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Due to the high application of dissimilar Al-Cu joints in different industries, pure copper and the AA2024 alloy with SiC nanoparticles were successfully welded by friction stir spot welding (FSSW). The comparison of microstructure evolution and mechanical properties of the aluminum and copper in the weld zone of the joint has been analyzed by light microscopy, electron backscattered diffraction (EBSD), and scanning electron microscopy. According to the results, both continuous and discontinuous dynamic recrystallizations occurred during conventional FSSW. On the other hand, during the FSSW with SiC nanoparticles, continuous dynamic recrystallization was the prominent mechanism in the microstructure evolution of the stir zone. The grain size decreased as SiC nanoparticles were used during the conventional FSSW process because of the pinning effects. Mechanical properties of the weld zone, such as tensile load and hardness, increased during the FSSW with SiC nanoparticles, due to the higher amounts of the strengthening mechanisms of grain boundaries and dislocations along with the formation of hard and brittle intermetallic compounds (IMCs). Fracture surface analysis showed ductile fractures for joint samples with SiC nanoparticles.

Grain refinement can make conventional metals several times stronger, but this comes at dramatic loss of ductility. Here we report a heterogeneous lamella structure in Ti produced by asymmetric rolling and partial recrystallization that can produce an unprecedented property combination: as strong as ultrafine-grained metal and at the same time as ductile as conventional coarse-grained metal. It also has higher strain hardening than coarse-grained Ti, which was hitherto believed impossible. The heterogeneous lamella structure is characterized with soft micrograined lamellae embedded in hard ultrafine-grained lamella matrix. The unusual high strength is obtained with the assistance of high back stress developed from heterogeneous yielding, whereas the high ductility is attributed to back-stress hardening and dislocation hardening. The process discovered here is amenable to large-scale industrial production at low cost, and might be applicable to other metal systems.

After the heavy reduction (HR) process was carried out at the solidification end of the continuous casting slab, the austenite grains were refined by recrystallization, which improved the thermoplasticity of the slab. However, the reduction in deformation during the HR process initiated stress concentration at the slab surface, and the crack risk increased. To effectively evaluate the risk of slab surface cracks under these complex conditions, the effect of the HR on the austenite recrystallization and thermoplasticity of a microalloyed slab surface was investigated by 15-pass reduction thermal simulation according to the wide and thick slab continuous casting process. The softening fraction was introduced as a global internal variable to quantitatively analyze various recrystallized re-refined grains. After the critical strain reaches the critical strain of dynamic recrystallization, a variety of recrystallization modes alternately occur. Among them, the contribution rate of dynamic crystallization to the later refinement reaches more than 50%. The contribution rates of static recrystallization and metadynamic recrystallization to grain refinement are almost the same. The thermoplasticity of the slab surface first increases and then decreases with increasing reduction pass. It was verified by transmission electron microscopy that the main reason for the decrease in thermoplasticity is that the dislocation multiplication rate increases, resulting in a sharp increase in stress and a decrease in thermoplasticity.

The technology of high-density electropulsing has been applied to increase the performance of metallic materials since the 1990s and has shown significant advantages over traditional heat treatment in many aspects. However, the microstructure changes in electropulsing treatment (EPT) metals and alloys have not been fully explored, and the effects vary significantly on different material. When high-density electrical pulses are applied to metals and alloys, the input of electric energy and thermal energy generally leads to structural rearrangements, such as dynamic recrystallization, dislocation movements and grain refinement. The enhanced mechanical properties of the metals and alloys after high-density electropulsing treatment are reflected by the significant improvement of elongation. As a result, this technology holds great promise in improving the deformation limit and repairing cracks and defects in the plastic processing of metals. This review summarizes the effect of high-density electropulsing treatment on microstructural properties and, thus, the enhancement in mechanical strength, hardness and corrosion performance of metallic materials. It is noteworthy that the change of some properties can be related to the structure state before EPT (quenched, annealed, deformed or others). The mechanisms for the microstructural evolution, grain refinement and formation of oriented microstructures of different metals and alloys are presented. Future research trends of high-density electrical pulse technology for specific metals and alloys are highlighted.

Excellent ductility is crucial not only for shaping but also for strengthening metals and alloys. The ever most widely used eutectic alloys are suffering from the limited ductility and losing competitiveness among advanced structural materials. Here we report a distinctive concept of phase-selective recrystallization to overcome this challenge for eutectic alloys by triggering the strain hardening capacity of the duplex phases completely. We manipulate the strain partitioning behavior of the two phases in a eutectic high-entropy alloy (EHEA) to obtain the phase-selectively recrystallized microstructure with a fully recrystallized soft phase embedded in the skeleton of a hard phase. The resulting microstructure fully releases the strain hardening capacity in EHEA by eliminating the weak boundaries. Our phase-selectively recrystallized EHEA achieves a high ductility of ∼35% uniform elongation with true stress of ∼2 GPa. This concept is universal for various duplex alloys with soft and hard phases and opens new frontiers for traditional eutectic alloys as high-strength metallic materials. The ever most widely used eutectic alloys often suffer from limited ductility. Here the authors propose a distinctive concept of phase-selective recrystallization to significantly improve their ductility and strength and pave the way for new applications of the widespread eutectic alloys.

Experiments are carried out in CIMPLE-PSI, to understand the recrystallization behavior of tungsten (W) exposed under very-high target temperature and ITER relevant long He+-fluence. The effect of helium bubbles on possible retardation of the recrystallization process is also studied. W samples were simultaneously exposed under He plasma and annealed by the plasma heat-load, in contrast to previously reported experiments in literature, which were carried out sequentially. Exposed samples are characterized by field emission scanning electron microscopy (FESEM), Vickers surface micro-hardness, nano-hardness and electron backscattered diffraction (EBSD). It is observed that the sample exposed to plasma under the highest temperature (1866 K) suffered acute retarded grain growth. This also contained small, unrecovered grains on the exposed surface. FESEM imaging of the cross-sections confirms that relatively smaller helium bubbles still form even at very high temperature conditions, which can impede the grain growth locally, whenever they are forming right on the grain boundaries. This results in an inhomogeneous mixture of surface grains with sizes ranging from a few micrometers to a few tens of micrometers. EBSD estimates that the plasma exposed surface was only 34% recrystallized. The second sample exposed at a lower temperature (1699 K) but for three times higher fluence (ion fluence: 1.19 × 1027 m−2) was almost fully recrystallized, which shows retardation diminishes very fast with the duration of the exposure. Hardness measurements were undertaken to understand the variation with plasma exposure/annealing temperature and the extent of recrystallization, with three different probing length scales, spanning from a few hundred nanometers to several micrometers. Both helium plasma exposed W samples are observed to undergo retarded softening up to a depth of a few hundred nanometers from the surface, compared to when the metal may be recrystallized by simple heating, without any plasma exposure.

Interrupted and continuous hot compression tests were performed for eutectoid steel over the temperature range of 850 to 1050 °C and while using strain rates of 0.001, 0.01, 0.1, and 1 s−1. The interrupted tests were carried out to characterize the kinetics of static recrystallization(SRX) and determinate the interpass time conditions that are required for initiation and propagation of dynamic recrystallization (DRX), while considering that the material does not contain microalloying elements additions for the recrystallization delay. Continuous testing was used to investigate the evolution of the austenite grain size that results from DRX. The results indicate that carbon content accelerates the SRX rate. This effect was observed when the retardation of recrystallization due to a decrease in deformation temperature from 1050 to 850 °C was only about one order of magnitude. The expected decelerate effect on the SRX rate when the initial grain size increases from 86 to 387 µm was not significant for this material. Although the strain parameter has a strong influence on SRX rate, in contrast to a lesser degree of strain rate, both of the effects are nearly independent of the chemical composition. The calculated maximum interpass times that are compatible with DRCR (Dynamic Recrystallization Controlled Rolling), for relatively low strain rates, suggest that the onset and maintaining of the DRX is possible. However, while using the empirical equations that were developed in the present work to estimate the maximum times for high strain rates, such as those observed in the wire and rod mills, indicate that the DRX start is feasible, but maintaining this mechanism for 5% softening in each pass after peak strain is not possible.

The annealing kinetics of cold-rolled AA5182 and AA5657 aluminum alloy sheets have been investigated and compared. The microstructures of a series of partially recrystallized samples are characterized by electron back scattered diffraction and key stereological parameters including volume fraction recrystallized, interfacial areas and contiguity are determined. The overall recrystallization kinetics as well as the nucleation and growth rates are thereby quantified. The results reveal that the nuclei develop in clusters. This matches well with the observation of a low kinetics exponent n = 2 in AA5182. Much more surprising is that n is 2.8 in AA5657, even though the nucleation is clustered. Also, it is higher than almost all recrystallization kinetics investigations which generally find values significantly below 3. Effects of nucleation rate, spatial distribution of nuclei and growth rate are discussed and used in a quantitative analysis of recrystallization kinetics in the two alloys.Graphic Abstract

In metallurgy, mechanical deformation is essential to engineer the microstructure of metals and to tailor their mechanical properties. However, this practice is inapplicable to near-net-shape metal parts produced by additive manufacturing (AM), since it would irremediably compromise their carefully designed geometries. In this work, we show how to circumvent this limitation by controlling the dislocation density and thermal stability of a steel alloy produced by laser powder bed fusion (LPBF) technology. We show that by manipulating the alloy’s solidification structure, we can ‘program’ recrystallization upon heat treatment without using mechanical deformation. When employed site-specifically, our strategy enables designing and creating complex microstructure architectures that combine recrystallized and non-recrystallized regions with different microstructural features and properties. We show how this heterogeneity may be conducive to materials with superior performance compared to those with monolithic microstructure. Our work inspires the design of high-performance metal parts with artificially engineered microstructures by AM. The traditional way of beating metals to improve their properties is not practical to 3D printed parts with intricate geometry. Here, the authors demonstrate how to program microstructural modifications of metals site-specifically during 3D printing to tune their properties.