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13. Morphological, nanostructural, and compositional evolution during phase separation of a model Ni-Al-Mo superalloy: Atom-probe tomographic experiments and lattice-kinetic Monte Carlo simulations

Author

Tu, Yiyou, Mao, Zugang, Noebe, Ronald D., and Seidman, David N.

Subjects
Condensed Matter - Materials Science
Abstract

The details of phase separation of a Ni-6.5Al-9.9Mo aged at 978 K for aging times ranging from 0.125 to 1024 h are investigated by atom-probe tomography and lattice-kinetic Monte Carlo (LKMC) simulations. On the basis of the temporal evolution of the nanostructure, three experimental regimes are identified: (1) concomitant precipitate nucleation and growth (t less than 0.25 h); (2) concurrent coagulation and coalescence (t 0.25 to 16 h); and (3) quasi-stationary coarsening of gamma prime(L12)- precipitates (t 16 to 1024 h). The temporal dependencies of the mean precipitate radius and precipitate number density, Nv(t), are determined experimentally, 0.344 (0.012) and -0.95(0.02), respectively, following the predictions of quasi-stationary coarsening models. In this alloy aged at 978 K, Al partitions strongly to the gamma prime(L12)-phase with a partitioning coefficient 4.06(0.04), whereas Mo and Ni partition to the gamma(f.c.c.)-matrix with values of 0.61(0.01) and 0.90(0.01), respectively. In the quasi-stationary regime(t larger than 16h), the temporal exponents of the Al, Mo, and Ni supersaturations in both the gamma(f.c.c.)-matrix and gamma prime(L12)-precipitates are in reasonable agreement with a multi-component coarsening model's prediction of -0.33. Quantitative analyses of the edge-to-edge inter-precipitate distances demonstrate that coagulation and coalescence are consequences of the overlap of the diffusion fields surrounding the gamma prime(L12)-precipitates. Both 3-D APT and LKMC results demonstrate that the interfacial compositional width decrease with increasing values. And the interfacial compositional width at infinite ageing time are estimated to be 1.89(0.22) nm, 2.09(0.12) nm and 2.64(0.03) nm for Ni, Al and Mo, respectively., Comment: 70 pages, 13 figures

Published
2018

30. Atomic interactions between Si and Mn during eutectoid transformation in high-carbon pearlitic steel.

Author

Huang, Linghui, Zhang, Rui, Zhou, Xuefeng, Tu, Yiyou, and Jiang, Jianqing

Subjects
ATOMIC interactions, PEARLITIC steel, FERRITES, ATOM-probe tomography, PHASE partition, CHEMICAL stability, CHEMICAL bonds
Abstract

The atomic interactions between Si and Mn during eutectoid transformation in high-carbon pearlitic steel were investigated. Atom probe tomography and first-principles calculations were applied to evaluate and analyze the atomic interactions at the ferrite/cementite interface. In the initial stage of eutectoid transformation, enrichment of Si and Mn occurred at the ferrite and cementite sides, respectively, of the interface. This interfacial segregation phenomenon gradually diminished as the transformation proceeded. Calculations of the cohesive energy and formation energy revealed a clear enhancement in the chemical bonding and stability of the pearlite system when the Si atom was moved from the ferrite layer to the cementite layer and the Mn atom was moved in the opposite direction. The interfacial segregation of the Mn and Si atoms was mainly responsible for the insufficient diffusion and high hybridization degree of Fe, Mn, Si, and C atoms. Furthermore, the partitioning ratio of Mn in high-Si steel was greater than that in low-Si steel, leading to greater partitioning of Mn into the cementite phase. Calculations of the electronic structure revealed that the enrichment of Si in the ferrite phase promoted the partitioning of Mn into the cementite phase owing to the strong repulsive force between Mn and Si at the pearlitic interface. [ABSTRACT FROM AUTHOR]

Published
2019
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34. Optimization of thermal stability and mechanical properties of Mo2C carbides via multi-element doping: experimental and theoretical calculations.

Author

Sun, Chichi, Zheng, Yong, Fang, Feng, Tu, Yiyou, Jiang, Jianqing, and Zhou, Xuefeng

Subjects
THERMAL stability, MECHANICAL behavior of materials, METALLIC bonds, MOLYBDENUM, IONIC bonds, DOPING agents (Chemistry), ALLOYS
Abstract

The physical properties of molybdenum semi-carbide (Mo2C) that usually serves as reinforced phases of engineering alloys are crucial for mechanical properties of bulk materials. In this work, multi-element doping is employed to modify physical properties of Mo2C. The thermodynamic stability, mechanical and electronic properties of doped L′3- and ζ-(Mo, M)2C (M = V, Cr, Fe and W) were examined by first-principles calculations and experiments. The results show that W stabilizes both L′3- and ζ-(Mo, M)2C, whereas Cr and Fe destabilize them. Among the four dopants, W greatly enhances the elastic anisotropy and Cr increases the ductility while retaining high hardness. Compared with L′3-(Mo, M)2C, ζ-(Mo, M)2C exhibits higher thermodynamic stability and stronger elastic anisotropy. Multi-element doping alters the relative fractions of covalent, ionic and metallic bonds in the compounds, accounting for the correlation between various dopants and physical properties. Further experiments confirm the structural and compositional dependence of thermal stability, which agrees well with the theoretical predictions. [ABSTRACT FROM AUTHOR]

Published
2022
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