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SCRIPTA

An integrated computational materials engineering (ICME) approach was used to design a fully Austenitic, Mn-stabilized steel strengthened by three nano-scale precipitate phases: β-NiAl, FCC-Cu, and M₂₃C₆ carbides. Thermo-Calc was used to predict the effects of Ni, Al, and Cu additions on phase stability in a Fe-17.7Mn-4.7Cr-0.48C (wt.%) base Austenitic composition. These predictions were verified experimentally by (1) synthesizing alloys of this Austenitic base alloy modified with Cu, Ni+Al, or Cu+Ni+Al, (2) measuring the mechanical properties by microindentation, and (3) characterizing the nano-scale microstructures by atom probe tomography (APT). After ageing at 580°C, selected based on Thermo-Calc modeling, only minor hardening was observed in samples modified with either Cu or Ni+Al. However, the alloy with combined additions of Cu together with Ni+Al exhibited significant hardening (490 HV, corresponding to an estimated σ_y ≈ 1200 MPa), which was attributed to the formation of FCC-Cu and β-NiAl nano-precipitates.

SCRIPTA

Alpha-beta titanium alloys excel for aeroengine applications but are typically limited to ~550°C. An alternative strategy is reinforcement with the ordered-beta TiFe intermetallic, toward ‘β-Ti superalloys’, however, there has been minimal study of TiFe precipitation in the binary system.

Here, a Ti-20Fe (at.%) alloy was homogenised at 1050°C in the β-Ti phase field and aged at 600°C where the Fe supersaturation promoted TiFe precipitation. Curiously, as the TiFe volume fraction increased, the alloy hardness decreased, due to an interplay of mechanisms: (1) Fe solid solution strengthening, which reduces as the β-Ti Fe content falls to 16.2% on ageing; (2) ω precipitation strengthening, as ω-like incommensurate modulated domains were identified by transmission electron microscopy in the homogenised β-Ti parent phase and are suggested to change in size and structure after ageing, resulting in reduced ω-strengthening; (3) softening as softer TiFe and α-Ti phases precipitate from the harder ω?strengthened β-Ti parent phase.

SCRIPTA

Zinc and its alloys have received increasing attention due to their potential to be applied as biodegradable implants. However, the distinct tension-compression yield asymmetry of zinc and its alloys after plastic forming is undesirable. Here we identify the strong tension-compression yield asymmetry in as-extruded pure zinc and reveal that higher {10-12}⟨-1011⟩twinning activity under tension is the underlying reason. Based on the finding, weakening texture and refining grains are put forward as effective strategies to reduce the tension-compression yield asymmetry.

SCRIPTA

Direct energy deposited (DED) Ti-5Ni (wt.%) alloys promote significant grain refinement with fine equiaxed grains (~30-50 μm); they have discontinuous grain boundary α, fine eutectoid α laths (~1 μm width), and α + Ti₂Ni phases. Post-heat treatment below beta transus for 24 h stimulates the formation of near equiaxed α, and the temperature significantly influenced the microstructural constituents, which included pre-eutectoid α + retained β + Ti₂Ni (760°C), pre-eutectoid α + α’ + Ti₂Ni (800°C), and fully α’ + Ti₂Ni (850°C) phases. The tensile properties of the as-fabricated and the heat-treated alloy remarkably improved compared with the other conventionally processed eutectoid Ti-Ni binary alloys. Thus, the proposed approach opens up new opportunities and possibilities for fabricating other eutectoid-forming alloy systems with improved manufacturing efficiency, which can overcome limitations in the other manufacturing processes.

SCRIPTA

Atomistic scale interface structures could tailor the dislocation behaviors at interface, as well as interface strengthening. In this work, we design a disordered interface by introducing an ultrathin amorphous layer in the Cu/Nb multilayers. Compared to the sharp interface, the disordered interface can smear the impinging dislocation with relaxing the in-plane and out-of-plane Burgers vectors, thus inducing a remarkable hardening in the Cu/Nb multilayers. The findings not only prove the hardening effect induced by dislocation interface smearing, but also shed light on developing advanced materials with ultrahigh hardness via disordered interface.

SCRIPTA

Plastic instability manifested as the serrated flow on stress-strain curves of metal alloys is generally detrimental to their processing and manufacturing. The flow serration in the HfNbTaTiZr alloy is reported in the present work and its dependence on temperatures (573-823K) and strain rates (5×10^-5 s^-1-1× 10^-3 s^-1) is investigated with experiments. The notable transition of the onset strain of the serrated flow from a declining (normal behavior) to rising (inverse behavior) trend with increasing temperature at applied strain rates is modeled successfully with a theoretical framework. The success of the model in predicting the onset strain as a function of temperature and strain rate represents a possibility of theoretically constructing a process window for the alloy under examination and beyond.

Vol. 200, 15 July. 2021, 113913

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