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The influence of prior-β-grain (PBG) size on the tensile behavior of a laser-deposited α/β Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy was investigated to reveal the strengthening mechanism of PBGs. Results show that the strength increases with increasing PBG size. This special coarse-PBG strengthening predominantly depends on the strong constraints resulting from the strain incompatibility among PBGs rather than PBG boundary resistance to dislocation motion. With increasing PBG size, increasing constraints are developed in the adjacent PBGs, and thus the applied stress needed to maintain the compatible deformation among these PBGs increases. In particular, the strengthening effect is not significantly affected by increasing temperature, which reveals a potential strengthening method that can strengthen a laser-deposited α/β Ti alloy at room and elevated temperatures simultaneously by controlling its PBG size. [ABSTRACT FROM AUTHOR]

Abstract: The compositional variation and the microstructural evolution in laser additive manufactured Ti/Ti–6Al–2Zr–1Mo–1V graded structural material were investigated. An anomalous compositional variation and its good correlation with hardness were found. Although, during the forming process, the ratio of Ti–6Al–2Zr–1Mo–1V in feeded powders abruptly changes from 0% to 100%, all the elements exhibit a gradual variation, which is preferred to a graded material because such gradient change can decrease the discontinuity of its microstructures and properties and can remarkably reduce the costs due to the simplified manufacturing process and the rational distribution of materials. [Copyright &y& Elsevier]

Every strengthening/hardening method has its own limit. However, it is difficult to couple multiple extreme hardening in a bulk material, especially for highly strengthened ones. Here we demonstrate a strategy to further harden an ultrastrong precipitation-hardened high entropy alloy (HEA) with ultrafine-grain (UFG)/nanograin (NG) bands. These UFG/NG bands with nanoprecipitates were obtained by combining local severe plastic deformation (SPD) from cryogenic rolling and the pinning effect of precipitates during recrystallization. We found that the bands, with combined hardening of NGs and nanoprecipitates, provide an amazing yield strength of ~2.8 GPa and an ultrahigh hardness of ~9.7 GPa. Such nanoprecipitation-hardened UFG/NG bands in the bulk HEA contribute to an extra strengthening close to 300 MPa and an extremely high tensile strength of more than 2.2 GPa. This research presents a possibility for obtaining NG structure in bulk metals which will open new avenues for developing stronger alloys. [ABSTRACT FROM AUTHOR]

Deformed alloys commonly soften after annealing because of the decrease in defects. This work reports an annealing hardening that occurs in a cryo-rolled CoCrFeNi high-entropy alloy. This abnormal behavior is related to continuous twinning in nanoscale, which would have occurred in cryo-rolling but was belated until annealing was performed. This is because the stress accumulated in cryo-rolling was not enough to activate the reaction between dislocations and stacking faults, so as to require additional thermal activation (by annealing) for supplying the energy requirement for twinning. Due to the extra strengthening from stable twin boundaries, the annealed samples exhibit an ultrahigh yield strength close to 1.4 GPa (~6 times higher than the solid-solution counterpart), even after annealing for 24 h. These results confirm a possibility for introducing numerous stable nanotwins in bulk FCC metals to improve their mechanical properties. [ABSTRACT FROM AUTHOR]

Including ductile solid-solution phases into NiAl intermetallic alloys can toughen these materials, but based on conventional alloying strategy, this approach either degenerates the strength or has no visible improvement in the room-temperature (RT) ductility. Here we show a feasible strategy to improve the RT ductility of NiAl alloys by replacing conventional dilute solid solutions with complex concentrated (i.e. , high-entropy) solid solutions as the ductilizing phases. The resulting alloy possesses a composite nanostructure, which combines B2-structured Ni(Al, Fe) phase with body-centered-cubic (BCC) and face-centered-cubic (FCC) structured high-entropy phases (HEPs) in different scales. We find that the applied stress is mainly partitioned to the B2 phase, contributing the alloy to high strength, while the ductile BCC and FCC HEPs, although bearing low loading, cooperate with the stress-induced martensitic transformation to accommodate the deformation, enhancing the sample ductility. Accordingly, our alloy presents superior tensile properties in a wide temperature range, including high strength (>1500 MPa at RT and >360 MPa at 700 °C) and good ductility (>9% at RT and ~8% at −50 °C), which achieves a simultaneous improvement in the ductility and strength compared with the conventional multiphase NiAl alloys. • Proposing a strategy to improve the ductility of multiphase NiAl alloys. • Revealing the distinctive ductilizing effect of high-entropy solid solutions. • Presenting the contribution of B2→L1 0 martensitic transformation to the ductility • Achieving a simultaneous improvement in the ductility and operating temperature. [ABSTRACT FROM AUTHOR]

Laser surface quenching was conducted on the 1.0C-1.5Cr steel with different initial microstructures. The effects of initial microstructure on the cementite dissolution and grain size were studied, and the distributions of temperature and stress were simulated. The results indicated that the thickness of hardened layer was larger when the initial microstructure is martensite, which is mainly because the martensite-to-austenite transformation is displacive transformation during laser heating. Within hardened layer, the volume fraction of cementite (VFC) increases with the increase of depth. For the initial microstructure composed of ferrite matrix and cementite, the refining of cementite will facilitate its dissolution, and within hardened layer, mean diameter of cementite (MDC) decreases first and then increases with the increase of depth. For the initial microstructure composed of martensite matrix and cementite, the MDC changes slightly at deeper position of hardened layer. Tensile stress will form in the heat affect zone, and due to the high sensitivity to tensile stress, the impact resistance and bending properties of spheroidized microstructure are worse after laser surface quenching. Under identical laser parameters, the impact absorbed energy of spheroidized microstructure is about 50% of tempered sorbite and 60% of martensitic microstructure, and its bending strength is about 60% of tempered sorbite, and 40% of martensitic microstructure. The impact absorbed energy can be increased by at least 26% through preheating at 160 °C. In addition, the refining of initial martensitic microstructure will enhance bending resistance. [ABSTRACT FROM AUTHOR]

The 1.0C-1.5Cr steel was subjected to conventional quenching and laser surface quenching treatment. A process combing double quenching and laser surface quenching was proposed for enhancing surface hardness and obtaining finer microstructure. The cementite dissolution and grain growth behavior in the austenitizing process of single quenching, double quenching, and laser surface quenching were studied. The results indicated that compared with single quenching, mean diameter of undissolved cementite particles (UCP) was much finer in double quenching, and the final prior austenite grain size (PAGS) could be decreased by nearly 40% to about 4.5 μm. Both grain and cementite particles near the surface will coarsen after laser surface treatment. Compared with single quenching, the PAGS within hardened layer can be decreased by at least 11% through double quenching, and the mean diameter of UCP at the bottom of hardened layer can be decreased by about 20%. Compared with conventional quenching, surface hardness was enhanced by about 20% through laser surface quenching, contributing to the wear resistance. However, the hard and brittle surface layer tends to be crack source during the impact process, leading to the deterioration of final impact toughness. Under the identical laser parameters, the impact absorbed energy is similar in both single and double quenching, which is about 25% of that before laser surface quenching. The impact absorbed energy can be increased from 22J to 28J by preheating at 160 °C during the laser surface quenching. [ABSTRACT FROM AUTHOR]