Competing deformation mechanisms in nanocrystalline metals and alloys: Coupled motion versus grain boundary sliding

Plastic deformation of nanocrystalline Pd and Cu as well as the demixing systems Cu–Nb and Cu–Fe is studied by means of atomic-scale computer simulations. The microstructures are specifically chosen to facilitate mesoscopic grain boundary sliding. The influence of segregating solutes on the deformation mechanisms is studied and different cases of solute distributions are compared. We find that the competition between mesoscopic grain boundary sliding and coupled grain boundary motion is controlled by the concentration and distribution of segregating solutes. By analyzing the microstructural evolution and dislocation activity we make a connection between the atomistic solute distribution and the mechanisms of deformation, explaining the observed stress–strain behavior. The detailed analysis of the normal grain boundary motion reveals a stick–slip behavior and a coupling factor which is consistent with results from bicrystal simulations.     

FEM–MD combined method for evaluation of microscopic stress distribution by considering microscopic heterogeneity and deformation in close vicinity to grain boundary of steels

Finite element method (FEM)–molecular dynamics (MD) combined method is proposed for the microscopic stress analysis of steels. In this numerical method, FEM is applied to the stress analysis inside grains, and MD is applied to the calculation of the atomic configuration near the grain boundary in order to consider the microscopic heterogeneity and the deformation near the grain boundary that influences the stress distribution. Slip length between two grains caused by the mismatch of the displacement near the grain boundary is calculated by FEM. Slip resistance, which is necessary to calculate slip length, is obtained by calculating the atomic configuration near the grain boundary by MD. The combination of FEM and MD is realized by using slip resistance in FEM and slip length in MD. The validity of modelling of the deformation near the grain boundary is investigated by comparing the deformation near the grain boundary calculated by FEM–MD combined method to that observed in the experiment in the case of a load applied to the specimen. Calculated slip length coincides with measured slip length. FEM–MD combined method is applied to the investigation of the influence of change in the grain shape caused by the thermal history such as the weld zone upon the strength characteristic. The high stress region tends to increase the incidence of larger grain diameter and it is indicated that grain coarsening due to the weld thermal history increases the possibility of the crack initiation. FEM–MD combined method is expected to be helpful in investigating the mechanism of fracture or the strength characteristic of the complicated microstructure such as the weld zone by evaluating the microscopic stress distribution.     

Simulation of stress concentration in Mg alloys using the crystal plasticity finite element method

A crystal plasticity finite element method (CPFEM), considering both crystallographic slip and deformation twinning, was developed to simulate the spatial stress concentration in AZ31 Mg alloys during in-plane compression. A predominant twin reorientation (PTR) model was successfully implemented to capture grain reorientation due to deformation twinning in twin-dominated deformation. By using the direct mapping technique for electron backscatter diffraction (EBSD) data, CPFEM can capture the heterogeneity of stress concentration at the grain boundaries in AZ31 Mg alloys during in-plane compression. The model demonstrated that deformation twinning enhances the local stress concentration at the grain boundaries between untwinned and twinned grains.     

Serrated flow and stick–slip deformation dynamics in the presence of shear-band interactions for a Zr-based metallic glass

In this paper, shear-band interactions (SBIs) were introduced by a simple method and their effect on the dynamics of shear bands and serrated flow was studied for a Zr-based metallic glass. Statistical analysis on serrations shows that the stick–slip dynamics of interacting shear bands is a complex, scale-free process, in which shear bands are highly correlated. Both the stress drop magnitude and the incubation time for serrations follow a power-law distribution, presenting a sharp contrast to the randomly generated, uncorrelated serrated flow events in the absence of SBIs. Observations on the fracture morphologies provide further evidence and insights into the deformation dynamics dominated by SBIs. A stick–slip model for multiple shear bands with interactions is also proposed and numerically calculated. The results, in good agreement with the experimental results, quantitatively show how multiple shear bands operate and correlate, especially for those with large serrated flow events. Our studies suggest that one serration in the stress–strain curve may correspond to collective stick–slip motions of multiple shear bands for those ductile bulk metallic glasses where a large number of shear bands are observed during deformation.     

The effect of grain size and stability on ambient temperature tensile and creep deformation in metastable beta titanium alloys

The effect of grain size in the range of 100–500 μm on the ambient temperature tensile and creep deformation behavior was investigated in a metastable Ti–9.4 wt% Mn alloy at 95% yield stress. It was observed that, as grain size increases the creep strain increases, the slip lines become coarser and the density of slip increases. An interrupt creep test showed that new slip lines were generated with time and most of the deformation occurred during the first few hours of testing. The effect of β-phase stability was studied by comparing the creep and tensile behavior of Ti–9.4 wt% Mn in this study, with previously reported results obtained for Ti–14.8 wt% V and Ti–13.0 wt% Mn at ambient (room) temperature. The Molybdenum Equivalency, which is a measure of the stability of these alloys, are 9.9, 14.3, and 19.9, respectively. The main mode of deformation in Ti–9.4 wt% Mn alloy was observed to be slip. The modes of deformation changed with the increase in stability from slip accompanied by Stress Induced Plate formation to only slip. It was also found that metastable ω-phase was present in lower stable alloys, irrespective of the observed mode of deformation. At 95% yield stress, the amount of creep strain in 200 h was found to increase with a decrease in stability of the β-phase.     

Deformation mechanism at impact test of Al-11% Si alloy processed by equal-channel angular pressing with rotary die

Al-11%Si(mass fraction)alloy was transformed into a ductile material by equal-channel angular pressing(ECAP)with a rotary die.Two mechanisms at impact test,slip deformation by dislocation motion and grain boundary sliding,were discussed.The ultrafine grains with modified grain boundaries and the high content of fine particles(<1μm)were necessary for attaining high absorbed energy.The results contradict the condition of slip deformation by dislocation motion and coincide with that of grain boundary sliding.Many fine zigzag lines like a mosaic were observed on the side surface of the tested specimens.These observed lines may show grain boundaries appeared by the sliding of grains.     

Analysis of stress states in compression stage of high pressure torsion using slab analysis method and finite element method

High pressure torsion (HPT) is useful for achieving substantial grain refinement to ultrafine grained/nanocrystalline states in bulk metallic solids. Most publications that analyzed the HPT process used experimental and numerical simulation approaches, whereas theoretical stress analyses for the HPT process are rare. Because of the key role of compression stage for the deformation of HPT, this paper aims to conduct a theoretical analysis and to establish a practical formula for stress and forming parameters of HPT process using the slab analysis method. Three equations were obtained via equations derivation to describe the normal stress states corresponding to the three zones of plastic deformation for HPT process as stick zone, drag zone and slip zone. As to the compression stage of HPT, the stress distribution results using the finite element method agree well with those using the slab analysis method. There are drag and stick zones on the contact surface of the HPT sample, as verified by the finite element method (FEM) and slab analysis method.     

Fretting wear behavior of liquid nitrided structural steel,En24 and bearing steel,En31

Fretting wear failures are reported in many bearing-shaft assemblies. Fretting wear characteristics of structural steel, En24 in as-received, hardened–tempered and liquid nitrided condition, when fretted with hardened–tempered bearing steel, En31 is reported in this paper. Tests were conducted at different normal loads and at constant slip amplitude under unlubricated conditions. Coefficient of friction under fretting conditions and wear resistance were measured. Surface chemistry influences the fretting wear behavior more than the hardness of the material. The mono phase epsilon iron nitride structure of compound layer formed in liquid nitriding process offers improved fretting wear resistance against bearing steels. The fretting wear resistance also depends upon the normal load and the nature of contact, stick–slip or gross slip.     

冲击接触载荷下45钢的微观塑性变形特征与损伤

选取退火４５钢为研究对象，考察冲击接触载荷下亚表层微观形变特征与损伤，结果表明，冲击接触载荷下，铁素体的变形以多滑移为特征；滑移在晶界受阻产生应力集中并产生力偶使晶粒发生相对转动，从而在晶界产生裂纹；亚晶界和亚晶粒的形成是不同区域的不同滑移系统的交互作用的结果。     

Crack opening due to deformation twin shear at grain boundaries in near-γ TiAl

Microcrack nucleation has been observed at apparent deformation twin interactions with grain boundaries in a duplex near-gamma TiAl specimen deformed to surface tensile strain of about 1.4%. To prove that these microcracks are a result of twins and not dislocation slip bands, detailed characterization of the surface topography and crystallography associated with the microcracks was conducted and analyzed. Electron backscatter diffraction patterns were used in conjunction with selected area channeling patterns to determine the crystallography near the observed microcracks. Transmission electron microscopy of a selected twin, extracted using a focused ion beam, and atomic force microscopy were used to show that the observed microcracks could only have been caused by local strain heterogeneities caused by twin interactions with grain boundaries and not by dislocation slip bands.     

Symmetric and Asymmetric Rolling Pure Copper Foil: Crystal Plasticity Finite Element Simulation and Experiments

A systematic study has been conducted aiming to attain an insight into the influence of coefficient of roll speed asymmetry, crystal orientation and structure on the deformation behavior, and crystallographic orientation development during foil rolling. Simulations were successfully carried out by using crystal plasticity finite element method(CPFEM),and a novel computational framework is presented for the representation of virtual polycrystalline grain structures. It has been found that asymmetric rolling(ASR) is more efficient in producing plastic deformation since it develops additional shear strain and activity of slip system compared with symmetric rolling(SR). For ASR, increase in the length of the shear zone, and decrease in the amount of the pressure and roll force would lead to further reduction. The shear strain path in SR and ASR is strictly influenced by the misorientation of neighbor grains, and corresponding {1 1 1} pole figures offer direct evidence of the spread of crystallographic orientation around the normal direction. The activity of slip systems was examined in detail and found that the predicted results are consistent with the surface layer model. The accuracy of the developed CPFEM model is verified by the fact that the simulated results of roll force coincide well with the experimental results.     

Evaluating the Plastic Anisotropy of AZ31 Using Microscopy Techniques

The tensile deformation mechanism of a rolled AZ31 alloy at 50°C, 150°C, and 250°C was investigated by a combination of in situ tensile testing, electron backscatter diffraction analysis, and ex situ atomic force microscopy analysis. With increasing temperature, there was a significant difference in the activity of the various deformation modes, along with a decrease in the plastic strain ratio. Extension twinning was only observed at 50°C, while at higher temperatures, a combination of basal and prismatic slip accounted for a large percentage of the observed deformation activity. Prismatic slip was prevalent at all testing temperatures and exhibited increased activity with increasing temperature. The activity of pyramidal 〈c + a〉 slip increased from 50°C to 150°C and then decreased at 250°C. Ex situ atomic force microscopy measurements suggested that the contribution from grain boundary sliding to the overall strains increased with increasing temperature. Overall, the in situ experiments combined with atomic force microscopy suggested that grain boundary sliding contributed more to the reduction in plastic strain ratio with increasing temperature than nonbasal slip activity.     

多晶Nb的纳米压痕研究

晶界硬化和滑移越过晶界传播的机制是材料变形研究中的两个重要问题，纳米压痕技术是研究滑移与晶界关系的有效手段之一．本文利用这一技术研究了多晶Nb晶界附近的变形行为，结果表明，当压痕打在一些晶界附近时，诱导了一种位移突变现象即晶界pop-in现象．与对应于材料初始塑性的pop—in不同，这种晶界pop-in现象与晶界二侧晶粒取向差密切相关．晶界pop—in与晶界附近硬度的变化无明显关联．     

Grain Boundary Responses to Heterogeneous Deformation in Tantalum Polycrystals

The evolution of heterogeneous deformation in a tantalum polycrystal was examined during a three-point bending experiment using electron backscatter pattern mapping. Slip bands formed at strains as low as 1%, and they became more intense with strain. Heterogeneous deformation was evident as intragranular orientation gradients as large as 30° were observed after a strain of about 8%. Nonmonotonic changes in the local average misorientation distribution were observed, implying that dislocation substructure developed in a complex manner. Slip bands were analyzed using plane traces computed from local orientation information. With the assumption of uniaxial stress, Schmid factors for favorable slip systems were identified for each grain and compared with observations, showing evidence for macroscopic activity on both {110} and {112} slip systems. Reconstructed boundary data were used to estimate the geometric potential for slip transfer at grain boundaries. The correlations indicated that when active slip systems were favorably oriented for slip transfer across the boundary, it was often observed in the form of continuous slip bands aligned across the boundary. In boundaries where geometrical alignment and Schmid factors were not favorable for slip transfer, there was a higher likelihood to form ledges (topographic discontinuities) along the grain boundaries. Dislocation pileups at grain boundaries were also correlated with a low potential for slip transfer.     

Effect of electric current pulse on grain growth in superplastic deformation of 2091 Al-Li alloy

1　INTRODUCTIONThecharacteristicthatelectriccurrentpulsepro motesatomdiffusionanddislocationmotionwasgrad uallyrecognizedfromthefailureanalysisonintegrat edcircuitin 1970s[110 ] .Theelectriccurrentpulsewasusedtoincrease plastic property ,improvetheformabilityofless deformablemetals[11,12 ] ,andstudytheeffectsofcurrentpulseonrecrystallization[1318]respectivelyin 1980s .Inthisrespect ,ConradandLaihavedonealotofwork ,inwhichCu ,AlandTiweremainexperimentalmaterialsfordeformationemployedwitha…     

Simulation of deformation twins and deformation texture in an AZ31 Mg alloy under uniaxial compression

To investigate deformation twins and the evolution of deformation texture during plastic deformation, uniaxial compression tests on a hot-rolled AZ31 Mg alloy were carried out at 200 °C. Cylindrical specimens were then compressed in both the rolling and the normal directions. The findings revealed that texture evolution, work hardening and macroscopic anisotropy are strongly dependent on the loading direction. Electron backscattered diffraction analysis was used to examine the orientation of parent grains and twin bands in the AZ31 Mg alloy under uniaxial compression. A viscoplastic self-consistent model (VPSC) was theoretically employed to calculate the relative activities of slip and twin systems in polycrystalline hexagonal aggregates under uniaxial compression. Each deformed grain exhibited an independent number and type of twin variants under uniaxial compression. Neutron diffraction was used to measure the macroscopic texture of the AZ31 Mg alloy. The VPSC model was used to simulate texture evolution, work hardening and macroscopic anisotropy during the uniaxial compression. A modified predominant twin reorientation (PTR) scheme was suggested to explain the gradual increase in twin volume in deformed grains.     

Role of discrete intragranular slip on lattice rotations in polycrystalline Ni: Experimental and micromechanical studies

In this paper, a new micromechanical approach accounting for the discreteness of intragranular slip is used to derive the local misorientations in the case of plastically deformed polycrystalline nickel in uniaxial tension. This intragranular microstructure is characterized in particular single slip grains by atomic force microscopy measurements in the early stage of plastic deformation. The micromechanical modelling accounts for the individual grain size, the spatial distances between active slip bands and the magnitude of slip in bands. The slip bands are modelled using discrete distributions of circular super glide dislocation loops constrained at grain boundaries for a spherical grain boundary embedded in an infinite matrix. In contrast with classic mean field approaches based on Eshelby’s plastic inclusion concept, the present model is able to capture different intragranular behaviours between near grain boundary regions and grain interiors. These theoretical results are quantitatively confirmed by local electron backscatter diffraction measurements regarding intragranular misorientation mapping with respect to a reference point in the centre of the grain.     

Atomistic simulation of tension deformation behavior in magnesium single crystal

The deformation behavior in magnesium single crystal under c-axis tension is investigated in a temperature range between 250 K and 570 K by molecular dynamics simulations. At a low temperature, twinning and shear bands are found to be the main deformation mechanisms. In particular, the {1012} tension twins with the reorientation angle of about 90° are observed in the simulations. The mechanisms of {1012} twinning are illustrated by the simulated motion of atoms. Moreover, grain nucleation and growth are found to be accompanied with the {1012} twinning. At temperatures above 450 K, the twin frequency decreases with increasing temperature. The {1012} extension twin almost disappears at the temperature of 570 K. The non-basal slip plays an important role on the tensile deformation in magnesium single crystal at high temperatures.     

The role of extrusion texture on strength and its anisotropy in a Mg-base alloy composed of the Long-Period-Structural-Order phase

A Mg–Y–Zn alloy composed almost completely of the Long-Period-Stacking-Ordered (LPSO) phase has been prepared by casting and extrusion to flat bar. An elongated microstructure is obtained with different grain orientations along the extrusion, transverse and normal directions, leading to a strong orientation dependence of strength. Differences of tensile and compressive stress are also noted. The importance of basal slip and other dislocation-dependent deformation mechanisms is discussed. The large tension-compression strength differences are considered to imply that twinning plays an important role in determining strength. The analysis of deformation mechanisms and strengthening is supported by metallographic studies of surface slip bands and dislocation distributions.