Effect Of Deformation Twinning Research Articles

Effects of deformation twins on microstructure evolution, mechanical properties and corrosion behaviors in magnesium alloys - A review

Due to lattice reorientation, grain segmentation, induced recrystallization, twins play a very important role in regulating texture, refining grains, improving mechanical properties and corrosion resistance, and has received more extensive attention. Numerous studies have shown that {10-12}<10-11> tensile twins (TTWs) are easily activated in large quantities due to the lower critical resolve shear stresses (CRSS). Introduction of TTWs under uniaxial compression improved the strength, ductility, and formability of magnesium (Mg) alloys. Moreover, TTWs produced by multi-directional impact forging (MDIF) can optimize the microstructure by dividing grains and promoting recrystallization, resulting in significant improvement of mechanical properties. Although {10-11}<10-12> compressive twins (CTWs) and {10-11}-{10-12} double twins (DTWs) can promote dynamic recrystallization (DRX), they are also favorable nucleation sites for cracks. In addition, the type and volume fraction of twins can affect the corrosion resistance, and they also play different roles in the corrosion process of different Mg alloys. Twins have shown great potential for improving structure and properties, but a comprehensive and critical discussion of twins in Mg alloys is still lacking. Therefore, based on previous studies, this article reviews the common types and variants of twins in Mg alloys, influencing factors, and their effects on the microstructure, mechanical properties and corrosion resistance. In addition, some interesting ideas are being proposed for further research.

Effect of pre-strain on hydrogen embrittlement of high manganese steel

The effect of deformation twins on the hydrogen embrittlement (HE) of a twinning-induced plasticity (TWIP) steel with various pre-strain levels, coupled with subsequent annealing treatment was systematically investigated by a slow strain rate tension test after cathodic hydrogen charging. The results revealed that specimens with high twin volume fraction had relatively better HE resistance. The high-density twin boundary not only plays an important role in hindering hydrogen diffusion, but also can refine the grains and inhibit the accumulation of hydrogen, so as to relieve stress concentration and avoid premature fracture of the specimen.

Effect on corrosion resistance of microstructure of 316L austenitic stainless steel subjected to combined torsion–tension deformation

Abstract The influences of combined torsion-tension deformation on the microstructural evolution and corrosion resistance of 316L stainless steel were investigated. The microstructure and corrosion behavior of the deformation samples were studied in detail. The results showed that the grains were more significantly refined under combined deformation than under tensile deformation. However, further increase in pre-torsion led to little change in grain size. The results of transmission electron microscopy and corrosion tests results indicated that high-density dislocations were detrimental, whereas the corrosion resistance, grain refinement and deformation twins were beneficial to corrosion resistance. Furthermore, the effect of deformation twins on corrosion resistance was greater than that of dislocation density, which, in turn, was greater than the influence of grain size.

Effect on corrosion resistance of microstructure of 316L austenitic stainless steel subjected to combined torsion–tension deformation

Abstract The influences of combined torsion-tension deformation on the microstructural evolution and corrosion resistance of 316L stainless steel were investigated. The microstructure and corrosion behavior of the deformation samples were studied in detail. The results showed that the grains were more significantly refined under combined deformation than under tensile deformation. However, further increase in pre-torsion led to little change in grain size. The results of transmission electron microscopy and corrosion tests results indicated that high-density dislocations were detrimental, whereas the corrosion resistance, grain refinement and deformation twins were beneficial to corrosion resistance. Furthermore, the effect of deformation twins on corrosion resistance was greater than that of dislocation density, which, in turn, was greater than the influence of grain size.

The effect of deformation twins on the quasi-cleavage crack propagation in twinning-induced plasticity steels

The present work investigates the crack propagation behaviour in a twinning-induced plasticity (TWIP) steel. At room temperature, the dominant fracture mechanism of TWIP steel is quasi-cleavage, which is assisted by the deformation twins since such fracture mode is circumvented at 573 K as deformation twins are prohibited. The quasi-cleavage fracture mechanism is responsible for the high notch sensitivity and low essential work of fracture (EWF) of TWIP steel at room temperature. Furthermore, the crack propagation process of TWIP steel at 298 K has been characterized quantitatively by in-situ digital image correlation (DIC) technique and J resistance curve (J-R curve). The results show that the cracking process of TWIP steel at 298 K is stable, even though cleavage patches are observed on the fracture surface. A physical model based on the dislocation behaviour at the crack tip has been proposed to explain qualitatively the mechanism responsible for the stable cleavage cracking in TWIP steels.

On the effect of deformation twinning and microstructure to strain hardening of high manganese austenitic steel 3D microstructure aggregates at large strains

The hardening and deformation characteristics of Hadfield microstructure are studied to investigate the effect of microstucture to the material behavior. A crystal plasticity model including dislocation slip and deformation twinning is employed. The role of deformation twinning to the overall strain hardening of the material is evaluated for two different grain structures. Large compressive strains are applied on 3D microstructural aggregates representing the uniform and non-uniform grain structures of Hadfield steels. The grain structure has an effect on the strain hardening rate as well as on the overall hardening capability of the microstructure. A major reason causing the difference in strain hardening arises from the different twin volume fraction evolution influenced by intra-grain and inter-grain interactions. A mixture of large and small grains was found to be more favorable for twinning and thus resulting in a greater hardening capability than uniform grain size.

Low-cycle fatigue behaviors of pre-hardening Hadfield steel

Low-cycle fatigue behaviors of the pre-hardening (PH) and the water-quenching (WQ) Hadfield steel were studied using optical microscopy, transmission electron microscopy, and electron backscatter diffraction technique. The effect of the PH treatment on low-cycle fatigue behavior of the Hadfield steel was analyzed through comparing the cyclic hardening/softening behaviors and the changing regulations of stress amplitude, internal stress, and effective stress at different total strain amplitudes. Results showed obvious differences in fatigue behaviors between the PH (with a cold rolling deformation degree of 40%) and the WQ Hadfield steels. Transient hardening followed by cyclic stability behavior occurred in the PH Hadfield steel under cyclic loading, whereas cyclic softening behavior was barely observed. The fatigue life of the PH Hadfield steel was higher than that of the WQ Hadfield steel at relatively low strain amplitudes, while a contrary result was obtained at relatively high strain amplitudes. At low strain amplitudes, the deformation twins induced in the PH Hadfield steel could enhance the multiplication and slip process of dislocations, which actually improved the deformation uniformity. The long-range motion of dislocations was intensified at high strain amplitudes. However, the dislocation motion was also blocked by twin boundaries. As a result, the interactions between dislocations and deformation twins enhanced, finally causing severe dislocation accumulation. These two effects of deformation twins on dislocation motion eventually resulted in different low-cycle fatigue behaviors of the PH Hadfield steel.

Rate-dependent hardening model for pure titanium considering the effect of deformation twinning

This paper is concerned with the strain hardening behavior of commercially pure titanium for a wide range of strain rates. Pure titanium has been described as having three stages of strain hardening behavior during compressive deformation. The stress–strain curve of pure titanium can be divided into three stages according to the strain hardening rate, and the strain hardening rate can be determined by the slope of the stress–strain curve. In the first stage, the strain hardening rate decreases as deformation continues due to dynamic recovery. The strain hardening rate shows a trend similar to general industrial metals. In the second stage, however, the strain hardening rate begins to increase as deformation continues. Following the second stage, the strain hardening rate decreases again in the third stage. Many researchers have determined that a sudden increase in the strain hardening rate in the second stage is caused by the generation of deformation twins, and the strain hardening behavior of pure titanium is influenced by the tendency of twin generation and evolution. In this paper, the effect of deformation twinning on the strain hardening behavior of pure titanium is investigated using OIM (Orientation Imaging Microscopy) analyses. EBSD (Electron Backscatter Diffraction) analyses are conducted to quantitatively observe the generation and evolution of deformation twins. The strain rate effect on the strain hardening is also investigated. Both tensile and compressive tests are conducted at strain rates ranging from 0.001/s to 10/s, and the effect of the strain rate on the three stages of the strain hardening behavior is quantified by observing the micro-structures of deformed specimens at the various strain rates. A novel rate-dependent hardening model is proposed by keeping trace of deformation twins with increase in the compressive strain, which induces the variation of the strain hardening rate. The proposed model is defined with a function of the plastic strain and the strain rate based on OIM results in terms of twin volume fraction and grain size distribution. The three stages of strain hardening behavior of titanium for a wide range of strain rates can be represented by one equation, and this equation can provide useful and simple way that can be applied to the numerical analysis.

On the effect of deformation twins on stability of B19′ structure in NiTi martensite

The present study is focused on discrepancy between theoretically predicted (base centered orthorhombic B33) and experimentally observed (monoclinic B19′) ground state structures of NiTi martensite. In particular, we deal with a hypothesis on possible effect of presence of twins on the martensite structure. Several models of (100) twins in NiTi martensite (representing several twin densities) are studied from first principles. Computational supercells are fully optimized in order to reach almost stress-free state for each twin model. Our results show that the presence of twins has a significant influence on crystal geometry that tends to stabilize B19′ structure rather than the B33 one, at least for higher twin densities.

The influence of Al content and annealing on vibration fracture properties of wrought Mg–Al–Zn alloys

Abstract This study investigated the vibration fracture behavior of two Mg–Zn–Al alloys, AZ31 and AZ61 in as-extruded and as-annealed states. The effects of deformation twins, precipitates and Al concentration on vibration properties were examined. As-extruded AZ31 specimens showed a longer vibration life than the others, mainly because of better damping capacity and a lack of intergranular Mg17Al12 precipitates.

The twofold effects of deformation twins on fracture behaviors of PST crystals of TiAl alloys

The twofold effects of deformation twins on fracture behaviors of PST crystals of TiAl alloys

Mechanical properties and deformation mechanisms of a commercially pure titanium

The mechanical behavior of a commercially pure titanium (CP-Ti) is systematically investigated in quasi-static (Instron, servohydraulic) and dynamic (UCSD's recovery Hopkinson) compression. Strains over 40% are achieved in these tests over a temperature range of 77–1000 K and strain rates of 10 −3–8000/s. At the macroscopic level, the flow stress of CP-Ti, within the plastic deformation regime, is strongly dependent on the temperature and strain rate, and displays complex variations with strain, strain rate, and temperature. In particular, there is a three-stage deformation pattern at a temperature range from 296 to 800 K, the specific range depending on the strain rate. In an effort to understand the underlying mechanisms, a number of interrupted tests involving temperature jumps are performed, and the resulting microstructures are characterized using an optical microscope. Based on the experimental results and simple estimates, it is concluded that the three-stage pattern of deformation at temperatures from 296 to 800 K, is a result of dynamic strain aging, through the directional diffusion of dislocation-core point defects with the moving dislocation at high strain rates, although the usual dynamic strain aging by point defects segregating outside the dislocation core through volume diffusion is also observed at low strain rates and high temperatures. The microscopic analysis shows that there is substantial deformation twinning which cannot be neglected in modeling the plastic flow of CP-Ti. The density of twins increases markedly with increasing strain rate, strain, and decreasing temperature. Twin intersections occur, and become more pronounced at low temperatures or high strain rates. In sum, the true stress–true strain curves of CP-Ti show two stages of deformation pattern at low temperatures, three stages at temperatures above 296 K, and only one stage at temperatures exceeding 800 K, although all three stages may exist even at 1000 K for very high strain rates, e.g. 8000/s. While the dislocation motion is still the main deformation mechanism for plastic flow, the experimental results suggest that dynamic strain aging should be taken into account, as well as the effect of deformation twinning.

The effects of deformation twins and strain-induced ϵ-martensite on mechanical properties of an Fe-32Mn-12Cr-0.4C cryogenic alloy

In this research, the tensile properties of a Fe-32Mn-12Cr-0.4C cryogenic alloy were investigated at temperatures between {minus}196 C to 250 C. This study was intended to clarify the influence of temperature on tensile properties and to correlate the deformed microstructures with the tensile elongation. The formation of strain-induced changes, deformation twins and strain-induced {var_epsilon}-martensite, was analyzed at various temperatures. The effects of the strain-induced changes on strain hardening rate were discussed to explain the variation of tensile elongation of Fe-32Mn-12Cr-0.4C cryogenic alloy.

The Effect of Deformation Twins on the Microhardness in a Two-Phase Alloy Steel

The close correlation of strength properties with twin density in 304 stainless steel and manganese steel, has suggested indirectly that twin boundaries act as significant hardening mechanisms by acting as dislocation barriers. The presence of twins can be regarded as a grain-size refinement which can be viewed in terms of the Hall-Petch relation as producing an increase in the flow stress.In the present study, 60 mil thick sheets of Uniloy 326, having a residual grain size of 1 to 2 μ were explosively shock loaded. The shock loading was performed in sandwich assemblies containing 304 and 316 stainless steels as reference materials utilizing techniques previously described.