Selection and change in deformation mode to maintain continuity of strains and slip/twinning planes at lamellar boundaries in fatigued TiAl polysynthetically twinned crystals

Change in deformation mode in six types of γ domain (A_M–C_T) and α₂ plates in TiAl polysynthetically twinned (PST) crystals fatigued at a loading axis parallel to lamellar planes with stress amplitude (Δσ) of 420–450 MPa was examined by the transmission electron microscope focusing on continuity of macroscopic strains and slip/twinning planes at lamellar boundaries. At Δσ = 420 and 450 MPa, the strain continuity is always maintained at lamellar boundaries by activation of one of the symmetric twinning systems in A-type domain and selection of the dominant deformation mode between ordinary dislocations and twins in (B and C)-type γ domain. The (B and C)-type γ domains of B_M,B_T,C_M and C_T behave as two sets of (B_M,C_T) and (B_T,C_M) because each set selects either the deformation mode of ordinary dislocations or twins as a dominant system in order to keep macroscopic strain continuity. The set (B_T,C_M) which accounts for a larger volume fraction than the set (B_M,C_T) in TiAl-PST crystals used in this study selected a twinning system at Δσ = 450 MPa, while ordinary dislocations were selected at Δσ = 420 MPa. At Δσ = 450 MPa, twinning deformation prevented the further motion of ordinary dislocations with a Burgers vector parallel to lamellar boundaries, and rapid fatigue hardening occurred accompanied by reduction of the accumulative plastic strain energy. Anomalous change in strain energy during fatigue is infiuenced by the volume fraction of a set of (B and C)-type domain and the anomalous behavior in fatigued TiAl-PST crystals may disappear when each type of γ domain is equally distributed.     

Current Progress of Mechanical Properties of Metals with Nano-scale Twins

Focus on face-centered cubic （fcc） metals with nano-scale twins lamellar structure, this paper presents a brief overview of the recent progress made in improving mechanical properties, including strength, ductility, work hardening, strain rate sensitivities, and in mechanistically understanding the underling deformation mechanisms. Significant developments have been achieved in nano-twinned fcc metals with a combination of high strength and considerable ductility at the same time, enhanced work hardening ability and enhanced rate sensitivity. The findings elucidate the role of interactions between dislocations and twin boundaries （TBs） and their contribution to the origin of outstanding properties. The computer simulation analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating TBs as internal interfaces and allowing special slip geometry arrangements that involve soft and hard modes of deformation. Parallel to the novel mechanical behaviors of the nano-twinned materials, the investigation and developments of nanocrystalline materials are also discussed in this overview for comparing the contribution of grain boundaries/TBs and grain size/twin lamellar spacing to the properties. The recent advances in the experimental and computational studies of plastic deformation of the fcc metals with nano-scale twin lamellar structures provide insights into the possible means of optimizing comprehensive mechanical properties through interfacial engineering.     

Effect of strain-induced precipitation on the low angle grain boundary in AA7050 aluminum alloy

Effects of the particles induced by strain on dynamic recrystallization and microstructure of the AA7050 aluminum alloy were investigated during hot deformation using X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD). Experimental results showed that partial recrystallized grains containing little sub-structure were produced during the solution treatment. Numerous particles were successfully obtained by the strain-induced precipitation during first-pass deformation at 573 K. The deformation promoted spheroidization and refinement of the precipitate particles. Then these particles pinned dislocations and grain boundaries inhibiting dynamic recrystallization during second-pass high-temperature deformation at 673 K and low angle grain boundary fraction was increased significantly to 83.8%. Furthermore, the tensile test indicated that microstructure with numerous low angle boundaries (LAGBs) and 5 μm sub-grains had increased the strength and ductility of the AA7050 aluminum alloy.     

Analysis of strengthening mechanisms in a severely-plastically-deformed Al–Mg–Si alloy with submicron grain size

Methods of severe plastic deformation of ductile metals and alloys offer the possibility of processing engineering materials to very high strength with good ductility. After typical amounts of processing strain, a submicrocrystalline material is obtained, with boundaries of rather low misorientation angles and grains containing a high density of dislocations. In the present study, an Al–Mg–Si alloy was severely plastically deformed by equal channel angular pressing (ECAP) to produce such a material. The material was subsequently annealed for dislocation recovery and grain growth. The strength of materials in various deformed and annealed states is examined and the respective contributions of loosely-arranged dislocations, many grain boundaries, as well as dispersed particles are deduced. It is shown that dislocation strengthening is significant in as-deformed, as well as lightly annealed materials, with grain boundary strengthening providing the major contribution thereafter.     

Strain effects on microstructure behavior of 7050-H112 aluminum alloy during hot compression

Effects of strain on microstructure behavior of 7050-H112 aluminum alloy was investigated by means of hot compression conducted at 450 °C and strain rate of 1 s⁻¹. The true stress–true strain behavior shows that it appears dynamic soft after acquired peak stress. The microstructure evolutions are mainly characterized by dislocation substructures with low misorientations which increase with deformation at low to medium strains, but decrease at high strain. It is concluded that the main soft mechanism is dynamic recovery with partial dynamic recrystallization at low to moderate strain, and then dynamic recrystallization at high strain. At last, the substructure behavior which is mainly affected by dislocation migrations is discussed in detail. At low deformation, dislocation migration can destroy grain boundaries and their junctions, resulting in formation of low angle boundaries. However, the interactions of dislocations increase with increasing of deformation, leading to a evolution of high-angle boundaries.     

Comparison of microstructures in electroformed copper liners of shaped charges before and after plastic deformation at different strain rates

Transmission electron microscopy observations of the recovered slugs of electroformed copper liner materials that had undergone high-strain-rate deformation show the existence of a wide range of crystal defects, including vacancy clusters and porosity. Cellular structures formed by tangled dislocations and subgrain boundaries consisting of dislocation arrays were also detected. Electron backscattering Kikuchi pattern technique analysis reveals that the fibrous texture observed in the as-formed copper liners of shaped charges disappeared after explosive detonation deformation. In a specimen that had been plastically deformed at a normal strain rate (4×10⁻⁴ s⁻¹), a high density of dislocations was observed within grains. These experimental results indicate that dynamic recovery and recrystallization play an important role during high-strain-rate deformation by virtue of a temperature increase in the deformation process, whereas the conventional slip mechanism operates during deformation at the normal strain rates.     

Nanostructured interfaces for enhancing mechanical properties of composites: Computational micromechanical studies

Computational micromechanical studies of the effect of nanostructuring and nanoengineering of interfaces, phase and grain boundaries of materials on the mechanical properties and strength of materials and the potential of interface nanostructuring to enhance the materials properties are reviewed. Several groups of materials (composites, nanocomposites, nanocrystalline metals, wood) are considered with view on the effect of nanostructured interfaces on their properties. The structures of various nanostructured interfaces (protein structures and mineral bridges in biopolymers in nacre and microfibrils in wood; pores, interphases and nanoparticles in fiber/matrix interfaces of polymer fiber reinforced composites and nanocomposites; dislocations and precipitates in grain boundaries of nanocrystalline metals) and the methods of their modeling are discussed. It is concluded that nanostructuring of interfaces and phase boundaries is a powerful tool for controlling the material deformation and strength behavior, and allows to enhance the mechanical properties and strength of the materials. Heterogeneous interfaces, with low stiffness leading to the localization of deformation, and nanoreinforcements oriented normally to the main reinforcing elements can ensure the highest damage resistance of materials.     

Mechanistic model for grain size and temperature dependence of flow stress in 316L austenitic stainless steel

Abstract

During plastic deformation of a polycrystalline material, both the grain interior and the grain boundary regions exhibit distinctly different dislocation behaviours at a given strain and temperature. Studying the variation of experimental flow stress with temperature, it seems that the flow stress of a fine grained polycrystalline material is mainly controlled by dislocation dynamics at and in the vicinity of grain boundaries. At low temperatures in a polycrystalline material, the dislocations are piled up at grain boundaries and the density of dislocations increases significantly in the grain boundary region, while at high temperatures the annihilation of dislocations take place at and in the vicinity of the grain boundaries during deformation. Therefore, the flow stress behaviour of a polycrystalline material can be understood in terms of the process of accumulation and annihilation of dislocations at and in the vicinity of grain boundaries at a given strain and temperature.     

Continuous dynamic recrystallization during severe plastic deformation

Severe plastic deformation (strains > 100%) has been shown to create significant grain refinement in polycrystalline materials, leading to a nanometric equiaxed crystalline structure for such metals as aluminum, copper and nickel alloys. This process, termed continuous dynamic recrystallization, is governed by evolution of the dislocation structure, which creates new grain boundaries from dislocation walls. In the proposed model, plasticity occurs which firstly involves dislocation multiplication, leading to strain hardening limited by dynamic recovery. After a critical dislocation density is reached new grain boundaries are formed by condensation of walls of dislocations, creating a new stable configuration that is favored due to a reduction of the system free energy. This evolution of the microstructure continues to develop, with a consequent progressive decrease in the average grain diameter. The proposed model provides a quantitative prediction of the evolution of the average grain size, as well as the dislocation density, during continued plastic strain. The model can be calibrated by use of results from any experiment that involves large plastic deformation of metals, subject to negligible annealing effects. In this paper, the model has been calibrated, and consequently validated, through experiments on machining of Al 6061-T6.     

On the nature of plastic deformation generated by hydrostatic pressure in silicon single crystals

Macroscopic plastic deformation of silicon single crystals, caused by annealing at hydrostatic pressure and high temperature, was studied by X-ray topography and transmission electron microscopy. The analysis is given of elastic and thermal properties of material around surface cracks and scratches from which deformation process is propagated. The idea of elastic misfit between damaged self-strained material at cracks and scratches and defect-free silicon matrix, is introduced. On the basis of theoretical and experimental data it is concluded that the plastic deformation of silicon at high pressure consists of two processes. The first is a loss of coherency of cracks and scratches by the emission of dislocations at misfitting second phase precipitates present in silicon. The second is the macroscopic yielding from incoherent cracks and scratches at lower elastic strain energies.The presented mechanism explains also the deformation behaviour of silicon crystals subjected to tensile stress at high temperatures; the generation and propagation of dislocations at oxide precipitates before the macroscopic yielding [3].     

Estimation of lattice strain in nanocrystalline silver from X-ray diffraction line broadening

The lattice strain contribution to the X-ray diffraction line broadening in nanocrystalline silver samples with an average crystallite size of about 50 nm is studied using Williamson-Hall analysis assuming uniform deformation, uniform deformation stress and uniform deformation energy density models. It is observed that the anisotropy of the crystallite should be taken into account, while separating the strain and particle size contributions to line broadening. Uniform deformation energy density model is found to model the lattice strain appropriately. The lattice strain estimated from the interplanar spacing data are compared with that estimated using uniform-energy density model. The lattice strain in nanocrystalline silver seems to have contributions from dislocations over and above the contribution from excess volume of grain boundaries associated with vacancies and vacancy clusters.     

Structure of grains and grain boundaries in cryo-mechanically processed Ti alloy

A Ti5Ta1.8Nb alloy with the major phase as α (hcp) Ti has been subjected to severe plastic deformation by means of cryo-rolling. Significant grain refinement (from ~5 μm to ~60 nm) has been observed. The mechanism of grain refinement was studied by analysis of lattice strain variations with increase in cold work using XRD technique. Various intermediate stages, such as hardening, alignment of dislocations, cell formation and criticality before new grain formation, were identified. Formation of cells with dislocations alignment at the boundaries and then finally forming an ultra-fine grain structure was confirmed by transmission electron microscopy studies. Detailed grain boundary characterisation has been carried out using high-resolution transmission electron microscopy studies and crystallographic texture analysis. The grain-refined structure was found to possess a large fraction of high angle boundaries identified also as special boundaries by evaluating the misorientation angle/axis sets for a pair of adjacent grain boundaries.     

Cryorolling effect on microstructure and mechanical properties of Fe–25Cr–20Ni austenitic stainless steel

Microstructure and mechanical properties of the Fe–25Cr–20Ni austenitic stainless steel after cryorolling with different reductions were investigated by means of optical, scanning and transmission electron microscopy, X-ray diffraction and mini-tensile testing. High density tangled dislocations and a small amount of deformation twins formed after 30% deformation. After 50% strain, a large amount of deformation twins was generated. Meanwhile, interactions between the twins and dislocations started to happen. As the strain increased to 70%, many deformation twins were produced and the interactions between the twins and dislocations were significantly enhanced. When the cryorolling was 90%, the grain size was refined to the nanometer scale. XRD analysis indicated that the diffraction peaks of the samples became broader with the strain increase. The yield strength and the ultimate strength increased from 305 MPa and 645 MPa (before deformation) to 1502 MPa and 1560 MPa (after 90% deformation), respectively. However, the corresponding elongation decreased from 40.8% to 6.4%. The tensile fracture morphology changed from typical dimple rupture to a mixture of quasi-cleavage and ductile fracture. After 90% deformation, the microhardness was 520 HV, which increased by 100% compared with the original un-deformed sample.     

Dislocation-mediated migration of interphase boundaries

Faceted interphase boundaries (IPBs) are commonly observed in lath-shaped precipitates in alloys consisting of simple face-centred cubic (fcc), body centred-cubic (bcc) or hexagonal closed packed (hcp) phases, which normally contain one or two sets of parallel dislocations. The influence of these dislocations on interface migration and possible accompanying long-range strain field remain unclear. To elucidate this, we carried out atomistic simulations to investigate the dislocation-mediated migration processes of IPBs in a pure-iron system. Our results show that the migration of these IPBs is accompanied with the slip of interfacial dislocations, even in high-index slip planes, with two migration modes were observed: the first mode is the uniform migration mode that occurs only when all of the dislocations slip in a common slip plane. A shear-coupled interface migration was observed for this mode. The other interfaces propagate in the stick-slip migration mode that occurs when the dislocations glide on different slip planes, involving dislocation reaction or tangling. A quantitative relationship was established to link the atomic displacements with the dislocation structure, slip plane, and interface normal. The macroscopic shear deformation due to the effect of overall atomic displacement shows a good agreement with the results obtained based on the phenomenological theory of martensite crystallography. Our findings have general implications for the understanding of phase transformations and the surface relief effect at the atomic scale.     

Hot deformation behavior and microstructural evolution of a modified 310 austenitic steel

To study the hot deformation behavior and microstructural evolution of a new modified 310 austenitic steel, hot compression tests were conducted at the temperature range from 800 to 1100 °C with strain rate of 0.1–10 s⁻¹ and strain of 30–70% using Gleeble 3500 thermal–mechanical simulator. The results showed that the serrated flow curves were caused by the competitive interaction between solute atoms and mobile dislocations. There were some coarsened precipitates on the high angle grain boundaries (HAGBs), which facilitated the nucleation of dynamic recrystallization grains. But these precipitates inhibited the growth of the recrystallization grains, and changed the deformation texture in the matrix. Low angle grain boundaries (LAGBs) decreased, while twin GBs and random HAGBs and increased as dynamic recrystallization occurred. Dynamic recrystallization occurred more readily at evaluated temperature or high strain rate. The true stress decreased with the reduction of LAGBs percent. The internal connections between mechanics and microstructures were also discussed.     

Recrystallization and Grain Growth in Ultrafine‐Grained Materials Produced by High Pressure Torsion

Ultrafine‐grained (UFG) materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of UFG materials. For this purpose a Cu–0.17 wt%Zr alloy was subjected to high pressure torsion (HPT) with a maximal pressure of 4.8 GPa at room temperature. The microstructures of the specimens were characterized using electron back scatter (EBSD) measurements, transmission electron microscopy (TEM), and hardness measurements. During annealing of the samples, dispersoids were formed which improved the thermal stability of the alloy. At higher strain levels the fraction of high angle grain boundaries (HAGBs) increased above 70% of the total grain boundaries.     

Features of the strain hardening of nickel titanium in the region of thermoelastic martensite transformation

The inheritance of strain hardening in a nickel titanium (TiNi) alloy that possesses shape memory has been studied using a sample subjected to plastic deformation in the austenite state prior to a transition to the martensite state. The results show that the strain hardening created in the TiNi alloy in the austenite state is not inherited in the martensite state and the sample behaves as if there were no significant plastic deformation. It is suggested that this behavior can be related to the fact that the alloy lattice defects (primarily, dislocations generated during pre-deformation in the austenite state) are not effective obstacles to the motion of domain boundaries in the martensite state.     

Effect of microstructure on micromechanical performance of dry cortical bone tissues

The mechanical properties of bone depend on composition and structure. Previous studies have focused on macroscopic fracture behavior of bone. In the present study, we performed microindentation studies to understand the deformation properties and microcrack–microstructure interactions of dry cortical bone. Dry cortical bone tissues from lamb femurs were tested using Vickers indentation with loads of 0.245–9.8 N. We examined the effect of bone microstructure on deformation and crack propagation using scanning electron microscopy (SEM). The results showed the significant effect of cortical bone microstructure on indentation deformation and microcrack propagation. The indentation deformation of the dry cortical bone was basically plastic at any applied load with a pronounced viscoelastic recovery, in particular at lower loads. More microcracks up to a length of approximately 20 μm occurred when the applied load was increased. At loads of 4.9 N and higher, most microcracks were found to develop from the boundaries of haversian canals, osteocyte lacunae and canaliculi. Some microcracks propagated from the parallel direction of the longitudinal interstitial lamellae. At loads 0.45 N and lower, no visible microcracks were observed.     

Grain size altering yielding mechanisms in ultrafine grained high-Mn austenitic steel:Advanced TEM investigations

The underlying mechanism of discontinuous yielding behavior in an ultrafine-grained(UFG)Fe-31Mn-3Al-3Si(wt.％)austenitic TWIP steel was investigated by the use of advanced TEM technique with taking the plastic deformation mechanisms and their correlation with grains size near the macroscopic yield point into account.Typical yield drop mechanisms such as the dislocation locking by the Cottrell atmo-sphere due to the presence of interstitial impurities cannot explain the origin of this phenomenon in the UFG high-Mn austenitic TWIP steel.Here,we experimentally revealed that the plastic deformation mechanisms in the early stage of deformation,around the macroscopic yield point,show an obvious association with grain size.More specifically,the main mechanism shifts from the conventional slip in grain interior to twinning nucleated from grain boundaries with decreasing the grain size down to less than 1 μm.Our observation indicates that the grain size dependent deformation mechanisms transition is also deeply associated with the discontinuous yielding behavior as it could govern the changes in the grain interior dislocation density of mobile dislocations around the macroscopic yield point.     

Hydrogen-induced Modification in the Deformation and Fracture of a Precipitation-hardened Fe-Ni Based Austenitic Alloy

Hydrogen-induced modification in the deformation and fracture of a precipitation-hardened Fe-Ni based austenitic alloy has been investigated in the present study by means of thermal hydrogen charging experiment, tensile tests, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. It is found that the γ＇ particles are subjected to the multiple shearing by dislocations during plastic deformation, which promotes the occurrence of the dislocation planar slip. Moreover, the alloy will be enhanced by hydrogen resulting in the formation of strain localization at macroscale. So, the mechanisms of deformation and fracture in the alloy have been proposed in terms of serious hydrogen-induced planar slip at microscale which can lead to macroscopic strain localization.