The hydrogen embrittlement (HE) resistance of a fine-grained equiatomic CoCrFeNi high-entropy alloy (HEA) is investigated via tensile testing under electrochemical H charging. The HE behavior is ...compared with that of HEA specimens charged with 100 MPa of H gas. The fine-grained HEA shows > 40% elongation with a tensile strength of ~800 MPa under electrochemical H charging. Meanwhile, H gas-charged specimens with a uniform distribution of H show deformation twin-related intergranular cracks, whose initiation length decreases owing to grain refinement. Such small cracks, which feature blunted tips, do not significantly affect the fracture of the specimens. The electrochemically H-charged specimens exhibit numerous surface cracks because of their higher surface H content compared with that of the H gas-charged specimens. Nevertheless, similar to the case of the H gas-charged specimens, most of the cracks do not propagate significantly. In conclusion, fine-grained HEA exhibits remarkable resistance to H-related crack growth.
•Sr–B poisoning effect on Si modification is solved by La addition in Al–10Si alloy.•Ductility of refined Al–10Si–0.05La–0.02B–0.015Sr alloy is twice as La-free alloy.•La reacts with B to form LaB6 ...acting as an effective nucleation site of α-Al.•Multiply twinning of Si is caused by Sr atoms doping at twin boundaries.•Poisoning mechanism of Sr–B are revealed by thermodynamic calculations.
As-cast Al‒Si alloys always face low ductility because of coarse α-Al grains and large lamellar eutectic Si. Although B-containing refiners and Sr modifiers could refine the α-Al grains and eutectic Si respectively, simultaneous addition leads to an invalid effect of Sr because B-containing refiner has a poisoning effect (PE) on Sr modifier in the Al‒10Si‒B‒Sr alloys. The present work achieves both refinement of α-Al grains and modification of Si merely by the addition of 500 ppm La in Al‒10Si‒0.02B‒0.015Sr alloy, which greatly enhances the ductility and tensile strength. Sr‒B interaction in the Al‒10Si‒0.02B‒0.015Sr melt results in a consumption of Sr by forming Sr‒B intermetallic compound. CALPHAD-type calculations reveal that Sr concentration in liquid before eutectic reaction becomes a key parameter to estimate the alloying modification effect. The addition of La forms LaB6 and thereby releases Sr to protect the Sr modification effect. First-principles calculations illustrate that both Sr and La are beneficial to the formation of twin boundaries in Si particles due to their negative formation energy of twin boundaries. The formed LaB6 has a semicoherent interface with α-Al with the orientation relationship of 11¯0LaB6//11¯0Al and (11¯1)LaB6//(11¯1)Al, demonstrating that LaB6 is an efficient nucleation site for α-Al, which contributes to the refinement of α-Al in the Al‒10Si‒0.05La‒0.02B‒0.015Sr alloy. Our findings reveal the micro-mechanism of refinement during the solidification process and develop an effective and simple way to obtain high-performance Al alloys.
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Since cross rolling promotes the transformation of austenite to deformation-induced α΄-martensite and increases the dislocation density in the retained austenite, the reversion/recrystallization ...annealing after cross rolling might be more effective to achieve finer grain sizes in metastable austenitic stainless steels. Accordingly, the effect of cold rolling route and annealing for grain refinement and improvement of mechanical properties of AISI 316 L stainless steel was systematically investigated in the present work. It was found and formulated that by increasing strain, the grain refinement efficiency during annealing is improved, the effect being more pronounced if the cold rolling step is applied by cross rolling instead of unidirectional rolling. The evaluation of tensile properties revealed that all cold rolling and annealing routes lead to significant improvements of mechanical properties in terms of strength and plasticity, the influence of cross rolling being more prominent again. Moreover, the cross rolling and annealing route led to a remarkable strength-ductility balance, which revealed the opportunities that this route can offer for the improvement of mechanical properties of austenitic stainless steels. Accordingly, the results of the present work may shed light on the further utilization of cold rolling route and annealing treatment.
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•Finer annealed grains obtained by cross rolling compared to unidirectional rolling.•Grain size formula based on reduction in thickness and rolling route was proposed.•Excellent strength-ductility balance was obtained by cross rolling and annealing.
The CrMnFeCoNi alloy with a stable single face-centered cubic (FCC) phase, representing a typical class of high-entropy alloys (HEAs), has garnered considerable attention owing to their exceptional ...mechanical properties. However, the inherent low hardness has impeded their utilization in structural or contact components. This study attempted to improve the surface hardness of CrMnFeCoNi HEA via grain refinement realized by nanosecond pulse laser irradiation. The effects of laser parameters on the surface microstructures, chemical composition, and mechanical properties were investigated, and the mechanism for grain refinement was analyzed. The experimental results showed that the hardness of the laser-irradiated surface was significantly improved without altering the chemical composition. This study provides a simple but effective method to refine the grain of HEAs and improve their surface hardness.
•Grain refinement was achieved on CrMnFeCoNi HEA via nanosecond laser irradiation.•Surface hardness of the irradiated surface was significantly improved.•The effect of laser parameters on microstructures and hardness was investigated.
This study investigates the effects of the initial grain size and temperature (ranging from room temperature to 1100 °C) on the mechanical properties and microstructure evolution of Ti2AlC MAX phase. ...A Hall-Petch like relationship is observed between compressive strength and the grain size below brittle-to-plastic transition temperature (BPTT). However, the compressive strength of fine-grained MAX phase decreases more rapidly with increasing temperature resulting in inverse Hall-Petch effect above BPTT. Results from postmortem EBSD analysis reveal complex microstructural evolution in both fine- and coarse-grained microstructures during loading at different temperatures. The pronounced drop in compressive strength for fine-grained microstructures at temperatures close to BPTT is attributed to creep induced grain boundary sliding resulting in texture development with more grains oriented for easy slip. In coarse-grained microstructures, no significant texture development is observed even though grain refinement occurs at all temperatures. A mathematical model has also been formulated to predict the experimentally observed grain size and temperature dependent variation in the compressive strength of Ti2AlC over a wide range of grain sizes and test temperatures. The mathematical model accounts for the competing effects of Hall-Petch strengthening and high temperature creep induced softening mechanisms.
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An ultrafine microstructure consisting of α grains (~ 0.51 µm) and a small amount of β phase was successfully achieved in a friction stir-processed (FSPed) Ti-6Al-4V alloy. The fraction of high angle ...grain boundaries (HAGBs) with random crystallographic orientations reached 89.3% revealed that dynamic recrystallization was responsible for the ultra-grain refinement mechanism during friction stir processing (FSP). Low-temperature superplasticity (LTSP) of such an ultrafine microstructure was demonstrated in the temperature range of 550–650 °C and strain rates of 1 × 10−4–3 × 10−3 s−1. Specifically, an extremely superior LTSP of 1130% was achieved at 600 °C and 3 × 10−4 s−1, which was explained by means of the ultrafine equiaxed grains, a large proportion of HAGBs with random orientations as well as the presence of β phase. The predominant superplastic deformation mechanism was considered as grain boundary sliding associated with grain boundary diffusion.
The microstructural response and grain subdivision process in commercially pure (CP) titanium subjected to multiple laser shock peening (LSP) impacts were investigated by means of optical microscopy ...(OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. The micro-hardness curves as a function of the impact time were also determined. The deformation-induced grain refinement mechanism of the close-packed hexagonal (hcp) material by laser shock wave was subsequently analyzed. Experimental results showed that uniform equiaxed grains with an average size of less than 50 nm were generated due to the ultra-high plastic strain induced by multiple LSP impacts. Special attention was paid to four types of novel deformation-induced microstructural features, including a layered slip band in the tension deformation zone, and inverse-transformation martensite, micro-twin grating and micro-twin collision in the compression deformation zone. Furthermore, the grain refinement mechanism in the near-surface layer of CP titanium subjected to multiple LSP impacts contains two types of simultaneous subdivision modes: multi-directional mechanical twin (MT)-MT intersections at (sub)micrometer scale, and the intersection between longitudinal secondary MTs and transverse dislocation walls at nanometer scale. In addition, both grain refinement (nanocrystallization) and the existence of a small amount of inverse-transformation martensite induced by multiple LSP impacts contribute to an increase in the micro-hardness of the near-surface layer.
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A grain refinement process for a GH4169 superalloy sheet was proposed with the following parameters: solid solution (1050 °C × 0.5 h), cold rolling (50% reduction rate), δ phase precipitation ...treatment (890 °C × 10 h), high-pressure torsion (HPT) (3, 4, and 5 GPa), and recrystallization annealing (950 °C × 3 h). An ultrafine-grained sheet with an average grain size of 0.8 μm was prepared. The microstructure transformation, recrystallization process, and grain refinement mechanism of the materials in each stage were examined. Results revealed that large amounts of subgrain structures and irregularly shaped δ phases existed in the GH4169 superalloy exposed to HPT. During the recrystallization annealing at 950 °C, the dispersed and uniformly distributed δ phase blocked the migration of the recrystallized grain boundaries. The distorted microstructure after HPT was replaced with the newly formed fine grain without distortion, eventually forming an ultrafine grain structure with high-angle grain boundaries. After HPT processing was performed at 5 GPa × 2 turns, a cubic texture in the {100} direction was produced in the GH4169 superalloy.
•A new grain refinement process for a GH4169 superalloy sheet was proposed.•Ultrafine-grained sheet with an average grain size of 0.8 μm was prepared.•Grain refinement mechanism of the GH4169 superalloy sheet was analyzed by EBSD.
High pressure torsion (HPT) was employed to fabricate nanostructured α-uranium successfully. The evolution of microstructures (grain size and dislocation density) and related mechanical properties ...(hardness and elastic modulus) varied with strain were quantified and analyzed. The steady-state microstructures and microtextures of the nanostructured α-uranium were characterized by transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD) analysis in scanning electron microscopy (SEM), respectively. The grain/subgrain size of the nanostructured α-uranium is below 100 nm, and a weak microtexture with four components is presented. The microstructure evolution of α-uranium during severe plastic deformation (SPD) could be divided into three stages, and it resonates with the variation of creep resistance as a function of strain. On account of the unique crystal structure of α-uranium, the grain refinement limit was discussed and compared with 25 pure metals from a statistical view.
Al-Ti-B based master alloys have been widely used for grain refining of Al-alloys in industry for many decades. However, the effectiveness of such grain refiners is severely compromised when a few ...hundred ppm of Zr is present in the Al melt, and this phenomenon is referred to as Zr poisoning in the literature. So far the exact mechanisms for Zr poisoning are not clear albeit significant research effort on the subject in the last few decades. In this work we investigated the mechanism for Zr poisoning through extensive examinations of the Al/TiB2 interface using the state-of-the-art electron microscopy and ab initio molecular dynamics simulations. We found that the presence of Zr in Al melts leads to (i) the dissolution of the Al3Ti 2-dimensional compound (2DC) formed on the (0 0 0 1) TiB2 surface during the grain refiner production process; and (ii) the formation of an atomic monolayer of Ti2Zr 2DC on the (0 0 0 1) TiB2 surface, which replaces the original Ti-terminated TiB2 basal surface. This monolayer of Ti2Zr not only has large lattice misfit (4.2%) with α-Al, but also is atomically rough, rendering the TiB2 particles impotent for heterogeneous nucleation of α-Al. This work, in combination of our previous work, demonstrates that heterogeneous nucleation can be effectively manipulated, either enhanced or impeded, by chemical segregation of selected alloying/impurity elements at the liquid/substrate interface.
(a, b) High resolution STEM HAADF images showing an atomic monolayer of Ti2Zr 2-dimensional compound (2DC) on (0 0 0 1) surface of TiB2 being viewed along (a) 1 1 -2 0TiB2 and (b) 1 0 -1 0TiB2 direction respectively, and (c) 3D construction of the Ti2Zr 2DC on top of TiB2. Display omitted
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