► The performance of AlTi5B1 falls short of expectations with the AlSi10Mg and AlSi12Cu alloys. ► When inoculated with AlB3, their grains are very small for the entire range of holding times. ► Fine ...grains, invariably smaller than 200
μm, are readily obtained with the addition of 200
ppm B. ► AlB3 offers remarkable grain refining efficiency and a strong resistance to fading.
The potential of AlB3 master alloy in the grain refinement of AlSi10Mg and AlSi12Cu foundry alloys was investigated and compared with that of the AlTi5B1 master alloy, the standard grain refiner for most aluminium foundries. The latter refines the grain structures of both alloys. However, this performance is not nearly as good as that obtained in wrought aluminium alloys with the same grain refiner. The Ti-free AlSi10Mg and AlSi12Cu alloys, on the other hand, exhibit very small grains for the entire range of holding times when inoculated with AlB3. This implies a remarkable grain refining efficiency, typical of grain refined wrought aluminium alloys, as well as a strong resistance to fading of the grain refinement effect. Lack of Ti in the melt allows the entire B to form AlB
2 particles, the perfect substrates, shortly before α-Al starts to crystallize. Aluminium castings can enjoy grains as small as those of the wrought alloys, well below 200
μm, with an addition of 0.02
wt% B provided that their Ti content is controlled.
The high texture dependence of a Hall–Petch slope (k) for Mg alloys has been frequently reported. Several important equations used to calculate k have been previously developed, and although they ...seem to work well for fcc and bcc materials, they often fail to predict the highly texture-dependent k in Mg alloys. A new equation based on the dislocation pile-up model was developed in this study. The validity of this new equation was tested through a comparison of the predicted k values with the experimental values as well as the calculations from older equations. The results indicate that the new equation can achieve an accurate prediction for several previously reported texture effects on k, whereas the k values predicted by the older equations often exhibit a clear deviation. The reasons for this were analyzed and discussed. The strong deformation anisotropy for Mg alloys leads to a complex texture effect on k, including the effects from both external and internal stresses. Both effects are well expressed in the new equation. In contrast, the old equations consider the external stress effect, but do not express well the internal stress effect. In addition, the old equations consider only the predominant deformation mode. However, our results indicate that the activation of a portion of another deformation mode other than the predominant one plays an important role in the k value. In the new equation, all possible deformation modes and their fractions are considered in the calculation. Using the important parameters of the new equation, the mechanisms for several texture effects on k as previously reported were discussed and new understandings were obtained.
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Surface nanocrystallization is gaining increased attention due to its high potential of enhancing mechanical functionality and modulating material's interaction with the surrounding environment. ...Among mechanical approaches, severe shot peening has recently shown to be a very promising technique for surface grain refinement, considering its efficiency, eco-friendliness, relative low cost and minimum geometrical restrictions. This study evaluates the effect of severe shot peening on microstructural and mechanical properties of 316L stainless steel, which is widely used in biomedical, food preparation, structural and marine applications. 316L samples, shot peened with different peening parameters, were studied in terms of morphological and structural features, defect density, grain size, phase transformation, surface topography, surface wettability, residual stresses and microhardness. The results indicated that severe shot peening induced near surface grain refinement to nano and sub-micron range and transformed the austenite phase into strain-induced α’- martensite in a layered deformation band structure. Severe shot peening also induced compressive residual stresses and work hardening in the top surface layer. Surface roughness and surface wettability, both of which favorably contribute to modulating the interaction of material with biological environment, were also notably enhanced by severe shot peening.
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•Severe shot peening induced gradient grain refinement from surface to the core.•Austenite phase transformed into αʹ-martensite in the form of a layered structure.•Martensite grains showed preferred orientation parallel to peening direction.
Considerable studies on metal selective laser melting (SLM) have proved the necessity to refine microstructure parts fabricated by SLM in order to eliminate property anisotropy, hot-tearing and to ...increase the SLM-processability. In the present work, Ti nanoparticles, at the first time, were discovered to be an extremely effective inoculant for an SLMed 2024 aluminium alloy. 0.7 wt% addition of Ti nanoparticles was capable of substantially eliminating the hot-tearing cracks and columnar structure, and refining the grains in the SLMed 2024 alloy in a broad processing window. The substantial grain refinement in the Ti-inoculated 2024 alloy was attributed to the in-situ formation of Al3Ti nanoparticles with a L12 ordered structure, which formed a coherent interface with Al matrix and therefore significantly promoted the heterogeneous nucleation of the α-Al during solidification of melt pools in the SLM process. After a conventional T6 heat treatment, this SLMed alloy exhibited a superior balance of strength and ductility (tensile strength was up to 432 ± 20 MPa and elongation of 10 ± 0.8%), which was comparable to its wrought counterpart. This work can be considered as a breakthrough in research of fabricating high-strength aluminium alloys using SLM.
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This study focuses on the role of pyramidal 〈c + a〉 dislocations in the grain refinement mechanism in the Ti-6Al-4V alloy with an initial {112¯0}<101¯0>rolling texture. A large number of pyramidal ...〈c + a〉 dislocations were activated in the sample subjected to the severe shot peening process. Two important roles of pyramidal 〈c + a〉 dislocations were discovered. First, pyramidal 〈c + a〉 slip coordinates the large c-axis strain, thereby achieving generalized plastic flow, especially in nanograins. Second, the unique low-angle grain boundaries (LAGBs) with basal-pyramidal dislocation locks (prismatic 〈c〉 and prismatic 〈c + a〉 dislocations) were produced for the first time by pyramidal 〈c + a〉 interacting with basal 〈a〉 dislocations. This unique low-energy boundary greatly enhances the stability of the strain-induced grain boundary and dislocation density (~6.6 × 1015m − 2 in nanograins). The grain refinement process contains three types of subdivision modes: (I) dislocation walls with pyramidal 〈c + a〉 dislocations in coarse grains; (II) basal 〈a〉 intersecting with prismatic 〈a〉 dislocations in coarse grains; and (III) basal 〈a〉 intersecting with pyramidal 〈c + a〉 dislocations in coarse grains, ultrafine-grains and nanograins. The occurrence of slip modes depends on the initial texture and texture evolution during dynamic recrystallization. Besides, Hall-Petch breakdown at the nanoscale was found and is attributed to the decreasing critical resolved shear stress of pyramidal 〈c + a〉 slip at the nanoscale. This study provides a new approach for the design of stable nanostructured hexagonal close-packed metals by the unique LAGBs with basal-pyramidal dislocation locks.
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Si poisoning on Al-5Ti-B master alloys has been restraining the effectiveness of grain refinement of hypoeutectic Al-Si casting alloys for over 60 years, and yet the underlying mechanism of this ...phenomenon remains unclear. In this work, Si poisoning in Al-Si/Al-5Ti-B system was systematically investigated by combining state-of-the-art electron microscopy, first-principles calculations and thermodynamic calculations. Different from the common belief that silicides coat and therefore poison TiB2, this study demonstrates that the segregation of Si atoms at the TiB2/α-Al interface is likely the cause of Si poisoning. Silicide was found to be thermodynamically unfavorable to form even in an alloy with 10 wt.%Si. On the other hand, an appreciable amount of Si (5–20 at.%) was found to segregate in the TiAl3 two-dimensional compound (2DC) which is critical for triggering the nucleation of α-Al on TiB2. The formation of Ti-Si covalent bond within TiAl3 2DC disturbs its lattice and reduces its chemical interaction with α-Al, which both obstruct the epitaxial nucleation of α-Al and hence leads to Si poisoning. This study suggests that composition engineering of TiAl3 2DC and TiB2 with elements less attractive to Si could be a viable way to mitigate Si poisoning.
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A new (α + β) Ti-alloy, Ti-6Al-2V-1Cr-1Fe (wt%), with fine grain sizes, fine precipitates, together with the high strength and excellent ductility in its as-cast state is developed. As compared to ...the as-cast commercial Ti-6Al-4V alloy, the grain size and α lath thickness of the new alloy are substantially refined by 50∼75%, and the yield strength and ductility are increased by 19.7% and 51.8%, respectively. The grain size refinement is achieved by tuning the supercooling capacity of the alloy through alloying with the aid of CALPHAD calculations. In addition, the alloying of Cr and Fe substantially reduces the α lath thickness. This low-cost Ti alloy with the enhanced properties is anticipated to be highly suitable for various structural applications in its as-cast or as-printed state.
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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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