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The size distribution of grains is a fundamental characteristic of polycrystalline solids. In the absence of deformation, the grain-size distribution is controlled by normal grain growth. The canonical model of normal grain growth, developed by Hillert, predicts a grain-size distribution that bears a systematic discrepancy with observed distributions. To address this, we propose a change to the Hillert model that accounts for the influence of heterogeneity in the local environment of grains. In our model, each grain evolves in response to its own local environment of neighbouring grains, rather than to the global population of grains. The local environment of each grain evolves according to an Ornstein-Uhlenbeck stochastic process. Our results are consistent with accepted grain-growth kinetics. Crucially, our model indicates that the size of relatively large grains evolves as a random walk due to the inherent variability in their local environments. This leads to a broader grain-size distribution than the Hillert model and indicates that heterogeneity has a critical influence on the evolution of the microstructure.

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An analytic model describing substitutional solid solution strengthening in body-centered cubic (BCC) complex, concentrated alloys (CCAs) based on a/2<111> screw dislocation mobility is developed and presented. At lower temperatures it is assumed that dipole dragging is the rate limiting strengthening mechanism, and as the temperature increases this changes to a jog dragging mechanism. While both mechanisms are contained in Suzuki's classical model for substitutional solid solution strengthening developed for BCC alloys significant modifications are required to apply such a model to the refractory CCA's. First, two approaches for incorporating the net effects of the complex solute chemistry are developed and contrasted using a ternary NbTaTi alloy and four quaternary alloys, MoNbTaTi, WNbTaTi, CrMoNbTi and CrMoTaTi . Our model is extended to higher temperatures and applied to analyze experimental yield data as a function of temperature (0.15–0.7T_L, where T_L is the absolute liquidus temperature) and strain rate (10⁻⁵ to 10⁻²/s) in a range of BCC chemically complex alloys including stoichiometric NbTiZr and HfNbTaTiZr. We find that the model, based on just these two mechanisms, gives excellent agreement with experimental yield data in both these BCC CCAs, NbTiZr and HfNbTaTiZr. The experimental yield data of these two BCC CCA's as well as NbTaTi and four quaternaries are also analyzed using Maresca's edge dislocation model developed for these types of alloys. The screw model presented is useful in predicting BCC CCAs with adequate high temperature strength, which will accelerate development of high temperature, high strength BCC alloys for aerospace applications.

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The synergistic effects of Mo, Ti, and Cr on nanoscale precipitation and mechanical properties of maraging stainless steels were systematically studied using high-resolution scanning transmission electron microscopy, atom probe tomography (APT), thermodynamic and first-principles calculations, and mechanical tests. Our results reveal a notable precipitation pathway involving the co-precipitation of Ni₃Ti, Mo-enriched, and Cr-rich precipitates; their formations are not separated, but rather highly interacted. The APT results indicate that Mo partitions to the Ni₃Ti precipitate core in the early stage of precipitation, which doubles the number density of Ni₃Ti precipitates. Our calculations indicate that the Mo partitioning not only increases the chemical driving force, but also reduces the strain energy for nucleation, thereby accelerating Ni₃Ti precipitation. As the precipitation proceeds, Mo atoms are rejected from the Ni₃Ti precipitate core to the interface between the Ni₃Ti precipitates and matrix, which leads to the heterogeneous nucleation of Mo-enriched precipitates on the outer surface of the Ni₃Ti precipitates. This results in a substantial size refinement of Mo-enriched precipitates. In addition, the formation of Ni₃Ti precipitates consumes Ni from the matrix, which substantially inhibits the spinodal decomposition and refines the size of Cr-rich precipitates. The cooperative strengthening of Ni₃Ti, Mo-enriched, and Cr-rich co-precipitates leads to the development of new steels with a strength of 1.8 GPa; the contributions of these precipitates to the strengthening were quantitatively evaluated in terms of precipitate shearing and Orowan dislocation looping mechanisms.

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向上滑动阅览英文摘要

The deformation mechanisms that dictate the tribological performances of heat treated Ni₅₅Ti₄₅, Ni₅₄Ti₄₅Hf₁ and Ni₅₆Ti₃₆Hf₈ alloys are revealed through rolling contact fatigue (RCF) testing and transmission electron microscopy (TEM) analyses. Analysis of worn samples that passed a 5×10⁸ cycle RCF runout condition shows that damage is primarily confined to deformation bands that propagate several hundred nanometers – to – several microns beneath the surface. These bands nucleate via localization of dislocation slip within the B2 austenite phase of the alloys. For the Ni₅₅Ti₄₅ and Ni₅₄Ti₄₅Hf₁ samples, further damage and eventual spall failures occur by shearing and dissolution of the strengthening Ni₄Ti₃ nanoprecipitates within the deformation bands, followed by nanocrystallization that sometimes includes stress-induced nucleation of B19′ nanocrystals. Eventually, complete amorphization occurs prior to fracture. The relative 15 – 25% increase in RCF contact stress performance of the Ni₅₆Ti₃₆Hf₈ alloy correlates with a more limited depth of damaged material beneath the wear surface; heavily damaged material beneath spall failure sites only extends 1.5 µm into the sample, compared to > 6 µm for Ni₅₅Ti₄₅ and Ni₅₄Ti₄₅Hf₁ alloys. This superior RCF damage resistance of the Ni₅₆Ti₃₆Hf₈ alloy results from a lower fraction of B2 matrix phase (≤ 13 %) that is highly confined by both cubic Ni-rich NiTiHf and H-phase nanoprecipitates. Specifically, this microstructure limits the widths of deformation bands within the B2 austenite phase to less than the size of the strengthening nanoprecipitates, which in turn inhibits precipitate shearing and dissolution processes that precede nanocrystallization and amorphization.

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向上滑动阅览英文摘要

We investigated hardness and tribological properties of ultra-fine grain (UFG) aluminum samples prepared by accumulative rolling bonding (ARB) and by physical vapor deposition (PVD). We have found wear-induced grain refinement in this material for the first time and we have identified a transition from grain growth to grain refinement as a function of the mean contact stress. This trend is the same for the PVD and ARB samples, which means it does not depend on the synthesis method. The hardness was found to increase with a decreasing grain size, but the results deviated from the Hall-Petch relation. Among ARB samples, UFG Al with the highest initial hardness was found to be the most wear-resistant. However, PVD-grown UFG Al was found to be most wear-resistant among all samples, even though it was not the hardest. The reason lies in the different dislocation contents of the samples prepared by different methods and in a subsequent wear-induced evolution of dislocation networks. The mechanisms of microstructural evolution in UFG Al were analyzed using transmission electron microscopy and the main mechanism identified is the dynamic recrystallization (DRX), including both continuous DRX and discontinuous DRX.

Vol. 208，15 Apr. 2021, 116777

Vol. 208，15 Apr. 2021, 116769