High Nb-containing TiAl alloys have been deemed ideal candidates for demanding structural applications under high temperatures due to their appealing mechanical qualities and low density. ...Nonetheless, its inherent brittleness at room temperature remains a significant impediment to its processability. Herein, different concentrations of boron were added into the Ti45Al8Nb2Cr (Ti4582) alloy by vacuum arc melting to tailor the microstructure and corresponding mechanical properties. The coarse lamellar colonies were significantly refined via boron addition, while excessive boron addition did harm to the mechanical properties. The different morphologies of the borides (Bf or B27) profoundly influenced the microstructure and mechanical properties of as-prepared alloys. Further elemental distribution, phase constitution, lamellar colony size, and mechanical properties were systematically investigated. Noteworthily, trace boron additions effectively improved the mechanical properties of Ti4582-xB alloys with an optimal adjustment in Ti4582–0.2B, showing 673 MPa for room temperature ultimate tensile strength (UTS) with elongation of 1.3 % and 1623 MPa for ultimate compressive strength (UCS) with compressive strain of 32.5 %. The improved mechanical properties of Ti4582-xB were mainly attributed to grain refining and second phase strengthening. Moreover, the hindrance and accommodation for dislocations by deformed twins and reinforcements effectively enhanced the mechanical properties.
•Ti45Al8Nb2Cr-B (Ti4582-xB) alloys were prepared via vacuum arc melting, consisting of α2, γ, β and TiB.•Three-step heat treatment was performed to tailor the microstructures of titanium aluminide matrix composites.•Ti4582–0.2B exhibits good mechanical properties at ambient and elevated temperatures.•The enhanced mechanical properties of Ti4582-xB are mainly attributed to grain refining strengthening and second phase strengthening.
Single-phase CrCoNi medium-entropy alloys (MEA) are emerging recently as an interesting class of metallic materials, but the dynamic response of this MEA at high strain rates remains unknown. Here we ...have produced this MEA with various heterogeneous microstructures, using cold rolling followed by annealing at various temperatures. The high-strain-rate response of the MEAs was characterized using hat-shaped specimens in Hopkinson-bar experiments. A combination of high dynamic shear yield strength and large uniform dynamic shear strain was observed, exceeding all other metals and alloys reported so far. Even better dynamic shear properties was revealed when the experiments were conducted at cryogenic temperature. The strong strain hardening under dynamic shear loading can be attributed to the dynamic grain refinement and deformation twinning that accompany the homogeneous shear deformation. When compared to room temperature, the efficiency of grain refinement was found to be enhanced at cryogenic temperature, with a higher density of multiple twins, stacking faults, Lomer-Cottrell locks, and hcp phase via phase transformation inside the grains, which could be responsible for the better dynamic shear properties under cryogenic environment.
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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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Wind turbine gears are critical components within the internal gearbox of the wind turbines. However, the vulnerability of gears to wear adversely affects their operational lifespan. Here, we propose ...a novel approach utilizing ultrasonic shot peening to incorporate Al2O3 particles onto the surface of the 18CrNiMo7–6, termed the USG treatment. The tribological test shows that the coefficient of friction and wear rate of the USG-treated sample decreased by 14.4% and 58.3% respectively, as compared to the untreated sample. Experiments and detailed analysis reveal that 18CrNiMo7–6 not only undergoes grain refinement on its surface but also forms a wear-resistant layer enriched with Al2O3. The findings here provide a new insight into the design of wear-resistant layers on gear steel surfaces.
Metastable β titanium alloys possess excellent strain-hardening capability, but suffer from a low yield strength. As a result, numerous attempts have been made to strengthen this important structural ...material in the last decade. Here, we explore the contributions of grain refinement and interstitial additions in raising the yield strength of a Ti-12Mo (wt.%) metastable β titanium alloy. Surprisingly, rather than strengthening the material, grain refinement actually lowers the ultimate tensile strength in this alloy. This unexpected and anomalous behavior is attributed to a significant enhancement in strain-induced α’’ martensite phase transformation, where in-situ synchrotron X-ray diffraction analysis reveals that this phase is much softer than the parent β phase. Instead, a combination of both oxygen addition and grain refinement is found to realize an unprecedented strength-ductility synergy in a Ti-12Mo-0.3O (wt.%) alloy. The advantageous effect of oxygen solutes in this ternary alloy is twofold. Firstly, solute oxygen largely suppresses strain-induced transformation to the α’’ martensite phase, even in a fine-grained microstructure, thus avoiding the softening effect of excessive amounts of α’’ martensite. Secondly, oxygen solutes readily segregate to twin boundaries, as revealed by atom probe tomography. This restricts the growth of {332} deformation twins, thereby promoting more extensive twin nucleation, leading to enhanced microstructural refinement. The insights from our work provide a cost-effective rationale for the design of strong yet tough metastable β titanium alloys, with significant implications for more widespread use of this high strength-to-weight structural material.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Display omitted
Being lightweight, energy-efficient and environmentally benign, magnesium alloys present great potential for various industrial applications. However, they possess relatively low mechanical ...properties and need to be strengthened. During the last decade, significant effort has been directed towards preparation of strong nanocrystalline (NC) Mg alloys, although because of the limited plasticity inherent to HCP metals, the grain size of Mg was rarely refined below 1000 nm and the yield strength seldom exceeded 500 MPa. Here, by means of a conventional industrial method of rotary swaging, we prepared bulk NC Mg–Gd–Y–Zr alloys with an average grain size of 80 nm and a dimension of ∅3 mm × 1000 mm. The further-aged NC Mg alloy exhibits the yield strength of 650 MPa and the ultimate tensile strength of 710 MPa, the highest such values published for bulk Mg alloys. Fracture surface observation suggested a ductile inter-granular fracture in the NC Mg alloys. The high strength are attributed to nano-grain, intra-granular Gd rich clustering, inter-granular solutes segregation, β′ precipitation, dislocation and solution strengthening contributions, among which the nano-grain strengthening is dominant. The nano-grain formation results from the large number of mechanical twins, deformation bands and stacking faults induced by the high strain rate of swaging. Our work advances the industrial-scale production of bulk NC Mg alloys by exploring a simple and low-cost fabrication technique.
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In this study, we report the effect of elemental combinations on the friction stress and Hall-Petch relationship in medium entropy alloys (MEAs) and high entropy alloys (HEAs) which are defined as ...the alloys composed of four or less and five or more principal elements, respectively, with (near-) equi-atomic concentrations. The MEAs (CoCrFeNi, CoCrNi, etc.), which are subsystems of equi-atomic CoCrFeMnNi HEA, were highly deformed by high-pressure torsion (HPT) and subsequently annealed at different temperatures. The specimens with fully-recrystallized microstructures of FCC single-phase with various mean grain sizes down to sub-micrometer scale were obtained. Subsequently, tensile tests were performed at room temperature to obtain precise Hall-Petch relationships and friction stresses of the materials. Co20(CrNi)80 was successfully predicted as the alloy showing the highest strength among the MEAs by the modified Labusch model (so-called mean field Labusch model) for solution hardening. Experimental values of the friction stresses were found to fit with the model very well, indicating that the strength of the alloys was closely related to entirely distorted crystal lattice acting as high-density obstacles for dislocation motion in the alloys. At the same time, values of the average lattice distortion in the alloys were found comparable to those in some dilute alloys, although “severe” lattice distortion had been believed as a reason for the higher strength than dilute systems. Finally, a strengthening mechanism by element-element interaction was proposed as an additional mechanism to enhance the strength in FCC HEAs and MEAs.
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•Rare earth (RE) containing Mg–Gd–Y–Zr alloy was successfully multidirectional forged for 8 passes without any cracks.•Initial coarse grains was refined due to a combination of discontinuous and ...continuous dynamic recrystallization.•The alloy MDFed after 6 passes exhibited an excellent balance of strength and ductility at room temperature.
A homogenized Mg–10Gd–4.8Y–0.6Zr (wt.%) alloy was subjected to multidirectional forging (MDF) at 773K. It was shown that the average grain size decreased with increasing cumulative strain till 6 passes. Uniform fine-grained structure with an average grain size of 9.3μm was achieved after 6 passes, i.e., cumulative strain of 1.8. Results revealed that grain refinement was induced by a complicated combination of discontinuous dynamic recrystallization at low strain regions and continuous dynamic recrystallization or rotation recrystallization at medium-to-high strain regions. The alloy multi-directionally forged (MDFed) for 6 passes exhibited an excellent balance of strength and ductility at room temperature with ultimate tensile strength (UTS) of 336MPa and elongation to fracture of 21.0%.
Additive manufacturing (AM) is a new and promising production methodology adept at producing complex geometries, which can be optimized for lower weight and enhanced capabilities. The material ...properties of these additive components are dictated by the microstructures developed during processing, with a high sensitivity to grain structure and associated anisotropy. With this new processing modality comes the added difficulty of understanding the thermodynamics and kinetic mechanisms that dictate the evolution of microstructure. This research addresses the unique thermal conditions present in AM and the pathways for grain refinement in nanofunctionalized aluminum alloys. The Al-Ta system, in which Al3Ta intermetallic compounds are demonstrated to have substantial grain refining capacity, are the focus of this study. The grain size is shown to be reduced relative to pure aluminum by 1000X when tantalum is added at 1 vol%. The effectiveness of the Al3Ta intermetallic is dictated by the crystallography and availability of the inoculant phase under AM conditions.
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