A novel concept involving dual-refinement of solidification structure (referring to high-temperature ferrites δ-Fe and proeutectoid ferrites α-Fe) followed by dual-homogenization of solute atoms has ...been proposed accounting for the crystallization process of Nb/V-alloyed TRIP steels with Rare Earth (RE) addition. M(Nb,V)C carbides were predicted and observed easily precipitating on RE2O2S. Both RE2O2S and RE2O2S-MC can serve as heterogeneous nucleation sites for δ-Fe, and induce oriented epitaxy of α-Fe following certain orientation relationships. In above processes, RE segregation ahead of liquid-solid interfaces significantly hinders the movement of these interfaces thus promoting the intragranular dissolution of solute atoms. Furthermore, extensive precipitations of α-Fe from both austenite boundaries and grain interiors can also facilitate C/Mn homogenization. Such effects can be similarly reproduced in Nb/V-free TRIP steels. This study provides a strategy for microstructure tailoring of peritectic steels using RE-metamorphic products, and offers profound insights into alloy designing of RE-Nb(V) co-microalloying or replacing Nb(V) by RE.
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Laser oscillating welding method was employed to welding of Invar alloy so as to achieve grain refinement and decrease internal defects. Effects of frequency and amplitude of beam oscillation with ...sinusoidal trajectory on weld appearance and solidification microstructure were researched. The essential effect of beam oscillation could be attributed to change the laser energy deposition distribution on the processing surface. Optical metallographic and electron backscattered diffraction analysis results demonstrated that beam oscillation could achieve the transformation of solidification microstructure from oriented dendrite grain to equiaxed dendrite grain. Further, the application of beam oscillation might retard solidification cooling speed and strengthen the formation of γ -fiber texture that is conducive to the improvement of ductility. According to the acquisition of dynamic feature pictures with high-speed camera, the local rapid movement of laser spot resulted in a more stable keyhole without fluctuation and the intensification of melt flow. Temperature curves calculated by finite element method simulation showed that lateral laser oscillation made the temperature distribution of molten metals nearby the center of weld more uniform even negative temperature gradient. The comprehensive description of grain refinement mechanism caused by beam oscillation was presented on the basis of the experiment and simulation analyses.
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•Grain refinement of Invar alloy was achieved by laser oscillating welding.•Effect of oscillation parameters on laser energy distribution was researched.•Microstructure and mechanical property of oscillated welds were investigated.•Established heat flux model based on energy distribution to describe temperature field and discuss solidification condition.
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•Nd promotes the co-segregation of Zn and Nd at grain boundary combination with the nanoscale (Mg, Zn)3Nd precipitates significantly refine the grain size.•Ce significantly enhances ...the co-segregation of Zn and Ce at the grain boundary and yields a noticeable solute drag effect on the mobility of the grain boundary.•A noticeable improvement in strength and ductility is achieved by Ce addition in Mg-Zn alloy.
The role of alloying element segregation at grain boundary on the microstructure and mechanical properties is investigated in the case of Ca, Ce, and Nd addition in Mg-1Zn (wt.%) alloy. Segregation of alloying elements at grain boundary is demonstrated in all the designed ternary alloys, leading to a strong solute drag effect on the mobility of grain boundaries and ultimately the grain size. While the co-segregation of Zn and Ce is revealed in the Mg-Zn-Ce alloy, only the segregation of Zn is observed in the Mg-Zn-Ca alloy. The co-segregation of Zn and Nd in Mg-Zn-Nd alloy is demonstrated to lead to the relatively large solute drag pressure at grain boundary, which in combination with the Zener pressure emerging from nanoscale (Mg, Zn)3Nd precipitates significantly refines the grain size. The grain refinement in combination with the bimodal microstructure significantly contributes to the increased strength of the designed ternary alloys. Furthermore, the addition of Ca and Ce are found to simultaneously improve the strength and ductility, which is closely correlated with the segregation of alloying elements at the grain boundary. Among the considered alloying species, Ce is shown to efficiently improve the mechanical properties of Mg-Zn alloy.
Grain refinement of cast magnesium alloys, particularly in magnesium–aluminium (Mg–Al) based alloys, has been an active research topic in the past two decades, because it has been considered as one ...of the most effective approaches to simultaneously increase the strength, ductility and formability. The development of new grain refiners was normally based on the theories/models that were established through comprehensive and considerable studies of grain refinement in cast Al alloys. Generally, grain refinement in cast Al can be achieved through either inoculation treatment, which is a process of adding, or in situ forming, foreign particles to promote heterogeneous nucleation rate, or restricting grain growth by controlling the constitutional supercooling or both. But, the concrete and tangible grain refinement mechanism in cast metals is still not fully understood and there are a number of controversies. Therefore, most of the new developed grain refiners for Mg–Al based alloys are not as efficient as the commercially available ones, such as zirconium in non-Al containing Mg alloys. To facilitate the research in grain refinement of cast magnesium alloys, this review starts with highlighting the theoretical aspects of grain refinement in cast metals, followed by reviewing the latest research progress in grain refinement of magnesium alloys in terms of the solute effect and potent nucleants.
Metal additive manufacturing (AM) enables the direct manufacturing of complex-shape components for aeronautical engineering application. However, the intrinsic high thermal gradient grants the strong ...tendency towards columnar grains with detrimental crystallographic anisotropy and poor mechanical performance. Hybrid interlayer point-forging with laser-deposition (PF-LD) provides a novel approach to in-situ refine grain morphology and modify crystal orientation during AM process. The present work mainly focused on an essential processing parameter, interlayer press down volume (PDV) on microstructural evolution and mechanical properties of PF-LDed Ti2AlNb intermetallic alloy. Three types of grain morphology involving coarse-columnar, bamboo-like and fine-equiaxial were obtained by in-situ manipulating interlayer PDV from 0 mm to 0.2 mm. Combining with the intrinsic correlation between layer thickness and recrystallization penetration, the formation mechanism of three grain morphology was revealed. The consecutive increase of interlayer PDV also contributed to apparent texture weakening, abundant lattice defect and higher precipitation fraction. The overall tensile strength was improved significantly from 875.2 MPa to 930.1 MPa and 1001.5 MPa with simultaneous increase of strain elongation from 8.7 % to 10.9 % and 13.3 %, revealing a positive strengthening-toughening effect by higher interlayer PDV. The overall of this work provided an insight to in-situ microstructure-performance control during laser hybrid manufacturing Ti2AlNb intermetallic alloy.
•Combining laser-deposition and interlayer point-forging (PF-LD) is a novel hybrid 3D-printing technique.•Three Ti2AlNb samples were fabricated by hybrid PF-LD technique via different interlayer press down volume.•The combination of fine-equiaxial grains, high precipitation density and strong lattice defect benefited the comprehensive mechanical performance of large PDV sample.
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.
Boron modified titanium alloys Singh, Gaurav; Ramamurty, Upadrasta
Progress in materials science,
June 2020, 2020-06-00, 20200601, Letnik:
111
Journal Article
Recenzirano
Titanium and its alloys are extensively used in a variety of high performance applications with the α + β alloy, Ti-6Al-4V, being the most popular. Conventionally Ti alloys in the as-cast condition ...possess highly coarse prior β grains, whose size is in the order of ~mm. Such large grain sizes not only reduce the strength of the alloy, but also impair its workability. The addition of about 0.1% wt.% B can result marked reduction in the grain size. The advantages offered by microstructural refinements, thus induced by trace addition of B in Ti alloys, are reviewed in this paper. Processing response of the as-cast alloys improved as a result of grain refinement due to B addition, leading to the possibility of removal or minimization of primary ingot breakdown steps. This can significantly bring down the cost of the finished Ti alloy components. Microstructural refinements with B addition on the mechanical performance of the alloys both at room and elevated temperatures are reviewed with emphasis on the microstructure-property correlations.
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