Extent Of Dynamic Recrystallization Research Articles

Hot deformation characteristics and dynamic recrystallization of biomedical CoCrWCu alloy

In order to explore the dynamic recrystallization of biomedical CoCrW-2Cu alloy, the hot deformation behavior was investigated by isothermal compression tests. True stress-true strain curves showed that the true stress increased with increasing strain rate or decreasing deformation temperature. Meanwhile, obvious dynamic softening was observed. The constitutive equation and processing maps were constructed to elucidate the relationship between true stress and deformation parameter. The phase component and microstructure were investigated to reveal the relationship between microstructure evolution and deformation condition, which was beneficial to the optimization of the hot deformation parameter. The variation of deformation condition has a limited influence on the phase component. The strain rate has a significant influence on the extent of recrystallization. An obviously discontinuous dynamic recrystallization (DDRX) process was occurred in this study. Furthermore, the extent of dynamic recrystallization (DRX) of sample deformed at 1100 °C/0.01 s -1 was the highest. Negative value of efficiency of power dissipation could be found in the top left corner of processing maps, while high value of efficiency of power dissipation was observed in the region with high temperature and low strain rate. The processing maps predicted that the optimized deformation parameters were 1100~1150 °C and 0.01~0.1 s -1 .

Effect of microstructural variation on strain localization in double‐sided friction stir welded AA6061‐AA7075 joints

AbstractIn friction stir welding (FSW), the inhomogeneous microstructure significantly affects the mechanical performance of the joints. The present study investigates the influence of microstructural asymmetry along the thickness on strain localization during tensile test using the digital image correlation technique and fracture morphology for double‐sided FSW (DS‐FSW) AA6061‐AA7075 joints. In top and bottom slices of the transverse tensile sample, nonhomogenous strain localization is noted in heat affected zone (HAZ) of the advancing side (AS), that is, AA6061‐T6, and also presents higher tensile strength. However, in the middle slice of the transverse sample, larger region including thermo‐mechanically affected zone and HAZ undergoes strain localization and exhibits higher elongation at failure. In longitudinal specimens, the strain distribution is homogeneous up to uniform elongation followed by strain concentration at a localised region and fracture. Electron backscatter diffraction revealed that the extent of dynamic recrystallization on the retreating side (AA7075‐T61) is higher than that observed on the AS of the weld. The grain orientation spread map showed a high fraction of recrystallized grains at the weld centre. Presence of major shear textures components and both below the tool shoulder and weld centre regions are observed from pole figures. The recrystallized texture components P ({011}<112>), Goss ({110}<001>), Rotated Goss ({110}<110>), Cube ({001}<100>) and shear texture ({001}<110>) components is also noted at the weld centre. Middle slice both for longitudinal and transverse sample showed the finest dimple size on the fracture surfaces. The strain localization behaviour and tensile performance assessed for transverse and longitudinal samples can be helpful to find the load orientation dependency and safe design of DS‐FSW joints.

Effect of pre-strain temperature on the subsequent dynamic recrystallization of parent β phase and transformed α-variant selection in a novel near α titanium alloy

The microstructure and micro-orientation evolution of a novel high-temperature titanium alloy were characterized during a two-step deformation process. In the first step, the specimens were deformed at temperatures of 800, 850, 900 and 950 °C, and subsequently deformed in the same β-phase region of 1050 °C. The specimens pre-strained at 800 °C and 950 °C were fully recrystallized during the β-phase deformation process. By contrast, the specimens pre-strained at 850 and 900 °C were not fully recrystallized during the β-phase deformation process. This indicates that the extent of dynamic recrystallization did not increased monotonically with the pre-strain temperature. The extent of dynamic recrystallization was related to the stored energy accumulated in the α and β-phase during the pre-strain deformation. The change in stored energy was based on the difference in microstructure morphology produced by different pre-deformation temperatures,and stored energy was quantitatively estimated based on the local misorientation distribution. Meanwhile, the generation of dynamically recrystallized grains was beneficial to decreasing the generation possibility of the macro-zone. This decrease was attributed to the refinement of β-grains and randomly distributed orientation, which was also related to the decrease in the typically distributed “two parallel wings” orientation between the α and β phase induced by uncrystallized deformation. Additionally, the two-step deformation process with the pre-strain at the (α + β) middle regime was an effective strategy to promote the dynamic recrystallization and decrease the generation probability of the macro-zone.

Hot deformation behavior and microstructure evolution of an Fe–30Cr–2Mo ultra-pure super ferritic stainless steel

The hot deformation behavior and microstructure evolution of an Fe–30Cr–2Mo ultra-pure super ferritic stainless steel were investigated at the temperature range of 950–1150 °C and strain rate varying from 0.01 to 10 s−1. A strain compensated constitutive equation based on the Arrhenius-type model was established to predict the flow stress. The hot processing map based on the dynamic materials model was achieved to identify the optimum processing parameters. In addition, the features of microstructure evolution combined with the processing map were systematically investigated. The experimental results revealed that the flow stress increased with decreasing deformation temperature or increasing strain rate. Dynamic recovery was confirmed to be the predominant softening mechanism. The values of flow stress predicted by the strain compensated constitutive equation agreed well with the experimental values. The extent of dynamic recrystallization and recrystallized grain size increased with increasing deformation temperature or decreasing strain rate, and the continuous dynamic recrystallization was attributed to be the predominant mechanism of recrystallization during hot deformation. The optimum hot working parameters were determined to be the deformation temperature of 1070–1150 °C and strain rate of 0.1–1 s−1 with a peak power dissipation efficiency of 42%.

Mechanism of shear band formation and dynamic softening in a two-phase (α2 + γ) titanium aluminide

Abstract

Effect of 0.8 at.% H on the Mechanical Properties and Microstructure Evolution of a Ti–45Al–9Nb Alloy Under Uniaxial Tension at High Temperature

To investigate the effect of hydrogen on the high-temperature deformation behaviors of TiAl-based alloys, the high-temperature tensile experiment was carried out on a Ti–45Al–9Nb (at.%) alloy with the H content of 0 and 0.8 at.%, respectively. Then, the effect of hydrogen on the high-temperature mechanical properties of the as-cast alloy was studied, the constitutive relations among stress, temperature, and strain rate were established, and the microstructure was analyzed. The results indicated that, compared with the unhydrogenated alloy, the flow stress of the hydrogenated alloy was significantly reduced, and the peak stress of the hydrogenated alloy decreased by (16.28 ± 0.17)% deformed at 1150 °C/0.0004 s−1. Due to the presence of hydride (TiAl)Hx in the alloy, the elongation showed a decline trend with increasing strain rate at the same deformation temperature. Compared with the unhydrogenated alloy, the elongation of the hydrogenated alloy reduced by (26.05 ± 0.45)% (0.0004 s−1), (23.49 ± 0.38)% (0.001 s−1), and (14.23 ± 0.19)% (0.0025 s−1), respectively, indicating that 0.8 at.% H softened the Ti–45Al–9Nb alloy and reduced the high-temperature plastic deformability. Under the same deformation condition, the deformation extent of the hydrogenated alloy was less than that of the unhydrogenated alloy. There were more residual lamellae in the hydrogenated alloy, and the extent of dynamic recrystallization was lower than that of the unhydrogenated alloy.

Effect of Deformation Temperature on Dynamic Recrystallization and CSL Grain Boundary Distribution of Fe-36%Ni Invar Alloy

This paper investigated the effect of deformation temperatures on microstructure evolution and grain boundary character distribution (GBCD) of Fe-36%Ni Invar alloy during hot compression tests. The isothermal hot deformation tests were carried out in a temperature range of 850-1100 °C and a strain rate of 0.01 s−1. The grain boundary character distribution was based on the Σ3 n (n = 1, 2, 3,…) coincidence site lattice (CSL) boundaries. The experimental results show that the extent of dynamic recrystallization significantly expanded at the strain rate of 0.01 s−1 with the increase in temperature and this phenomenon was consistent with the evolution tendency of Σ3 n boundaries. To some degree, the increasing proportion of typical low-ΣCSL boundaries, especially the increasing trend of Σ3, positively promoted the wider process of dynamic recrystallization.

Microstructure, texture and mechanical properties of Mg-Gd-Y-Zr alloy annular forging processed by hot ring rolling

Hot ring rolling (HRR) was successfully applied to fabricate large-size seamless magnesium alloy annular forging in this work and the resultant microstructures, texture and mechanical properties of the ring are discussed. Results show that the microstructures, along the thickness direction of the ring, are heterogeneously distributed, with the extent of dynamic recrystallization (DRX) differing among different sampling regions, owing to the inhomogeneous strain and temperature distribution. Typical basal texture is identified in all sampling regions, but with different degree of inclination or scattering of the {0002} basal poles. Due to the rotation of the basal poles towards some specific direction, the ring exhibits an obvious mechanical anisotropy. After aging treatment, the best comprehensive mechanical properties are obtained in the transverse direction of the outer region, with an ultimate tensile stress of 456MPa, yield stress of 426MPa and elongation to failure of 3.5%.

Mechanism of the formation of peripheral coarse grain structure in hot extrusion of Al-4.5Zn-1Mg

Microstructural evolution leading to peripheral coarse grain (PCG) structure in hot extruded Al-4.5Zn-1Mg rods is investigated. The extent of dynamic recrystallization (DRX) in the as-extruded product falls not in line with the basis over which the existing mechanisms for interpretation of PCG formation are built. A new mechanism is therefore proposed based on partial DRX during extrusion and nucleation and abnormal growth of statically recrsystallised grains.

Effect of Strain Rate on Hot Ductility Behavior of a High Nitrogen Cr-Mn Austenitic Steel

18Mn18Cr0.6N steel specimens were tensile tested between 1173 K and 1473 K (900 °C and 1200 °C) at 9 strain rates ranging from 0.001 to 10 s−1. The tensile strained microstructures were analyzed through electron backscatter diffraction analysis. The strain rate was found to affect hot ductility by influencing the strain distribution, the extent of dynamic recrystallization and the resulting grain size, and dynamic recovery. The crack nucleation sites were primarily located at grain boundaries and were not influenced by the strain rate. At 1473 K (1200 °C), a higher strain rate was beneficial for grain refinement and preventing hot cracking; however, dynamic recovery appreciably occurred at 0.001 s−1 and induced transgranular crack propagation. At 1373 K (1100 °C), a high extent of dynamic recrystallization and fine new grains at medium strain rates led to good hot ductility. The strain gradient from the interior of the grain to the grain boundary increased with decreasing strain rate at 1173 K and 1273 K (900 °C and 1000 °C), which promoted hot cracking. Grain boundary sliding accompanied grain rotation and did not contribute to hot cracking.

The effect of hot rolling on the microstructure, texture and mechanical properties of twin roll cast AZ31Mg

In this study, a commercially available twin roll cast AZ31 Mg sheet was hot rolled at 400°C to investigate the influence of rolling parameters (speed and percent thickness reduction) on microstructure, texture and mechanical properties. As compared to the twin roll cast condition, the microstructure of the successfully rolled materials was much finer and relatively less bimodal but contained profuse twinning and varying fractions of dynamically recrystallized grains. The extent of dynamic recrystallization (DRX) during rolling was found to be significantly influenced by percent thickness reduction. In the parameters investigated here, the rolling speed had a weak effect on DRX. The results showed that hot rolling strengthened the weak basal texture of the twin roll cast material and in addition to necklace grain formations at grain boundaries, DRX inside the twins was also identified as a key recrystallization mechanism. Mechanical testing revealed that at room temperature hot rolling improved the yield strength with retention of limited tensile ductility (<15%). However at 200°C, the rolled material exhibited more a balanced mechanical properties with large tensile elongations. Microstructural analysis of the failed tensile tested specimens showed that the deformation behavior was dependent on grain size and crystallographic texture.

Enabling shear textures and fine-grained structures in Magnesium sheet by machining-based deformation processing

The production of Mg alloy AZ31B sheet in a single deformation step by large- strain extrusion machining (LSEM) is detailed. LSEM imposes intense simple shear in a narrow zone by constrained chip formation. The confined deformation and the associated in situ adiabatic heating are found to be the key factors in production of the Mg sheet without need for external (pre-) heating. A range of shear textures with basal planes inclined to the sheet surface are achieved by this processing. The basal plane inclination could be varied by controlling the strain path. Microstructures, both ultrafine-grained (100-500 nm) and conventional fine-grained (2-5 μm), have been obtained by controlling the adiabatic heating and the extent of dynamic recrystallization. The LSEM sheet with shear texture and fine grain size shows superior combinations of formability and strength compared to rolled sheet.

Dynamic Recrystallization in a Ti Modified Austenitic Stainless Steel During High Strain Rate Deformation

The article discusses the dynamic recrystallization (DRX) behavior in a 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel (commonly known as alloy D9) during high strain rate deformation. Hot compression tests were conducted in a Gleeble thermomechanical simulator in the temperatures range 1173 K–1373 K with a strain rate 10 and 100 s−1 to different strains. Hot deformed microstructures were characterized using Electron Back Scatter Diffraction (EBSD) technique. Recrystallized grains were partitioned from the deformed grains employing grain orientation spread approach. Extent of dynamic recrystallization (DRX) was found to be minimal at 1173 K. DRX was predominantly found to happen at and above 1273 K which increases with increase in strain. The fraction of Σ3 and Σ9 boundaries was found to increase with %DRX. The origin and the possible role of these boundaries on DRX in alloy D9 are also explained.

Hot deformation behavior of SiCp/AZ91 magnesium matrix composite fabricated by stir casting

The uniaxial compressive deformation behavior of a 10 vol.% SiC particulate reinforced AZ91 magnesium matrix composite (SiCp/AZ91) fabricated by stir casting is investigated at elevated temperature (250–400 °C). Peak stresses and flow stresses decrease as temperatures increase and strain rates decrease. The extent of dynamic recrystallization (DRX) becomes less as temperatures decrease at 250–350 °C or strain rates increase, and recrystallization occurs mainly within the intergranular regions rich of particles. Dynamic recrystallization accomplishes at 400 °C even at the strain rate of 1 s −1. An analysis of the effective stress dependence on strain rate and temperature gives a stress exponent of n = 5 and a true activation energy of Q = 99 kJ/kJ. The value of Q is close to the value for grain boundary diffusion in Mg. It is concluded that the deformation mechanism of SiCp/AZ91 composite during hot compression is controlled by the dislocation climb.

Crystal plasticity and superplasticity in quartzite; A natural example

Plastically deformed quartzites from the Betic Movement Zone (Betic Cordilleras, Spain) exhibit microstructures indicative of crystal plasticity on a mineral grain scale. Quartzites with dynamically recrystallized grain sizes larger than 10 μm have strong crystallographic preferred orientations, narrow grain boundaries, little creep damage, and an inverse proportionality of dislocation density and grain size. Mylonites with grain sizes smaller than 10 μm have low crystallographic preferred orientations, wide grain boundaries (up to 1000 Å), abundant creep damage, and decreasing dislocation density with diminishing grain size. This is thought to reflect a clear-cut shift in deformational regimes from dislocation creep to superplastic flow at 10 μm grain size. Superplasticity can be acquired by quartzites which suffer dynamic recrystallization to grain sizes smaller than 10 μm during an initial dislocation creep stage. Dislocation motion is the major accomodating mechanism for strain incompatibilities that arise during grain-boundary sliding in the mylonites. It seems reasonable to estimate flow stresses from unbound dislocation densities and dynamically recrystallized grain sizes in the tectonite specimens. In the mylonites, dynamically recrystallized grain size probably reflects the stress magnitude before the shift in deformational mechanisms, and an estimate for late stage stresses is provided by unbound dislocation densities. In both deformational regimes the flow strength appears to depend on the extent of dynamic recrystallization.