Solute stabilization of nanocrystalline tungsten against abnormal grain growth

Abstract

Microstructure and phase evolution in magnetron sputtered nanocrystalline tungsten and tungsten alloy thin films are explored through in situ TEM annealing experiments at temperatures up to 1000°C. Grain growth in unalloyed nanocrystalline tungsten transpires through a discontinuous process at temperatures up to 550°C, which is coupled to an allotropic phase transformation of metastable b-tungsten with the A-15 cubic structure to stable body centered cubic (BCC) a-tungsten. Complete transformation to the BCC a-phase is accompanied by the convergence to a unimodal nanocrystalline structure at 650°C, signaling a transition to continuous grain growth. Alloy films synthesized with compositions of W-20 at.% Ti and W-15 at.% Cr exhibit only the BCC a-phase in the as-deposited state, which indicate the addition of solute stabilizes the films against the formation of metastable b-tungsten. Thermal stability of the alloy films is significantly improved over their unalloyed counterpart up to 1000°C, and grain coarsening occurs solely through a continuous growth process. The contrasting thermal stability between W-Ti and W-Cr is attributed to different grain boundary segregation states, thus demonstrating the critical role of grain boundary chemistry in the design of solute-stabilized nanocrystalline alloys. transition with a focus on harsh environment sensors produced by additive manufacturing processes. Professor Trelewicz's research is on the science of interface engineered alloys with particular emphasis on high-strength and radiation-tolerant nanomaterials for extreme environment applications. His group couples in situ and analytical characterization tools with large-scale atomistic simulations to explore the thermal stability, mechanical behavior, and radiation tolerance of solute-stabilized nanocrystalline alloys, crystalline-amorphous nanolaminates, metallic glass matrix composites, and other unique hierarchical metallic structures. Professor Trelewicz is a recipient of the 2017 DOE Early Career

Evidence for a High Temperature Whisker Growth Mechanism Active in Tungsten during In Situ Nanopillar Compression

“…This is consistent with anticipated coarsening of the nanograined tungsten via grain growth as observed by a significant reduction in diffraction spots. Grain growth in tungsten films has been shown to becomes facile above 650 °C and should grow to the lengthscale of the pillar diameter by that temperature [ 34 ]. The observation of shear steps at the surface, thus, suggest that the grain diameters have approached the width of the pillar by 850 °C.…”

Phase Field Modelling of Abnormal Grain Growth

“…To establish in more detail AGG conditions, it may be useful to apply machine-learning techniques to the analysis of simulation data bases for the three-grain type systems. So far, all the simulations have been conducted in 2D and provide an important insight into 2D abnormal grain growth observed e.g., in thin films [25][26][27][28]. The simulation results also provide guidance for future 3D simulations to specify in more detail AGG conditions for bulk materials.…”

“…Abnormal grain growth (AGG) refers to a subset of grains that will grow excessively at the expense of surrounding normal grains leading to an obvious size advantage of the abnormal grains [1,2]. The AGG phenomenon has been observed in many materials, including steels [3][4][5][6][7][8][9][10][11], aluminum alloys [12,13], super alloys [14][15][16], ceramics [17][18][19][20][21], nanocrystalline materials [22][23][24][25] as well as thin films [25][26][27][28]. AGG is of important technological relevance and must be controlled in the thermal processing of polycrystalline materials.…”

Thermodynamic Stability of Nano-grained Alloys Against Grain Coarsening and Precipitation of Macroscopic Phases

“…[20][21][22][23][24][25][26][27][28] That is why the purpose of this paper is to develop a model for thermodynamic stability of such NG alloys. Although there is plenty of previous literature on both the synthesis [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] and on modeling the stabilization of NG alloys ) (see also reviews [89][90][91][92][93] ), the present paper is novel as it puts this question into a wider framework of the nano-Calphad method, [94,95] i.e., into the thermodynamic framework originally developed by Gibbs. [96] All previous models on GB stability apply the simplest Langmuir-McLean model [97,98] (see also Reference 99) for modeling grain boundary (GB) energy.…”