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The hot extrusion process can effectively refine the coarse grains of metastable β titanium alloy billet, thereby enhancing the material's internal microstructure and mechanical properties. This study explored the hot deformation behavior of as-cast Ti-6554 samples at temperatures ranging from 900 °C to 1150 °C and strain rates between 0.001 s-1 and 1 s-1 by a Gleeble-3500 tester. The Arrhenius constitutive model was established to simulated the hot extrusion process. Parameters such as the degree of deformation (x), metal plastic flow (μ), and deformation temperature (θ) were introduced to quantify the uniformity of distribution under various processes. Based on the simulation results, the optimal extrusion conditions were determined to be an extrusion ratio of 10, an extrusion angle of 60°, an extrusion velocity of 40 mm/s, a preheating temperature of 1100 °C, and an extrusion load of 2500 tf. Examination of the microstructure of the hot-extruded bar revealed that grain sizes consistently decreased from the centers towards the edges. The grains were refined from 2000~5000 μm to 100~200 μm after extrusion, demonstrating that the deformation degree, plastic flow and deformation temperature were well coordinated under these conditions.

This paper studies the grain refinement mechanisms of 5A06 aluminum alloy sheets in cold rotary forging (CRF). The results show that the grains are clearly refined from 25.1 µm to 11.8 µm during the CRF process. The grain refinement mechanism can be divided into two modes: (1) The grains with a small Schmid factor (SF) are activated by multi-slip systems, and dense dislocations are segregated along the boundaries of interior regions with different slip systems, which results in a rapidly increasing strain localization along these boundaries. Since the strain localization restrains the coordinate slip deformation between different interior regions, the grains are directly separated into several finer grains. (2) The grains with a large SF are primarily activated by a single slip system, and the dislocation migrates smoothly along most microband boundaries. Then, a more severe lattice rotation causes a transformation to a hard orientation and multi-slip system activation, which contributes to an increase in the rapid misorientation across microband boundaries and thus promotes significant SF grain refinement.

The multi-principal-element alloys (MPEAs), also referred to as high-entropy alloys (HEAs), have attracted extensive attention during the last decade and a half due to their unique and excellent properties. However, many MPEAs show coarse and anisotropic columnar grains in the as-cast state. While constitutional supercooling (CS)-driven parameters have been widely used to evaluate and predict the effect of solutes on columnar-to-equiaxed transition (CET) and grain refinement of diluted binary alloys, similar studies are lacking on MPEAs. Due to the multiple solute elements (solutes) and their high concentrations, the CS-driven parameters for MPEAs are different from those proposed for diluted binary alloys. Here, we derived the CS-driven parameters, including undercooling parameter and growth restriction factor, for MPEAs based on their physical significances, with the help of calculated phase diagrams. The calculated CS-driven parameters were then used to predict the effect of a solute on CET and grain refinement in NiCoFeCr MPEAs. Additional alloying solutes Nb, Ti, and V in the NiCoFeCr MPEA were also evaluated for their different CS-driven characteristics. The grain size of the as-solidified microstructures of NiCoFeCr with and without Nb, Ti, and V were compared and interpreted with the predicted tendency of the CS-driven parameters.

Al alloy parts fabricated by powder bed fusion (PBF) have attracted much attention because of the degrees of freedom in both shapes and mechanical properties. We previously reported that the Si regions in Al-Si alloy that remain after the rapid remelting process in PBF act as intrinsic heterogeneous nucleation sites during the subsequent resolidification. This suggests that the Si particles are crucial for a novel grain refinement strategy. To provide guidelines for grain refinement, the effects of solidification, remelting, and resolidification conditions on microstructures were investigated by multiphase-field simulation. We revealed that the resolidification microstructure is determined by the size and number of Si regions in the initial solidification microstructures and by the threshold size for the nucleation site, depending on the remelting and resolidification conditions. Furthermore, the most refined microstructure with the average grain size of 4.8 µm is predicted to be formed under conditions with a large temperature gradient of Gsol = 106 K/m in the initial solidification, a high heating rate of HR = 105 K/s in the remelting process, and a fast solidification rate of Rresol = 10−1 m/s in the resolidification process. Each of these conditions is necessary to be considered to control the microstructures of Al-Si alloys fabricated via PBF.

This paper investigates the microstructural evolution and grain refinement kinetics of a solution-treated Cu–0.1Cr–0.06Zr alloy during equal channel angular pressing (ECAP) at a temperature of 673 K via route BC. The microstructural change during plastic deformation was accompanied by the formation of the microband and an increase in the misorientations of strain-induced subboundaries. We argue that continuous dynamic recrystallization refined the initially coarse grains, and discuss the dynamic recrystallization kinetics in terms of grain/subgrain boundary triple junction evolution. A modified Johnson–Mehl–Avrami–Kolmogorov relationship with a strain exponent of about 1.49 is used to express the strain dependence of the triple junctions of high-angle boundaries. Severe plastic deformation by ECAP led to substantial strengthening of the Cu–0.1Cr–0.06Zr alloy. The yield strength increased from 60 MPa in the initial state to 445 MPa after a total strain level of 12.

Friction stir processing (FSP) stands as an effective approach designed for grain refinement and site-specific microstructural modification. The evolving microstructure during FSP is determined by various variables out of which rate of sample cooling is the key parameter. More often, FSP is conducted in naturally flowing air; however, a large number of studies are conducted by researchers across the world; stressing the importance of additional sample cooling strategy for tailoring the material microstructure. Such strategies vary not only in terms of the cooling medium used but also with regard to various other compliant conditions that must be fulfilled for the cooling process to make them successful and economically viable. This work critically reviews the most prevalent methods practiced by various researchers and industries for controlled sample cooling during and after FSP. The underlying mechanisms; advantages; disadvantages; and limitations of each procedure along with the resulting microstructure and material performances are discussed and recommendations are provided

In order to improve the strength and toughness of Q690E steel sheets, the effect of rare earth element Ce on the strength and toughness of Q690E steel was studied by means of transmission electron microscopy, scanning electron microscopy, and metallographic microscope. The results showed that the addition of Ce in steel limited the combination of S with Mn and Ca, transformed Al 2 O 3 inclusion into spherical CeAlO 3 inclusion, and modified the precipitate form of some composite inclusions of TiN and sulfide oxides into TiN precipitation alone. The inclusions were spheroidizing. The size of inclusions was decreased from 3–5 μm to 1–2 μm, and the distribution was dispersed. Ce played a role in purifying molten steel through desulphurization and deoxidization. Meanwhile, the addition of Ce in steel effectively increased the nucleation particles in the liquid phase, improved the nucleation rate, enlarged the equiaxed grain refinement area, and limited the development of columnar crystals. The average grain size of slab decreased from 45.76 to 35.25 μm, and the proportion of large grain size (> 50 μm) decreased from 40.41% to 23.74%. The macrostructural examination of slab was improved from B0.5 to C2.0, which realized the refinement of the solidified structure and reduced the banded structure of hot rolled plate. In addition, due to the inheritance of refined structure in the upstream, the recrystallization of deformed austenite and the growth of grain after recrystallization were restrained, and a refined tempered sorbite structure was obtained. When rare earth element Ce was added, the width of the martensite lath bundle was narrowed from about 500 nm to about 200 nm, which realized a remarkable grain refinement strengthening and toughening effect. Mechanical properties such as tensile, yield, and low-temperature impact toughness were significantly improved.

The effect of 0–1.0 at.% Al additions on grain refinement and phase transformation of the Mg-2.0Gd-1.2Y-0.5Zn-0.2Mn (at.%) alloy containing a long period stacking ordered (LPSO) phase was investigated in this work. The addition of Al promoted the formation of the Al2RE phase in the Mg-2.0Gd-1.2Y-0.5Zn-0.2Mn (at.%) alloy, and the dominant secondary phases in the as-cast Mg-2.0Gd-1.2Y-0.5Zn-0.2Mn-1.0Al (at.%) alloy were the Mg3RE phase, LPSO phase, and Al2RE phase. With increased Al addition, the area fraction of the Al2RE phase increased monotonously, while the area fraction of LPSO phase and Mg3RE phase decreased gradually. The orientation relationship between the Al2RE phase and the α-Mg matrix was determined to be Al2RE//<112¯0>α-Mg, {101}Al2RE//{101¯0}α-Mg, which was not affected by Zn and Mn concentrations in the Al2RE phase. Since the Al2RE particles with a size more than 6 μm located at the center of grains could act as nucleants for α-Mg grains, the average grain size of the as-cast alloys decreased from 276 μm to 49 μm after 1.0% Al addition. The effect of the Al addition on the grain refinement of the Mg-2.0Gd-1.2Y-0.5Zn-0.2Mn alloy was comparable to that of the Zr refined counterpart.

The history of grain refinement with AlB2 in Al–Si and Al–Si–Cu casting alloys is reconsidered in light of findings published recently by Zheng and co-workers. When using boron as a grain refiner, some problems may be encountered. These are considered briefly. Fundamental aspects of grain refinement are also discussed and useful future experiments proposed.

In this work, a novel body-centered tetragonal boron nitride (bct-BN)-reinforced 6351Al-based cast metal matrix composite of various reinforcement contents is synthesized for the first time adopting a low-cost hybrid route of liquid metal infiltration-cum-stir casting followed by subsequent treatments of forging and age hardening. Not only a new low-density ultra-hard reinforcement (bct-BN) is incorporated but also a new hybrid casting route pertinent to the low-density reinforcement incorporation is attempted for the first time in order to achieve high specific ultimate tensile strength (SUTS) value. As a noteworthy outcome, a significant value of SUTS (113 MPa/g cm −3 ) and modest ductility (%Elongation = 10) are achieved in 6351Al-12.3 vol.% BN composite on casting followed by forging and age hardening treatments that is quite comparable or superior to many common aluminum-based metal matrix composites and steels for structural application. In view of structure-property correlation, such an enhanced property (YS = 196 MPa, UTS = 291 MPa, % Elongation = 10, SUTS = 113 MPa/g cm −3 ) as compared to the initial property of cast 6351Al alloy (YS = 131 MPa, UTS = 144 MPa, % Elongation = 2, SUTS = 56 MPa/gcm −3 ) is attributed to reinforcement effect of coherent bct-BN particles, matrix grain refinement effect on bct-BN particle incorporation, evolution of nano-twinned region within BN particle on forging and the age hardening effect of coherent Mg 5 Si 6 ( β ″) precipitates. This asserts an emergence of a new cost-effective structural material in the form of 6351Al/bct-BN composite.