Twin Strengthening Research Articles

Effects of cryogenic treatment on microstructure and mechanical properties of AZ31 magnesium alloy rolled at different paths

In this study, the AZ31 Mg alloy sheets were fabricated by the traditional process of air cooling after rolling and a new process of cryogenic treatment after rolling. The effect of cryogenic treatment after rolling on the mechanical properties, texture and microstructure of AZ31 Mg alloys were studied. The results showed that the {10–12} extension twin, {10–11} contraction twin and {10–12}-{10–11} double twin were observed in the samples of cryogenic treatment after rolling. All samples mainly exhibited the {0001} <11–20> basal texture. Compared with the sample of air cooling after rolling, the basal texture intensity of cryogenic treatment after rolling samples was significantly weakened. The mechanical properties of the samples of cryogenic treatment after rolling were remarkably enhanced with respect to the air cooling after rolling sample. The sample of cryogenic treatment after the rolling that was rotated by 45° along the rolling direction (RD) exhibits the highest ultimate tensile strength (∼283 MPa) and increases by 18.4% compared to the air cooling after rolling sample. This is attributed to twin strengthening, fine-grained strengthening, precipitation strengthening, grain boundary strengthening and dislocation strengthening. The elongation of the sample of cryogenic treatment after the rolling that was rotated by 45° along the RD increased by 87.0% by contrast to the air cooling after rolling sample, reaching ∼25.8%, attributing to grain refinement, the weakening of texture and the activation of pyramidal <c+a> slip.

Effect of twin and grain size strengthening on tensile properties of a multi-directionally impact forged Mg–Gd–Y alloy

The microstructure and tensile properties of twin and grain size strengthening Mg–6.85Gd–5.79Y–0.44Zr alloy have been investigated and characterized by optical microscopy (OM), scanning electron microscopy (SEM), and electron back-scattering diffraction (EBSD). Their room temperature tensile results exhibit that twin strengthening can enhance the strength of Mg–Gd–Y–Zr alloy but sacrifice its ductility or toughness. This should be attributed that lenticular {10−12} twins with high-stress localization directly lead to a brittle failure. In contrast, the strength, ductility, and toughness were synchronously improved for the grain size strengthening Mg–Gd–Y–Zr alloy, i.e., the ultimate tensile strength, uniform elongation, and static toughness increased to 254 (259–245) MPa, 1.02 (1.40–0.55)%, and 5.7 (6.7–4.2) MJ/m3, respectively. This ductility increment should have resulted from fine recrystallization that was free from stress concentration and accommodated the tensile strain by more non-basal slip and grain boundary sliding or grain rotation.

Theory of twin strengthening in fcc high entropy alloys

Twinning in fcc High Entropy Alloys (HEAs) has been implicated as a possible mechanism for hardening that enables enhanced ductility. Here, a theory for the twinning stress is developed analogous to recent theories for yield stress. Specifically, the stress to move a twin dislocation, i.e an fcc partial dislocation moving along a pre-existing twin boundary, through a random multicomponent alloy is determined. A reduced elasticity theory is then introduced in which atoms interact with the twin dislocation pressure field and the twin boundary. The theory is applied to NiCoCr using results from both interatomic potentials and elasticity theory. Results are also used to predict the increased stress for the motion of (i) a single partial dislocation leaving a trailing stacking fault and (ii) adjacent partial dislocations involved in twin nucleation. Increased strength is predicted for all processes involved in the nucleation and growth of fcc twins. Comparison to single-crystal experiments at room temperature then suggests that twinning is controlled by twin nucleation, with reasonable quantitative agreement. When solute/fault interactions are neglected, the theory shows that twinning and lattice flow stresses are related. The theory also provides insight into how other dilute solute additions could suppress twinning, as found experimentally.

Hierarchical grain size and nanotwin gradient microstructure for improved mechanical properties of a non-equiatomic CoCrFeMnNi high-entropy alloy

This study explored a multi-mechanism approach to improving the mechanical properties of a CoCrFeMnNi high-entropy alloy through non-equiatomic alloy design and processing. The alloy design ensures a single-phase face-centered cubic structure while lowering the stacking fault energy to encourage the formation of deformation twins and stacking faults by altering the equiatomic composition of the alloy. The processing strategy applied helped create a hierarchical grain size gradient microstructure with a high nanotwins population. This was achieved by means of rotationally accelerated shot peening (RASP). The non-equiatomic CoCrFeMnNi high-entropy alloy achieved a yield strength of 750 MPa, a tensile strength of 1050 MPa, and tensile uniform elongation of 27.5%. The toughness of the alloy was 2.53 × 1010 kJ/m3, which is about 2 times that of the same alloy without the RASP treatment. The strength increase is attributed to the effects of grain boundary strengthening, dislocation strengthening, twin strengthening, and hetero-deformation strengthening associated with the heterogeneous microstructure of the alloy. The concurrent occurrence of the multiple deformation mechanisms, i.e., dislocation deformation, twining deformation and microband deformation, contributes to achieving a suitable strain hardening of the alloy that helps to prevent early necking and to assure steady plastic deformation for high toughness.

Role of slip and {10-12} twin on the crystal plasticity in Mg-RE alloy during deformation process at room temperature

The deformation mechanism of slips and twins has a considerable influence on the plasticity of magnesium alloys. However, the roles of slips and twins in the room-temperature deformation of Mg-rare earth (Mg-RE) alloys with high contents of rare earth elements is rarely investigated. Here, the microstructural evolution and deformation mechanism of an aged Mg-5Y-2Nd-3Sm-0.5Zr alloy during uniaxial compression at room temperature were systematically investigated using in-situ electron-backscattered diffraction and transmission electron microscopy. The results indicated that in the early stage of deformation, the Mg-RE alloy was mainly controlled by the slip of <a> dislocations in the basal plane and the coordinated c-axis strain of the {10-12} twin. With an increase in the strain, the grain orientation became more suitable for the initiation of pyramidal II <c + a> dislocations in the later stage of deformation; these dominated the deformation mechanism. In the twin evolution of the Mg-RE alloy, there were three types of twin-twin interaction behaviors: (i) single twin variant ‘parallel’ structure, (ii) single twin variant ‘cross’ structure, and (iii) multi twin variant ‘cross’ structure. In addition, three types of twin-grain boundary interaction behaviors were summarized: (i) twin ‘refracting through’ grain boundary, (ii) twin ‘parallel through’ grain boundary, and (iii) twin ‘fusing through’ grain boundary, which are expected to act as new means and solutions for the twin strengthening of magnesium alloys.

Microstructure and mechanical properties of novel CrCoNi–Al2O3P medium entropy alloy-matrix composites

In this study, novel CrCoNi–Al2O3 composites with 2.5–7.5 wt% Al2O3 were prepared by mechanical alloying (MA) plus rapid spark plasma sintering (SPS) method, and their microstructure and mechanical properties were systematically investigated. The results demonstrate that the Al2O3 nanoparticles are distributed homogeneously in the CrCoNi medium entropy alloy (MEA) matrix with ultra-fine grain size (≤0.37 μm). Moreover, the composites have clean, regular and well bonded matrix/α-Al2O3 interfaces. Interestingly, a large number of nano twin bundles, large-scale 9R phase and geometrically necessary dislocations (GNDs) were formed near the matrix/α-Al2O3 interfaces. The Vickers hardnesses of the 2.5–7.5 wt% Al2O3 composites reach 521–603 HV0.3, and the compressive yield strength is 1877–2359 MPa with fracture strains of 9.3–31.6%. The yield strength of the composites is by 65.4–107.8% higher than that of the pure CrCoNi matrix, meanwhile they still have considerable ductility, which indicates composite strengthening is a feasible method to improve mechanical properties of high/medium entropy alloys. The composite includes multiply strengthening mechanisms, namely grain refinement strengthening, twin strengthening, Orowan strengthening and dislocation strengthening. The contributions of various strengthening mechanisms were quantitatively analyzed, which were well consistent with the experimental values.

Composition design, microstructure and mechanical properties of novel multiphase Ti–Cu–Ni–Nb complex concentrated alloys

The compositions of novel Ti–Cu–Ni–Nb alloys were designed by the proportional mixing of the Ti2Cu Laves phase and Ni60Nb40 eutectic unit. The Ti28Cu14Ni34.8Nb23.2 alloy is composed of Ti2Ni, NiNb, NbNi3 phases, and a small amount of amorphous structure; the Ti30Cu15Ni33Nb22 alloy is composed of Ti2Ni and NiNb phases; the NiNb phase can form the twin structure with the different twin planes in the two alloys. The Ti32Cu16Ni31.2Nb20.8 alloy is composed of the Ti2Ni, NiNb and Nb phases. The high strength of Ti28Cu14Ni34.8Nb23.2 and Ti30Cu15Ni33Nb22 alloys are attributed to grain refinement and twin strengthening; in addition, the existence of NiNb twins improves the strength and hardness of the two alloys, compared with that of Ti32Cu16Ni31.2Nb20.8 alloy. In this study, the proportional mixing of the Laves phase and eutectic unit is an effective method for the composition design of high entropy alloys with high strength.

Effects of Silicon on the Microstructure and Mechanical Properties of 15–15Ti Stainless Steel

Effects of silicon on the microstructure and mechanical properties of 20% cold-worked 15–15Ti austenitic stainless steels are systemically investigated by uniaxial tensile tests, scanning and transmission electron microscope observations, and strength calculations. The results reveal that a large number of deformation twins are formed in the 20% cold-worked steels with various silicon additions. The volume fraction of deformation twins and the density of dislocations increase with silicon content, while the twin thickness slightly decreases. A better strength–ductility combination is achieved by silicon addition, since the yield strength of the steel with 2% silicon is 61 MPa higher than that of the steel with 1% silicon, while their uniform elongations are almost both equal to 16%. The yield strength of the 15–15Ti stainless steels is predominantly contributed by the solid solution, dislocation and deformation twin strengthening effects, which can be enhanced by silicon addition.

Structural characteristics of {10[formula omitted]1} contraction twin-twin interaction in magnesium

Twin interface characteristics play a crucial role in elucidating twin propagation, twin-twin and twin-dislocation interactions, and then affect their mechanical properties. Herein, focusing on magnesium as a model of a hexagonal close-packed structure, we investigate the structural characteristics of \\{101¯1} contraction twin, depending on crystallographic analysis, molecular dynamics simulations and transmission electron microscopy observations. The geometrical features reveal that it is preferential to form acute twin boundaries during the processes of twin-twin interactions. Subsequently, the dynamics simulations results demonstrate that the propagation process of {101¯1} contraction twin is mainly related to the migration of {101¯3} twin boundaries in twin tip, and its thickening process is dominantly associated with the movement of multiple-layered steps. Additionally, two twin-twin interaction modes- tip-twin boundaries crossing transmission and tip-tip collision-have been successfully constructed, dependent on the preferential acute growth mode of contraction twins. Finally, typically structural characteristics on both free-growth of single contraction twin and twin-twin interactions have been confirmed in the ultrahigh pressure pure magnesium by transmission electron microscopy observations. This work clarifies fundamental physics of {101¯1} contraction twins in hexagonal close-packed metals, which can also be applied to other systems to explain twin strengthening mechanisms.

ERCP is more challenging in cases of acute biliary pancreatitis than in acute cholangitis – an analysis of the Hungarian ERCP Registry data

In this study, we propose a novel defect engineering strategy for circumventing the strength–ductility trade-off of a β-type Ti–35Nb–7Zr–5Ta alloy and discuss its underlying mechanism. This strategy is based on the simultaneous introduction of dislocations and twins into the alloy structure through the thermal stress generated during additive manufacturing via laser powder bed fusion (LPBF). As a result, the LPBF-fabricated alloy contains high-density dislocations and two different types of nanosized {112}〈111〉 mechanical twins: unique zigzag-shaped and conventional lamellar ones. Interestingly, the produced alloy exhibits a high tensile yield strength of 816 MPa and large elongation of 16.5%, which significantly exceed the published values for other representative β-type titanium alloys fabricated by various material processing methods. Such a high yield strength is predominantly attributed to the introduced defects, and the relative contributions of dislocation strengthening and twin strengthening are equal to 58.0% and 26.1%, respectively. Meanwhile, the large elongation results from the inhibition effect of the produced twins on the dislocation motion as well as from their dislocation pass-through and growth. The obtained results can provide guidelines for the microstructural design and fabrication of novel β-type titanium alloys with excellent mechanical properties by defect engineering during additive manufacturing.

Deformation twinning in a mild steel: Loading dependence and strengthening

Deformation twinning in a mild steel is investigated under quasi-isentropic compression (IC) and shock compression (SC) to about 12 GPa, as regards the effects of pulse duration (plateau width) and strain rate (rising edge slope), and associated twin strengthening. The pulse duration ranges from 80 ns to 790 ns; and two rise times are explored, approximately 30 ns for SC and 300 ns for IC. Free-surface velocity histories are measured to obtain strain and strain rate. The postmortem samples are characterized with electron backscatter diffraction, and the yield strengths of the samples pre-deformed by impact are examined with a materials testing system. For SC, twin density and size increase with pulse duration up to 580 ns. At longer pulse durations, the increase in twin density is stagnated as a result of deviatoric stress relaxation, while twin size continues growing. For IC (410 ns), twin density and size are much larger than the SC counterpart, as a result of shallower rising edge. Increased deformation time can compensate the effects of reduced strain rate or applied stress for deformation twinning. Twin strengthening effect is strong in postmortem samples, depends on twin density instead of area fraction, and follows the empirical Hall–Petch relationship.

Impact and Rolling Abrasive Wear Behavior and Hardening Mechanism for Hot-Rolled Medium-Manganese Steel

The coupled impact and rolling wear behavior of the medium-manganese austenitic steel (Mn8) were studied by comparison with the traditional Hadfield (Mn13) steel. Scanning electron microscopy (SEM), X-ray diffractometer (XRD), and transmission electron microscope (TEM) were used to analyze the wear and hardening mechanisms. The experimental results show that the impact and rolling wear resistance of hot-rolled medium-manganese steel (Mn8) is better than that of high-manganese steel (Mn13) under conditions of low-impact load. The better work hardening sensitivity effectively improves the wear resistance of medium-manganese steel. Not only the coefficient of friction is low, but the mass loss and wear rate of the wear are lower than that of high-manganese steel. After impact and rolling wear, a hardened layer with a thickness of about 600 μm is formed on the wear surface. The highest microhardness of the subsurface layer for Mn8 is about 594 HV and the corresponding Rockwell hardness is about 55 HRC, showing the remarkable work hardening effect. The wear-resistant strengthening mechanism of medium-manganese steel is compound strengthening, including the deformation-induced martensitic transformation, dislocation strengthening, and twin strengthening. In initial stages of impact and rolling abrasion, dislocation strengthening plays a major role. When the deformation reaches a certain extent, the deformation-induced martensitic transformation and twinning strengthening begin to play a leading role.

Deformation twinning structure and interface in a FCC-based Al0.3FeNiCo1.2CrCu high-entropy alloy matrix composites

High entropy alloy composites (HEACs), Al0.3FeNiCo1.2CrCu reinforced by Titanium carbide (TiC) particles were produced by mechanical alloying (MA) and spark plasma sintering (SPS). The results from X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows that a single FCC solid-solution phase with certain nanoscale deformation twins are produced following SPS. Subsequent investigations characterize the mechanical properties and microstructure of nanocrystalline twin HEAs by high resolution electron microscope (HREM) and compression tests. The produced interface was found to be clear and well bonded. The compressive yield strength, fracture strength, plastic strain and hardness are found to be 1730 ± 15 MPa, 2260 ± 15 MPa, 29.50% and 645Hv, respectively. This is attributed to the twin strengthening and the presence of uniformly distributed TiC particles.

The strengthening effect of spinodal decomposition and twinning structure in MnCu-based alloy

Abstract The strengthening effects of spinodal and twining structures in Mn–20 at%Cu–5 at%Ni–2 at%Fe (M2052) alloy are investigated. In order to distinguish the respective roles in the strengthening of the alloy played by grain size, spinodal structure and twin structure, the M2052 alloys are solution heat treated at different temperatures, followed by an aging treatment for a different duration of time. The results show that the alloy exhibits a high strengthening effect after the aging treatment, which can be ascribed to the substructures of spinodal and twin structures formed during the aging process. The strengthening effect of spinodal structure and twin structure increases with the increase in aging time. However, the strengthening effect of spinodal structures is about 3 times which is caused by twin structures. In addition, the twin strengthening effect (twin density) shows grain-size dependence, while the spinodal strengthening effect does not.

A new method to strengthen turbine disc superalloys at service temperatures

TMW® superalloys have superior mechanical properties up to 725°C. Numerous annealing twins are present because these alloys have relatively low stacking fault energies. Nanoscale deformation twins are the dominant mechanism during creep and tensile testing at 725°C. In addition to traditional strengthening mechanisms enhanced by higher Ti addition, twinning structures enhanced by higher Co addition also play an important role in strengthening coarse-grained alloys at high temperatures. The effects of twin strengthening are discussed.

Introducing a hierarchical structure for fabrication of a high performance steel

The multifunctional diversities existing in nature provide clues to speculate the structure–property–function relationships. A hierarchically structured steel is designed by using principles derived from nature and fabricated in situ by a one-step method of surface mechanical attrition treatment (SMAT). The microstructure of the processed steel is characterized by multilayered structure with hard nanocrystalline surface and compliant inner-layer, in particular with a smooth mechanical gradient induced by dual-phase constituents and multiscale grain size distribution. The hierarchically structured steel exhibits simultaneously high stiff, strong and large ductility, which originate from the joint deformation mechanisms of distinct reinforcing layers. The four layers present their own unique deformation mechanisms, including second-phase hardening, transformation induced plasticity and twin strengthening. The unique spatial form of gradation can release stress concentration and improve energy-dissipation leading to exceptional mechanical properties compared with the uniform materials.

HOT DEFORMATION BEHAVIOR OF A HIGH STRENGTH CONTAINER STEEL COMPOUNDED WITH Nb-B

Hot deformation behaviors of high strength container steel compounded with Nb-B have been systematically studied through high temperature compression tests on the Gleeble-1500 thermal-mechanical simulator. In the present deformation conditions, there are different relationships between the strain hardening exponent (n) and the true strain (e) under different strain rates at the deformation temperature of 1423 K; n-ecurves have single peak and single valley at the strain rates of 0.1 s~(-1) and 1s~(-1), while n-ecurves have double peaks and double valleys at the strain rates of 5 s~(-1) and 15 s~(-1).During the heat work-hardening process, dislocation strengthening, twin strengthening and precipitation strengthening are the major work-hardening mechanisms of the tested steel. According to Zener-Hollomon and Ludwik equation, the experimental data have been regressed by using nonlinear method, the apparent deformation activation energy (Q) and stress exponent (n) were obtained to be about (428.188±20.109) kJ/mol and 4.923 in the temperature range of 1123 K—1423 K, respectively. Finally, an expression of Z parameter, hot deformation equation and dynamic recrystallization model diagram of the tested steel are established.

A Novel Strategy for the Design of Advanced Engineering Alloys—Strengthening Turbine Disk Superalloys via Twinning Structures

AbstractA novel strategy for designing advanced engineering superalloys using twin structure is presented. By inducing numerous annealing and deformation twins, a new advanced polycrystalline Ni–Co‐base superalloy (TMW‐4M3 alloy) has been developed, which has low stacking fault energy, enhanced tensile and creep strength without degrading other mechanical properties such as low cycle fatigue and crack growth resistance. Based on TEM analysis, the twin strengthening mechanism is proposed.