Resolved Shear Stress For Twinning Research Articles

The influence of temperature on the strain-hardening behavior of Fe-22/25/28Mn-3Al-3Si TRIP/TWIP steels

The influence of temperature and stacking fault energy (SFE) on the strain-hardening behavior and critical resolved shear stress for twinning was investigated for three Fe–22/25/28Mn–3Al–3Si wt.% transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The SFEs were calculated by two different methods, density functional theory and statistical thermodynamic modeling. The dislocation structure, observed at low levels of plastic deformation, transitions from “planar” to “wavy” dislocation glide with an increase in temperature, Mn content, and/or SFE. The change in dislocation glide mechanisms from planar to wavy reduces the strain hardening rate, in part due to fewer planar obstacles and greater cross slip activity. In addition, the alloys exhibit a large decrease in strength and ductility with increasing temperature from 25 to 200 °C, attributed to a substantial reduction in the thermally activated component of the flow stress, predominate suppression of TRIP and TWIP, and a significant increase in the critical resolved shear stress for mechanical twinning. Interestingly, the increase in SFE with temperature had a rather minor influence on the critical resolved shear stress for mechanical twinning, and other temperature dependent factors which likely play a more dominant role are discussed.

Tension/compression asymmetry in additive manufactured face centered cubic high entropy alloy

The high entropy alloy, Al0.3CoCrFeNi, has been fabricated by direct laser fabrication and its deformation behaviour studied. This alloy exhibited significant tension/compression asymmetry in its work hardening rate and ductility. The exceptionally high work hardening in compression has been attributed to profuse mechanical twinning which has been exacerbated by a strong texture in the as-cast material. The tensile deformed material did not exhibit any mechanical twinning, deformed exclusively by slip, and therefore showed minimal work hardening and poor ductility. The critical resolved shear stress for twinning of the alloy was found to be approximately 240MPa.

Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high-entropy alloy

At low homologous temperatures (down to cryogenic temperatures), the CrMnFeCoNi high-entropy alloy possesses good combination of strength, work hardening rate (WHR), ductility, and fracture toughness. To improve understanding of the deformation mechanisms responsible for its mechanical properties, tensile tests were performed at liquid nitrogen and room temperature (77 K and 293 K) and interrupted at different strains to quantify the evolution of microstructure by transmission electron microscopy. Dislocation densities, and twin widths, their spacings, and volume fractions were determined. Nanotwins were first observed after true strains of ∼7.4% at 77 K and ∼25% at 293 K; at lower strains, deformation occurs by dislocation plasticity. The tensile stress at which twinning occurs is 720 ± 30 MPa, roughly independent of temperature, from which we deduce a critical resolved shear stress for twinning of 235 ± 10 MPa. In the regime where deformation occurs by dislocation plasticity, the shear modulus normalized WHR decreases with increasing strain at both 77 K and 293 K. Beyond ∼7.4% true strain, the WHR at 77 K remains constant at a high value of G/30 because twinning is activated, which progressively introduces new interfaces in the microstructure. In contrast, the WHR at room temperature continues to decrease with increasing strain because twinning is not activated until much later (close to fracture). Thus, the enhanced strength-ductility combination at 77 K compared to 293 K is primarily due to twinning starting earlier in the deformation process and providing additional work hardening. Consistent with this, when tensile specimens were pre-strained at 77 K to introduce nanotwins, and subsequently tested at 293 K, flow stress and ductility both increased compared to specimens that were not pre-strained.

Investigation of the dependence of deformation mechanisms on solute content in polycrystalline Mg–Al magnesium alloys by neutron diffraction and acoustic emission

Influence of aluminium content on the deformation mechanisms in Mg–Al binary alloys has been studied using in-situ neutron diffraction and acoustic emission technique. It is shown that the addition of the solute increases the critical resolved shear stress for twinning. Further, the role of aluminium on the solid solution hardening of the basal plane and softening of non-basal planes are discussed using results of the convolutional multiple peak profile analysis of diffraction patterns. The results indicate that the density of both prismatic 〈a〉 and pyramidal 〈c+a〉 dislocations increases with increasing alloying content.

Mechanical twins in 304 stainless steel after small-charge explosions

Optical, scanning electron, and scanning tunneling microscopy, as well as X-ray diffraction, were employed to detect microstructural modifications caused in AISI 304Cu steel disks by explosions of small charges placed at close ranges. Explosions induced limited or no gross macro-deformation. NSP (plastic) explosive spherical charges of 54.5 and 109 g TNT equivalent mass and explosive-to-target distances in the range from 6.5 to 81.5 cm were used to achieve peak pressures in the 160–0.5 MPa range. Two alloy grain sizes (60 and 32 μm) were tested. Surface optical and scanning electron microscopy revealed partial surface melting, zones with indications of recrystallization, and intense mechanical twinning, which was detected also by X-ray diffraction. In the interior of the samples, twins were exclusively found. They can be seen up to some distance from the explosion-impinged surface and again, at the shortest charge-to-sample distances, in a thin layer around the reflecting surface. For metal object mapping after explosions, an important source of information in forensic science, the maximum charge-to-target distance at which the phenomena disappear has been singled out for each charge and alloy grain size and related to the critical resolved shear stress for twinning.

Metal Objects Mapping After Small Charge Explosions. A Study on AISI 304Cu Steel with Two Different Grain Sizes

Evidence of exposure of a metal component to a small charge explosion can be detected by observing microstructural modifications; they may be present even if the piece does not show noticeable overall plastic deformations. Particularly, if an austenitic stainless steel (or another metal having a face-centered cubic structure and a low stacking fault energy) is exposed to an explosive shock wave, high-speed deformation induces primarily mechanical twinning, whereas, in nonexplosive events, a lower velocity plastic deformation first induces slip. The occurrence of mechanical twins can be detected even if the surface is damaged or oxidized in successive events. In the present research, optical metallography (OM) and scanning electron microscopy (SEM), and scanning tunneling microscopy (STM) were used to detect microstructural modifications caused on AISI 304Cu steel disks by small-charge explosions. Spherical charges of 54.5 or 109 g TNT equivalent mass were used at explosive-to-target distances from 6.5 to 81.5 cm, achieving peak pressures from 160 to 0.5 MPa. Explosions induced limited or no macro-deformation. Two alloy grain sizes were tested. Surface OM and SEM evidenced partial surface melting, zones with recrystallization phenomena, and intense mechanical twinning, which was also detected by STM and X-ray diffraction. In the samples' interior, only twins were seen, up to some distance from the explosion impinged surface and again, at the shortest charge-to-sample distances, in a thin layer around the reflecting surface. For forensic science locating purposes after explosions, the maximum charge-to-target distance at which the phenomena disappear was singled out for each charge or grain size and related to the critical resolved shear stress for twinning.

Mechanical twins, their development and growth

Mechanical or deformation twinning is first explained as a shearing mechanism and compared to crystallographic slip. Then twinning in several metals is discussed. A twin can be represented as a thin layer, bound by an upper and lower plane. The average width of a disk-shaped twin is either given by the average diameter of a microregion (grain) or is calculated in the case of twin nuclei. An energy balance is outlined in detail for the untwinned and twinned status of a representative volume element. The elastic strain energy due to the twinning shear eigenstrain is studied in detail both analytically and numerically. An energy criterion for the stability of equilibrium allows to formulate a twinning condition which yields a minimum thickness in the case of a deformation twin. In the case of a twin nucleus Onsager's principle of maximum dissipation rate is engaged as a further criterion to find the dimensions of the twin nucleus. Comparisons with experimentally observed twins in TiAl intermetallics are reported finally. Further consequences of the study are listed, such as the estimation of the critical resolved shear stress for twinning.

Constitutive model of the TWIP effect in a polycrystalline high manganese content austenitic steel

The TEM study of our steel with a high manganese content reveals that mechanical twining (TWIP effect) occurs during the deformation at room temperature. Microtwins are organised into parallel stacks and two systems are sequentially activated in each grain. They participate to the deformation and they are strong obstacles for the dislocations and for other twins, leading to the decrease of the effective grain size. Thus, TWIP provides our alloy a very good ductility and a high hardening rate.Our constitutive modelling deduced from a model proposed by Bouaziz and Guelton [4] integrates this typical organisation of microtwins. Twinning is quantified in each grain by the partial volume fraction of twins in each system. A nucleation law for the microtwins is introduced which depends on the local stress and the stress relaxation due to pre‐existing twins. The flow stress is deduced from the dislocation density, which evolves with the dynamical recovery and the decrease of the mean free path (MFP). The MFP takes into account the grain and twin boundaries and the forest dislocations. The strain is calculated by adding the contributions of dislocation glide and twinning accounting the orientation of the grain. To treat the polycristal, the behaviours of different grain orientations are mixed by assuming at each strain step that the increment of elastic energy stored is the same in each grain.The model was successfully applied to describe the mechanical properties of our alloy, for two different grain sizes. Some microstructural parameters are yet fitted. This leads to an insufficient prediction of the evolution of the microstructure. In further developments, we expect to introduce numerical simulation results on local characteristics of microtwins (thickness, critical resolved shear stress for twinning) and experimental results on the rate of twin nucleation.

Determining deviatoric stress tensors from calcite twins: Applications to monophased synthetic and natural polycrystals

Theoretically, calcite twins analysis makes possible the full determination of the deviatoric stress tensor responsible for the formation of the twin lamellae, that is, the orientations of the principal stress axes and the three differential stress values (σi–σj) may be calculated, assuming conditions of constant resolved shear stress for twinning. Inverse method using nonlinear regression and robust estimation is proposed.Applications to monophased synthetic polycrystals that are generated numerically by solving the direct problem and to a natural sample from the Quercy (southwest of France) have given good results. Comparison with the first method proposed by Turner (1953), modified by Nissen (1964) for principal stress orientations and by Jamison and Spang (1976) for differential stress values, clearly emphasizes the considerable interest of this new method.

Continual mechanical twinning: Part I: Formal description

A model-independent description of mechanical twinning is presented. It is shown that slip, which may be induced concurrent with twinning, serves as an energy dissipation process. The relative case of this dissipation governs the characteristics of the total deformation process and predicts phenomena such as a negative strain-rate sensitivity and a positive temperature dependence for the deformation. Details of the description are tested by experiments on several cubic metals and appropriate experimental support from subsequent papers is quoted here. The alloy Fe-25 at. % Be is of particular interest because it deforms almost exclusively by twinning; moreover, when in an ordered state the alloy can untwin spontaneously even under opposing stresses. The formal description of continual mechanical twinning can be made sufficiently general to account in detail for such events, as well as the more standard observations of work hardening, orientation effects, etc. The existence of a critical resolved shear stress for twinning (which must, however, depend on the state of the material through dislocation mobility) is observed and forms an integral part of the description. No particular model for twin nucleation is necessary in the formal development since most results are adequately explained without specifically accounting for nucleation.

Nucleation and growth of twins in dislocation-free zinc crystals

Zinc whiskers and platelets with their largest faces in the basal plane and thin enough (~½ μ ) to be transparent to 100 kV electrons were strained in tension inside the electron microscope and the nucleation and growth of twins observed by the transmission technique. These observations were then related to the various stages of stress-strain curves obtained in a high-sensitivity tensile machine. All of the crystals were found to be initially completely free of dislocations. Twin nucleation occurred in a region where the applied stress was concentrated, namely at the specimen grips, at a corrosion pit or at a growth step. A critical resolved shear stress for twinning of ~ 50 Kg/mm 2 was found, which is a reasonable value for the theoretical twinning stress. Twins propagated at a rate controlled by the strain rate, at a stress of 2 to 20 Kg/mm 2 and by the nucleation and movement of twinning dislocations. Usually a single twin was formed and grew until it transformed the entire crystal. Basal glide often occurred in a twinned region, and fracture took place in the twin by the propagation of a crack along a basal plane on which a large amount of glide had occurred. The transformation of a matrix containing dislocations with various possible Burgers vectors is considered and the differences between twinning in bulk crystals and in whiskers and plates are discussed.