superplastic deformation are observed for the fine-grained commercial Al–Mg based alloys. In this study, microstructural evolution during the deformation at a temperature of 0.97Ti.m.with a constant strain rate of 4 × 10−3 s−1 was investigated by scanning and transmission electron microscopy for Al–Mg based alloys with a different Mg content. A strain-induced evolutions of the grain and dislocation structure and the surface structure with FIB-milled grids during elevated-temperature deformation were analyzed. A decrease in the strain rate sensitivity m-coefficient from 0.6 to 0.4, dynamic grain growth with significant grains elongation to the tensile direction accompanied by a pronounced dislocation activity, subgrains formation, and the development of the precipitated depleted zones were observed during deformation. The intergranular and intragranular strains were measured to estimate the contributions of superplastic deformation mechanisms. An increase in solute Mg from 4.8wt% to 6.5–7.6wt% inhibited dynamic grain growth and increased a mean GBS contribution from ~24% to ~40%. “Striation” zones developed near the transverse grain boundaries, and precipitation-depleted zones accumulated up to 50% of strain for the alloy with 4.8wt%Mg and 20–35% for higher Mg alloys. Grain body deformation via dislocation clip/creep provided 20–30% of total strain. The results confirmed the critical nature of the diffusional creep and dislocation slip/creep mechanisms for superplastic deformation of the studied alloys.

Syn‐Shearing Deformation Mechanisms of Minerals in Partially Molten Metapelites

AbstractWe investigated an experimentally sheared (γ = 15, = 3 × 10−4 s−1, 300 MPa, 750°C) quartz‐muscovite aggregate to understand the deformation of parent and new crystals in partially molten rocks. The scanning electron microscope and electron backscatter diffraction analyses along the longitudinal axial section of the cylindrical sample suggest that quartz and muscovite melted partially and later produced K‐feldspar, ilmenite, biotite, mullite, and cordierite. Quartz grains became finer, and muscovite was almost entirely consumed in the process. With increasing γ, melt and crystal fractions decreased and increased, respectively. Among the new minerals, K‐feldspar grains (highest area fraction and coarsest) nucleated first, whereas cordierite and mullite grains, finest and least in number, respectively, nucleated last. Fine grain size, weak crystallographic preferred orientations, low intragranular deformation, and equant shapes suggest both initial and new minerals deformed dominantly by melt‐assisted grain boundary sliding, which is further substantiated by higher misorientations between adjacent grains of quartz, K‐feldspar, and ilmenite.

Deformation behavior of the Al-5.9Zn-2.1Mg-2.1Cu (wt%) alloy during isothermal pressure-holding after pre-deformation

Load-holding after the forming load reached the limit of hydraulic press is a sufficient method to promote further forming and ensure the filling quality of large Al-5.9Zn-2.1Mg-2.1Cu (wt%) alloy die forgings. To investigate the deformation behavior of the alloy during load-holding process, the thermophysical simulation tests at the holding pressure range of 20–60 MPa, temperature range of 350–450 ℃ with the pressure-holding time of 5–30 s and/after compression tests with the strain rate of 1 s−1 and strain of 0.231 were conducted on a Gleeble-3500 isothermal simulator. The strain rate variation during pressure-holding reveals that the critical holding pressure of accelerating deformation is between 40 and 60 MPa at the temperature higher than 400 ℃. The microstructures of the tested specimens were analyzed by optical microscopy (OM) and electron back-scattered diffraction (EBSD). The pre-deformation promotes the intragranular deformation and hinders grain rotation during following pressure-holding process. Besides, the processing map based on Kumar-Prasad criterion is not as accurate as that based on Murty-Rao criterion in predicting the instability during pressure-holding. Based on the processing map and microstructure analysis, the best pressure-holding conditions are recommended to be 400 ℃&40 MPa, 450 ℃&40 MPa and 450 ℃&60 MPa.

Microstructure and twin behavior of Ti-2Al-2.5Zr during cold pilgering

The microstructure and twin behavior of Ti-2Al-2.5Zr tube during cold pilgering were investigated. During the deformation, the intragranular deformation is not uniform, and the microstructure characteristics are similar to low-cycle shear fatigue. Under alternating stress, the rotation tendency of grain is mainly related to the ratio of circumferential to radial strain. As the mainly primary twin, tensile {10–12}〈10–11〉 is activated in large number, which selection of twinning variant conforms to the Schmidt factor law. However, the selection of the secondary twinning variant is related to the slip system activated by the adjacent grains to accommodate the shear strain caused by the twins. The Schmidt factor of the secondary twin is determined by the slip system opened by the adjacent grains.

Influence of residual stress distribution and microstructural characteristics on fatigue failure mechanism in Ni‐based Superalloy

AbstractThe influence of residual compressive stress (RCS) depth and magnitude generated through surface treatments such as shot peening (SP), deep cold rolling (DCR), and vibro‐peening (VP) on fatigue crack mechanisms of Ni‐based superalloy is investigated. The fatigue performance with associated failure mechanisms is measured under strain‐controlled fatigue testing upto 104 cycles with total strain in the range of 0.9%–1.4% at an R ratio of 0.1 and 400°C followed by load controlled fatigue until failure. In‐depth understanding of the failure mechanism is obtained through fractography analysis, cyclic stress–strain plot, and microstructural features. A pronounced improvement in fatigue life tested at low strain range (0.9%–1.1%) is achieved after inducing RCS up to 400 μm depth. However, the fatigue life is reduced when RCS increased to 800–1000 μm depth. Failure is primarily driven by micro‐cracks formed due to balancing tensile stresses and high intensity stress concentration generated as the result of dislocation pile‐ups and slip bands. Results are discussed in detail through the evidence of grain refinement, addition of low angle grain boundaries (LAGBs), strain accumulation, and intragranular deformation in the sub‐surface.

Microstructures generated in AISI 316L stainless steel by Vickers and Berkovich indentations

Abstract The work aimes for studying the specifics of deformation of AISI 316L stainless austenitic steel under the conditions of micro severe plastic deformation (mSPD) created locally at the submicro and micro levels using Berkovich and Vickers indenters. The deformation was carried out by instrumented (depth-sensing), quasi-static indentation and microscratching methods in the load interval P = 10–2000 mN. Studies have shown that various mSPD methods (submicro-, microindentation and microscratching) create similar patterns of plastic deformation in thin surface layers of the material. The stick-slip nature of the scratching process has been demonstrated. It has been established that in the region of loads below 50 mN, the intragranular deformation mechanism has the main contribution to the formation of indentations, while at loads above 50 mN, intragranular and intergranular mechanisms participate in the process, and the role of the latter becomes greater with P increase. Load growth leads to a decrease in hardness under submicro-, microindentation and microscratching for all applied mSPD methods. The type of mSPD method, as well as the type of indenter and the magnitude of the applied load are the factors affecting the mechanical characteristics that should be taken into account depending on the practical purpose of the AISI 316L austenitic steel products.

Dynamic recovery observed in distinct grains within a polycrystalline nickel-based superalloy during cyclic high temperature fatigue via high energy X-ray diffraction microscopy

Elastic micromechanical fields are tracked for a nickel-based superalloy polycrystal via high energy X-ray diffraction microscopy (HEDM) and corresponding intragranular deformation metrics are determined with peak broadening analysis during cyclic high temperature loading. A low solvus, high refractory (LSHR) sample with 252 grains was subjected to uniaxial tension and held under fixed displacement while thermally cycling between 460 °C and 770 °C with intermittent HEDM characterization to study the grain average response to thermo-mechanical deformation. Elevated temperatures were found to allow for heterogeneous amounts of recovery amongst distinct grains within the sampled region, indicating complex grain interactions; the extent of recovery was greater at higher temperatures.

Grain-boundary and intragranular deformation in ultrafine-grained aluminum-based alloy at high strain rate

Fast-forming superplastic aluminum-based alloys are in demand. A deep understanding of high-strain-rate superplasticity is essential for developing novel alloys with improved properties. Herein, we analyzed the superplastic deformation mechanisms of a novel fast-forming aluminum-based alloy. The strain-induced microstructural evolution of the bulk and that of the focused-ion-beam-gridded surfaces of samples, deformed at 0.2 s−1, were studied. The results indicated that both grain-boundary sliding and dislocation slip/creep contribute equally to the total strain during high-strain-rate superplastic deformation. Higher intragranular strains and dislocation activity result in dynamic recrystallization, which has a determining effect on the high-strain-rate superplastic deformation of the alloy.

Quantifying microscale drivers for fatigue failure via coupled synchrotron X-ray characterization and simulations

During cyclic loading, localization of intragranular deformation due to crystallographic slip acts as a precursor for crack initiation, often at coherent twin boundaries. A suite of high-resolution synchrotron X-ray characterizations, coupled with a crystal plasticity simulation, was conducted on a polycrystalline nickel-based superalloy microstructure near a parent-twin boundary in order to understand the deformation localization behavior of this critical, 3D microstructural configuration. Dark-field X-ray microscopy was spatially linked to high energy X-ray diffraction microscopy and X-ray diffraction contrast tomography in order to quantify, with cutting-edge resolution, an intragranular misorientation and high elastic strain gradients near a twin boundary. These observations quantify the extreme sub-grain scale stress gradients present in polycrystalline microstructures, which often lead to fatigue failure.

Comparison of Contributions of the Mechanisms of the Superplastic Deformation of Binary and Multicomponent Brasses

The microstructural changes on the surface of the samples of a binary brass and of brasses alloyed with iron and manganese or aluminum, taken place in the process of superplastic deformation at 550°C at a constant strain rate of 1 × 10–3 s–1 have been analyzed in this work. The contributions of the intergranular and intragranular deformation to the total elongation have been determined. It has been shown that the additional alloying of brass leads to a decrease in the contribution of the grain boundary sliding, which is decreased from 54% in the binary brass to 17–23% in the alloyed brasses, and to an increase in the contribution of the intragranular deformation from 10 to 30%. Experiments based on the Kirkendall method have shown that alloying leads to a slowdown of the diffusion processes, which can presumably limit the grain boundary sliding in the investigated multicomponent brasses.

Experimental study of the superplastic deformation mechanisms of high-strength aluminum-based alloy

The role of different deformation mechanisms and the contributing factors behind them needs to be clearly defined to develop materials exhibiting high strain rate superplasticity. It is still not fully understood to what extent the deformation conditions and microstructural parameters affect the mechanisms of superplastic deformation. A novel Al–3.9Zn–4.1Mg–0.8Cu–2.8Ni–0.25Zr alloy is employed to determine what effects the testing conditions, elemental and phase composition and evolution of grain and sub-grain structure make towards deformation mechanisms. To analyze the deformation behavior and microstructure evolution, uniaxial tensile tests are performed following two deformation regimes with the different constant strain rates and temperatures: (1) 2 × 10-3 s-1, 480°С and (2) 2 × 10-2 s-1, 440°С. The elongation to failure exceeds 650% and the strain rate sensitivity coefficient m is near 0.5 at both deformation regimes. Electron microscopy and focused ion beam techniques are employed to analyze mechanisms associated with grain boundary sliding and intragranular strain by using the samples in a stable flow. Surface grids indicated that superplastic flow is accompanied by the formation of striated regions on the surface, whereas grain boundary sliding involves grain neighbor switching and grain rotation. Intragranular deformation with increased dislocation density and dynamic recrystallization are also observed. Weaker intergranular and intense intragranular deformation are revealed at high strain rate deformation regime. Solute effect and the presence of two types of secondary phases, nanoprecipitates of L12-Al3Zr and micron-sized eutectic Al3Ni, are discussed in comparison to conventional AA7475 alloy in different aspects of grain growth, dynamic recrystallization and strain-rate sensitivity.

Intragranular deformation mechanisms in calcite deformed by high-pressure torsion at room temperature

Polycrystalline calcite was deformed to high strain at room-temperature and confining pressures of 1–4 GPa using high-pressure torsion. The high confining pressure suppresses brittle failure and allows for shear strains >100. The post-deformation microstructures show inter- and intragranular cataclastic deformation and a high density of mechanical e left{01overline{1}8right} twins and deformation lamellae in highly strained porphyroclasts. The morphologies of the twins resemble twin morphologies that are typically associated with substantially higher deformation temperatures. Porphyroclasts oriented unfavorably for twinning frequently exhibit two types of deformation lamellae with characteristic crystallographic orientation relationships associated with calcite twins. The misorientation of the first deformation lamella type with respect to the host corresponds to the combination of one r left{10overline{1}4right} twin operation and one specific f left{01overline{1}2right} or e left{01overline{1}8right} twin operation. Boundary sections of this lamella type often split into two separated segments, where one segment corresponds to an incoherent r left{10overline{1}4right} twin boundary and the other to an f left{01overline{1}2right} or e left{01overline{1}8right} twin boundary. The misorientation of the second type of deformation lamellae corresponds to the combination of specific r left{10overline{1}4right} and f left{01overline{1}2right} twin operations. The boundary segments of this lamella type may also split into the constituent twin boundaries. Our results show that brittle failure can effectively be suppressed during room-temperature deformation of calcite to high strains if confining pressures in the GPa range are applied. At these conditions, the combination of successive twin operations produces hitherto unknown deformation lamellae.

Mechanisms of Superplastic High-Rate Deformation in the Al–Mg–Zn–Fe–Ni–Zr–Sc Alloy

The microstructure and acting superplastic deformation mechanisms in the high-strength Al–7.0% Zn–2.7% Mg–1.0% Ni–0.9% Fe alloy low-doped with Sc and Zr upon deformation at a temperature of 480°С and a strain rate of 1 × 10–2 s–1 at a stable flow stage in a true strain range of 1.1 to 1.6 have been investigated. To evaluate the contributions of superplastic deformation mechanisms to the total elongation, marker grids have been applied on the surface by ion etching, and microstructural changes of the surface have been analyzed. Grain boundary sliding and intragranular deformation play dominant roles. The contribution of each mechanism is 35–40%. The remaining 25% belongs to the diffusional creep mechanism, which is determined from the size of striated zones formed at the transverse grain boundaries on the surface of a deformed sample.

Lateral and Vertical Heterogeneity in the Lithospheric Mantle at the Northern Margin of the Pannonian Basin Reconstructed From Peridotite Xenolith Microstructures

AbstractThis study analyzes the microstructures and deformational characteristics of spinel peridotite xenoliths from the Nógrád‐Gömör Volcanic Field (NGVF), located on the northern margin of a young extensional basin presently affected by compression. The xenoliths show a wide range of microstructures, bearing the imprints of heterogeneous deformation and variable degrees of subsequent annealing. Olivine crystal preferred orientations (CPOs) have dominantly [010]‐fiber and orthorhombic patterns. Orthopyroxene CPOs indicate coeval deformation with olivine. Olivine J indices correlate positively with equilibration temperatures, suggesting that the CPO strength increases with depth. In contrast, the intensity of intragranular deformation in olivine varies as a function of the sampling locality. We interpret the microstructures and CPO patterns as recording deformation by dislocation creep in a transpressional regime, which is consistent with recent tectonic evolution in the Carpathian‐Pannonian region due to the convergence between the Adria microplate and the European platform. Postkinematic annealing is probably linked to percolation of metasomatism by mafic melts through the upper mantle of the NGVF prior to the eruption of the host alkali basalt. Elevated equilibration temperatures in xenoliths from the central part of the volcanic field are interpreted to be associated with the last metasomatic event, which only shortly preceded the ascent of the host magma. Despite well‐developed olivine CPOs in the xenoliths, which imply a strong seismic anisotropy, the lithospheric mantle alone cannot account for the shear wave splitting delay times measured in the NGVF, indicating that deformation in both the lithosphere and the asthenosphere contributes to the observed shear wave splitting.