Vol. 204 目录

SCRIPTA

Guided by solution thermodynamic modeling coupled with detailed experimental characterization, the present study establishes that the alternating FCC and BCC lamellar microstructure in the Al_0.7CoCrFeNi high entropy alloy, is a result of non-equilibrium partitionless solidification from the liquid to single B2 phase, followed by solid-state decomposition. Widmanstätten FCC lamellae form from the allotriomorphic FCC precipitates at the B2 grain boundaries, leading to a lamellar microstructure, divided into two distinct sub-systems. Isothermal annealing further drives these individual sub-systems towards equilibrium via precipitation of ordered intermetallic phases. The transformation in FCC lamellae initiates by the formation of metastable L12 precipitates at shorter annealing times, which are eventually replaced by the equilibrium BCC and B2 phases, forming composite B2+BCC laths, on long term annealing. These results further exemplify that interesting transformation pathways lead to hierarchical microstructures within HEAs, and the fact that as processed conditions in these alloys are often far-from equilibrium.

SCRIPTA

The distribution of Re in a Ni-based single crystal superalloy is specified by atom probe tomography after different heat treatment. At higher temperatures (950-1100℃), the Re diffusivity in γ matrix is becoming smaller than the actual effective diffusion for γ΄ growth, during the subsequent cooling process and the interface migration, consequently Re atoms are insufficient to diffuse away and keep accumulating, leading to forming segregation at γ/γ′ interface. While at low temperatures (800-900℃), the Re diffusion is capable of balancing the effective diffusion for γ′  growth. After introducing more alloys with different Re content and different aging temperatures, Re segregation is irrelevant to the Re content but highly temperature-dependent. These results provide a novel perspective on the comprehension of the Re effect on γ′ growth in Ni-based superalloy and are valuable to both improving the γ/γ′ microstructure stability and designing high-performance superalloys.

SCRIPTA

In this work, we report an anomalous phenomenon on superelastic cycling of NiTi shape memory alloys when deforming at the temperature close to the border of superelastic window. New unexpected effects are found—(i) critical stress for inducing martensite transformation during the second loading cycle is higher than that of the first cycle; (ii) the plateau stress of the second cycle decreases to the original level when the strain overcomes the limit of the first cycle; (iii) transition from good superelasticity in the first cycle to fully irreversible strain in the second. We propose that defects generated during the first superelastic cycle close to the border of superelastic window impede following stress-induced martensitic transformations, leading to the increase of critical stress beyond yield stress of the B2 matrix, and thus functional fatigue of NiTi alloys.

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SCRIPTA

Generalized stacking/planar fault energy (GSFE/GPFE) curves provide the intrinsic energy barriers (IEBs) for plastic deformation in FCC structures. In this study, we propose a new approach for estimation of these IEBs. A strong correlation between the unstable fault energies and shear modulus on the {111} planes is shown for a large set of pure FCC metals. Interestingly, data from the literature also obeys this correlation for a variety of FCC solid solutions. High-throughput estimation of the IEBs combining these correlations with a diffuse multi-layer fault model for stable fault energies is demonstrated for pure FCC metals and FCC (Ni_0.5Co_0.5)_1-xRu_x (x=0 to 0.5). Additionally, other important descriptors for alloy design, including critical stresses for slip and twinning, were estimated. This new approach opens avenues for the high-throughput design of multi-principal element alloys (MPEAs) based on the propensity for twinning and deformation pathways.

SCRIPTA

We demonstrate excellent superelasticity and giant elastocaloric effect in a directionally solidified Co_50.7V_33.3Ga₁₆ alloy. Owing to the formation of [001]_A preferentially oriented columnar grains, the as-grown alloy displays nearly perfect superelastic behaviors with fully reversible compressive strain of up to 5.4%, narrow stress hysteresis of about 40 MPa and a wide operating temperature window from 258 to 373 K. Simultaneously, a giant adiabatic temperature change ΔT_ad of -18.7 K is obtained on removing a uniaxial stress of 400 MPa at room temperature, which is in good agreement with the theoretical values based on Maxwell relation (-19.0 K) and calorimetric measurements (-19.9 K). A large |ΔT_ad| of greater than 12.5 K is obtained over a wide temperature span of at least 115 K. Moreover, both superelasticity and elastocaloric effect remain good functional stability without any evident degradation for 100 cycles.