COMPUTER SIMULATION OF EARLY STAGE OF DISLOCATION STRUCTURES IN A FATIGUED COPPER SINGLE CRYSTAL

The dislocation evolution wassimulated by using positiveand negative parallelstraightedgedislocationsrandomly distributing on the primary slip planes astheinitialconditions. Thevein and wallstructuresof dynamicequilibrium have been obtained. Abig dipolestructure wasfound inthesimulation.

Effect of dislocation structure evolution on low-angle grain boundary formation in 7050 aluminum alloy during aging

The effect of dislocation structure evolution on low-angle grain boundary formation in 7050 aluminum alloy during aging was studied by using optical microscopy, transmission electron microscopy, and electron backscatter diffraction analysis of misorientation angle distribution, cumulative misorientation and geometrically necessary dislocation （GND） density. Experimental results indicate that coarse spindle-shaped grains with the dimension of 200 μm- 80 μm separate into fine equiaxed grains of 20μm in size as a result of newborn low-angle grain boundaries formed during the aging process. More specifically, the dislocation arrays, which are rearranged and formed due to scattered dislocations during earlier quenching, transform into low-angle grain boundaries with aging time. The relative frequency of 3°-5° low-angle grain boundaries increases to over 30%. The GND density, which describes low-angle grain boundaries with the misorientation angle under 3°, tends to decrease during initial aging. The inhomogeneous distribution of GNDs is affected by grain orientation. A decrease in GND density mainly occurs from 1.83 × 10^13 to 4.40 × 10^11 m^-2 in grains with 〈111〉 fiber texture. This is consistent with a decrease of unit cumulative misorientation. Precipitation on grain boundaries and the formation of a precipitation free zone （PFZ） are facilitated due to the eroding activity of the Graft etchant. Consequently, low-angle grain boundaries could be readily viewed by optical microscopy due to an increase in their electric potential difference.

Effects of Re, Ta, and W in [110] (001) dislocation core of γ/γ' interface to Ni-based superalloys: First-principles study

The strengthening effects of alloying elements Re,Ta,and W in the[110](001)dislocation core of theγ/γ'interface are studied by first-principles calculations.From the level of energy the substitution formation energies and the migration energies of alloying elements are computed and from the level of electron the differential charge density(DCD)and the partial density of states(PDOSs)are computed.Alloying elements above are found to tend to substitute for Al sitesγ'phase by analyzing the substitution formation energy.The calculation results for the migration energies of alloying elements indicate that the stability of the[110](001)dislocation core is enhanced by adding Ta,W,and Re and the strengthening effect of Re is the strongest.Our results agree with the relevant experiments.The electronic structure analysis indicates that the electronic interaction between Re-nearest neighbor(NN)Ni is the strongest.The reason why the doped atoms have different strengthening effects in the[110](001)dislocation core is explained at the level of electron.

BAUSCHINGER EFFECT AND DISLOCATION STRUCTURE IN STAINLESS STEEL DURING CYCLIC DEFORMATION

Cyclic deformation in symmetrical push-pull mode was carried out at room temperature in air using a Schenck hydropuls machine.The total strain amplitude which was kept constant dur- ing the test ranged from ±0.004 to±0.012.The 0.2% offset yield stress σ_(0.2f) in tension and σ_(0.2r) in compression and peak stress σ were measured from the stress-strain hysteresis loop at various cycles.The Bauschinger strenghth differential factor(BSDF)was then calcu- lated from σ_(0.2f) and σ_(0.2r).The energy loss △E of each cycle was determined from hysteresis loop areas.These parameters,BSDF,σ and △E,appear to have two distinctively different stages. The dislocation structures were observed using TEM in specimens deformed cyclically,for various cycles.The observation shows that the dislocations pile-up mainly against grain boundaries and there exist large amount of deformation twins.The addition of 0.25 wt-% ni- trogen reduced the stacking fault energy of the alloy significantly.Cross-slip and climb are therefore rather difficult to occur during the cyclic deformation at room temperature,and well-defined dislocation cells and walls can only be seen at the final stage of fatigue.

Deformation and Dislocation Structure in L1_2 Titanium Trialuminide Alloys

The deformation behaviour and the nature of dislocations of the Al3Ti-base L12 alloya modified with Fe and Mn etc, were investigated. The results show that the deformation and fracture character istics are closely related to the alloy compositions. The effect of hot-working process on the room tem perature ductility is remarkable, not only resulting in an appreciable improvement of compressive properties but also showing a 0.28% plastic strain in tensile test. The SISF dissociation of a < 110>dislocations on {111} planes was found at room temperature. The determined dissociation scheme is consistent with the mechanical behaviour of these alloys in the lower temperature region.

Low Energy Dislocation Structure in Deformed Metals

In this paper,the dislocation distribution struc- ture in deformed metal is discussed.The flow stress of material for the heterogeneous dislocation distri- bution which tends to the flow stress for the homo- geneous dislocation distribution in the limiting case is derived.The causes and the effects of the long range internal stresses are discussed.The total deformation energy of material system is obtained and the trend of evolution of dislocation distribu- tion in deformed metals is discussed simultaneously. No micromechanisms of dislocations are involved in the discussion,therefore the theory developed in this paper is universal.

Fatigue Cracking Characterization of High Grade Non-oriented Electrical Steels

The fatigue behavior of 30 WGP1600 non-oriented electrical steel, which is generally used in the motors for electrical vehicles, was investigated. The detailed microstructure and dislocation configurations of the fatigue specimens were examined by OM, SEM, and TEM. The test results showed that fatigue cracks were commonly initiated from the surface grain boundaries, crystals plane, and inclusions. The rapid fatigue crack propagation was characterized by transgranular cleavage fracture, while most transient fracture exhibited ductile tearing characteristics. After cyclic deformation of the non-oriented electrical steels, various dislocation structures, such as short and thick lines, veins, persistent slip bands, cells, and labyrinth, were observed.

Multiscale simulations in face-centered cubic metals:A method coupling quantum mechanics and molecular mechanics

An effective multiscale simulation which concurrently couples the quantum-mechanical and molecular-mechanical calculations based on the position continuity of atoms is presented.By an iterative procedure,the structure of the dislocation core in face-centered cubic metal is obtained by first-principles calculation and the long-range stress is released by molecular dynamics relaxation.Compared to earlier multiscale methods,the present work couples the long-range strain to the local displacements of the dislocation core in a simpler way with the same accuracy.

CONSTITUTIVE MODELLING OF NONPROPORTIONAL CYCLIC PLASTICITY

A multiplicative hardening function and a unified evolution rule of the hardening factors are proposed.The hardening factor f_1 is introduced to describe cyclic hardening with respect to the plastic strain range,while f_2 and f_3 describe,respectively,instantaneous and hereditary additional hardening with respect to the nonproportionality of the plastic strain path.Two material dependent memory parameters α_1 and α_3 are introduced to keep the memory of the largest cyclic and additional hardening in the previous plastic deformation history.Different hardening mechanisms are then embedded into a thermomechanically consistent constitutive equation through the hardening function.The constitutive response of 304 and 316 stainless steels subjected to biaxial nonproportional cyclic loading is analyzed and the proposed model is critically verified by comparing the results with experimental results obtained by Tanaka et al.,and Ohashi et al.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Fe_3Al BASED ALLOYS

The promise for industrial applications offered by iron aluminides is today restricted by insufficient ductility at room temperature and mediocre strength and creep resistance at high temperatures. The tendency to embrittlement in the presence of hydrogen or water vapour limits the ductility even more. The atomic arrangements in binary and alloyed variants are examined here and related to the difficulties of dislocation propagation at room and at high temperatures. In this way the influence of intrinsic structure and alloying modifications on mechanical behaviour can be understood. Possibilities for further improving properties through structure control are considered.

PLASTIC DEFORMATION BEHAVIOR OF ELECTROFORMED COPPER LINER OF SHAPED CHARGE AT DIFFERENT STRAIN RATES

The paper deals with different plastic deformation behavior of electroformed copper liner of shaped charge, deformed at high strain rate (about 1×107s-1) and normal strain rate (4×10-4s-1). The crystallographic orientation distribution of grains in recovered slugs which had undergone high-strain-rate plastic deformation during explosive detonation was investigated by electron backscattering Kikuchi pattern technique. Cellular structures formed by tangled dislocations and sub-grain boundaries consisting of dislocation arrays were detected in the recovered slugs. Some twins and slip dislocations were observed in specimen deformed at normal strain rate. It was found that dynamic recovery and recrystallization take place during high-strain-rate deformation due to the temperature rising, whereas the conventional slip mechanism operates during deformation at normal strain rate.

Regular Pattern of Morphology Transition in Mixed Martensite Structures

The morphology and substructure of mixed martensites in ferrous alloys have been examined by using optical and transmission electron microscope. The results indicated that the main formation se- quence of martensitic morphology was butterfly→ plate→lath,with decreasing forming temperatures when the plastic accommodation takes place in the parent phase,which is affected by the transforma- tion strain fiélds.It was shown that the martensite morphology is not only decided by the forming temperature alone,but also by the dislocation struc- ture in austenite before the transformation.

The Study of Dual-phase Steel after Cyclic Deformation at Various Strain Amplitudes

Low cycle fatigue tests under plastic strain control were carried out with a dual-phase steel containing 23 Vol.-％ martensite. Specimens hardened rapidly at first few cycles followed by a slight softening to saturation stages when cycled at higher strain amplitudes, whereas at lower strain amplitudes the specimens presented continually hardening for a long time until saturation. TEM examination of the saturation dislocation structures show that clusters, parallel walls and cells were found at low, medium and high strain amplitude, respectively. It also has been found that the martensite/ferrite interfaces did not affect the dislocation structures signi- ficantly when a specimen was fatigued at lower strain amplitude. However, the dislocation struc- ture adjacent to the two-phase boundary is dif- ferent to some extent from that in the remote regions in the ferrite when a higher strain amplitude is applied.

PLASTIC DEFORMATION OF TiAl INTERMETALLICS WITH Mn ADDITION

The dislocation structures of arc-melted,near-equiatomic TiAl alloys containing 1.5-2.0 wt.-%manga-nese were analysed and compared with those of binary TiAl alloy processed in a similar fashion.It was found that the pinning effect of a/6[112]partial dislocations was eliminated by Mn addition,as a result,the movement of a[101]and a/2[112]superdislocations and twinning and played an important role in the plastic deformation and increased the ductility of TiAl alloys.In addition,the workability of the Mn-ductilized TiAl alloy was evaluated with hot extrusion and forging.

EFFECT OF DIFFERENT DIMENSION STABILIZATION TECHNIQUES ON DIMENSION STABILITY OF SiC_(p)/Al COMPOSITES

The elTcct of different dimension stabilization treatments on the dimension stability of SiC_(p)/Al composites were investigated by the measurement of microyield strength,microcreep properties,dimension,residual stress and analyses of transmission electron microscopy.Results show that the heat cycle dimension stabilization techniques could efficiently decrease the residual stress,stabilize the dimension change,and increase resistance to microplastic deformation.The main reason was that the heat cycle dimension stabilization techniques might form stable dislocation structure in the SiC_(p)/A1 composites.

The Characteristic of Threading Dislocation in Different Structures GaN Films Grown by MOCVD

Three mechanisms to reduce threading dislocations(TDs) in GaN films as the epitaxial films grow thicker are suggested by SEM and

The dislocation structure of slip bands in deformed high entropy alloy nanopillars

Remarkable diversity is observed in dislocation interactions that are responsible for intermittent and sudden crystal slips.While large crystal slips can be easily observed on the surface of deformed crystals,unraveling the underlying dislocation interaction mechanisms,however,has been a longstanding challenge in the study of single-crystal plasticity.A recent study demonstrated that the sluggish dislocation dynamics in the high entropy alloy(HEA)of Al_(0.1)CoCrFeNi enables the observation of slip bands for a direct link to dislocation avalanches in a nanopillar.Here,we further examined the dislocation structure of slip bands in the HEA nanopillars oriented for single slip.Experimental evidence was provided on the dislocation organization in a slip band based on groups of primary dislocations,secondary dislocations,and dislocation pileups.The results were compared with the previously proposed slip band models.The unique aspects of the HEA that enable such observations were also investigated through an examination of the dislocation microstructure and its response to applied forces in the HEA nanopillars.

High Temperature Low Cycle Fatigue Behavior of GH4742 Alloy

High temperature low cycle fatigue tests on GH4742 superalloy were studied under the total strain-con- trolled conditions at 650 ℃. Combined with fatigue test data, fatigue properties of the alloy were analyzed. Fracture morphology and dislocation structure were observed by scanning electron microscopy and transmission electron mi- croscopy. The results showed that fatigue life and fatigue resistance of GH4742 alloy decreased significantly with in- creasing total strain amplitude. The cyclic hardening, cyclic softening and cyclic stability phenomena of the alloy oc- curred during the low cycle fatigue process. The increasing total strain amplitude is conducive to the formation of γ1 phase. Fatigue crack propagation is controlled jointly by ductile and brittle fracture. Inhomogeneous deformation and deformation restricted in slip bands are the main reasons for the reduction of fatigue life of GH4742 alloy.