Influence of alloying additions on grain boundary cohesion in tungsten: First-principles predictions. Xuebang Wu and C.S. Liu

Transcription

1 2016 Joint ICTP/CAS/IAEA School and Workshop on PMI in Fusion Device Influence of alloying additions on grain boundary cohesion in tungsten: First-principles predictions Xuebang Wu and C.S. Liu Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, PR China

2 Outline Background and objective GB models and Methodology Main results Summary

3 Background W is considered as the leading candidate for the PFM because of its excellent physical properties. W exhibits serious embrittlement, such as low-t brittleness, irradiation brittleness and recrystallization brittleness. Impurities, such as O and N, which segregate in the grain boundary (GB) are thought to be one of the main causes of intergranular fracture. Hurishita et al, Phys. Scr. T 159 (2014)

4 Background Some effective ways to improve the properties: Microalloying: solid solution with Ta, V, Re, Ti, Zr Nanostructured alloys: ODS, TaC, TiC, ZrC GB engineering: optimize GB structure, grain size and texture In all of these areas, understanding and controlling the properties of W at intergranular regions is highly valued, especially for the effect of impurities on the GB strength. Tan et al., JNM 441, (2013)

5 Background Since it is difficult to obtain effective information on the effects of impurity atoms on GB strength experimentally, theory study via FP calculations provides useful insight into the mechanism controlling the strength, toughness, and resistance to impurity embrittlement. Simulation studies have successfully used to study effects of solutes on GB strength in many metals, Fe, Al, Ni, Cu, Mg, and Zr. Fe Geng, Olson, Freeman, Gao Science PRB Al Lu, Yamamoto, Olson PRB, Acta Cu Finnis, Duscher, Kang Nature, NM, PRL Ni Sob, Yamaguchi Science, Prog. Mater. Sci. Mg Nie Science 2013 Zr Christensen JNM 2010

6 Background Two methods were adopted: 1. Enegetics based on Rice-Wang model: (Embrittling impurities have stronger binding to surface than to GB) E E E X seg GB Bulk + - X X E E E ( E E ) ( E E ) + - SE GB FS GB GB FS FS Yamaguchi, Metall Mater Trans A 42A, 319 (2011)

7 Background 2. FP computational tensile test: (a uniaxial tensile strain is applied in the direction normal to GB plane ) W W E frac E E0 max f ( ) S E frac e W W PRB 82, (2010); Acta Mater. 59, 6155 (2011); Metall Mater Trans A 42A, 330 (2011)

8 Objective We focus on the segregation and strengthening behaviors of transition metal elements (3d, 4d and 5d) in a series of low-σ symmetric tilt W GBs with [001], [110] and [111] tilt axes. To reveal the dependence of the GB strengthening on the GB structures and the solute itself To provide a reference in designing W-based materials with improved cohesion by solutes Solutes

9 Outline Background and objective GB models and Methodology Main results Summary

10 GB structures Vacuum Free Surface Grain Boundary Grain 2 Grain 1 Z Free Surface Y X 8 low-σ symmetric tilt W GBs with [001], [110] and [111] tilt axes 6 substitutional sites along the GB/FS (1-6)

11 Exper. Images Vs Models Σ5(310) GB (Novoselov et al, Physics of the Solid State, 2014, 56, ) Σ3(112) GB (Lejcek P, Grain boundary segregation in metals: Springer Science & Business Media; 2010, Page 12)

12 Methodology A first-principles method, DFT VASP code PAW-GGA Ecut: 500 ev Force on each atom : less than 10-3 ev/å

13 GB structures

14 GB sliding and migration Σ5(210)

15 Outline Background and objective GB models and Methodology Main results Summary

16 Segregation energy and strengthening energy Σ3(111)[110] E seg is negative for undersized solutes, indicating that these atoms are stable in GBs. And they leads to a GB cohesion enhancement. For oversized solutes, the situation is complex. E seg and ΔE SE shows a positive or negative value at different positions.

17 Common solutes Ti, V, Ta, Re, Hf and Ru γ, both E seg and ΔE SE. For Σ3(112), strengthening is weakest Ti, V and Ta do not show a strong segregation to GBs and their strengthening are less pronounced Hf and Ru remain in the GBs and increases the strength at Σ11(323) significantly

18 Dense segregation of atoms For the segregation of dopant dimers at the same layer into the GB E seg are almost twice the values of single atoms. This indicates that the impurity atoms will occupy the most stable positions until all available positions are exhausted Strengthening of dimers is roughly proportional to the planar concentration of the solute. Σ3(111)[110] For dense segregation of atoms to different layers of the GB Segregation sequence of multiple atoms and their effects on the GB strength were also studied, the results show that the GB strength does not vary significantly.

19 Dependence of ΔE SE on the metallic radii of solutes γ ΔE SE increases with the increasing radius of solute. Size effect plays a major role in the solute effect on GB strength. For a certain metallic radius, ΔE SE decreases with increasing γ. The solutes tends to increase the strength for GBs with larger γ values

20 General trend of ΔE SE of solutes in different metals A similar trend is observed for the change of ΔE SE of solutes across the TM series. Variation trend of ΔE SE may rely mainly on the properties of solutes but the magnitude depends on the solvent and the GB structures.

21 FP tensile tests Separation energy (J/m 2 ) (a) Stage II Stage I saturation Stage III W bulk W GB W GB+Ru(2) W GB+Ru(1) Separation / nm Tensile stress (GPa) Σ3(111)[110] W bulk W GB W GB+Ru(2) W GB+Ru(1) Tensile stress (b) Separation / nm Three distinctive stages exits during tensile test (Elastic, plastic, and broken). σ max is largest for bulk W (29.48 GPa), smaller for clean GB (26.52), and Ru segregation at site 1 causes a small reduction (26.38) and a slight increase (26.65) at site 2.

22 Charge density during straining I W(1)-W(4) <I W(1)-W(-4) <I W(1)-W(-2) <I W(1)-W(-2) <I Ru(2)-W(4) Elastic Plastic Broken

23 Summary The GB strengthening depends strongly on the GB structures. For all GBs the strengthening energies of elements correlate positively with their metallic radii, implying that the size effect plays a major role in the GB strengthening in W. The oversized solutes (Zr, Nb, Hf, Ta) act as cohesion enhancers when segregated in the GB plane, while the undersized elements (Ru, Rh, Re, Os, Ir) strengthen the GBs at the nearest neighbor positions. The presence of Zr/Ta/Re impurity leads to an increase of the theoretical tensile strength and the GB cohesion, suggesting that the improved fracture resistance by Re, Ta and Zr originates mainly from the GB strengthening.

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27 Test of GB structures Grain boundary energies (γ) of typical Σ3(111), Σ5(310) and Σ5(210) GBs with different numbers of atoms and vacuum thickness. GBs Σ3(111) Σ5(310) Σ5(210) No. of atoms d vacuum (Å) γ (J/m 2 )

28 GB models Schematic representation of the unit cells. (a) GB s. (b) bulk and (c) free surface

29 Segregation energies of multiple Ta or Re atoms at the different positions in the Σ3(111) GB. Case Positions of Ta atoms Total segregation energy (ev) Case Positions of Re atoms Total segregation energy (ev) Ta -6 Re Ta Re Ta Re (-2) (-2) (-3) (-3) -0.13

30 Possible fracture paths for (a) W GB with an impurity atom in site 1, (b) site 2, (c) two impurity atoms in sites 1 and 3, (d) sites 1 and 2. The red and gray spheres represent the impurity atom and W atoms. Fracture surfaces are marked with dashed lines.

31 Calculated fracture energies of pairs of 2Ta, 2Re, Ta/Re, and Ta/Ru atoms at the different positions in the Σ3(111) GB. The fracture energies for the preferred fracture paths are marked in bold. Case Positions of atoms Fracture path Fracture energy (J/m 2 ) Clean GB Ta Re Ta Re TaRe TaRu