Direct view of self healing in creep alloys

Transcription

1 Direct view of self healing in creep alloys TU Delft: Haixing Fang, Ekkes Brück, Sybrand van der Zwaag, Niels van Dijk ESRF: Peter Cloetens MPIE: Michael Herbig Dutch Materials 2018, Utrecht, The Netherlands 1

2 Creep damage in metals: σ time-dependent deformation occurs for T > 0.4 T melting (in K) under constant load Damage preferentially nucleate and grow at grain boundary (GB) σ σ σ σ Isolated Oriented Micro crack Macro crack 2

3 Self healing approach: manage the damage mobile solute atoms Cavity nucleates and grows at GB precipitation at free crack surface Supersaturated mobile atoms diffuse towards cavity Precipitation at cavity surface Precipitation completely close the open cavity Self healing model alloys fully filled partially filled unfilled Au-depleted zone 1 μm Fe-Au Fe-Mo S. Zhang, Adv. Eng. Mater., S. Zhang, Metall. Mater. Trans. A,

4 Requirements of element X to be healing agent Temperature ( o C) Schematic phase diagram of potential self healing Fe-X alloy BCC Solutionized temperature Service temperature BCC + Precipitate Could be solutionized at high T Supersaturation: w X > w X (sol.) Diffusivity: D(X) > D(Fe) A tendency to precipitate at free surface - Atomic radius: R(X) > R(Fe) - Atomic volume: V(X of prec.) > V(Fe) Super-saturation Weight % X 4

5 Preferred X elements for self healing Fe-X alloys Fe-Cu Fe-Au BCC+Cu BCC+Au Fe-Mo Fe-W BCC+Fe 2 W BCC+Fe 2 Mo 5

6 Nucleation at free surface than in the matrix is easier for larger atoms, e.g. T = 550 o C, Fe-(sat.+1) at.% X alloys 5.0x x10-15 Surface precipitation of gold in Fe-Au (a) Fe-Cu 0.0 (b) Fe-Au Gibbs energy (J) -5.0x x10-13 Precipitation in matrix Precipitation at free surface -1.5x x x x x10-14 Precipitation in matrix Precipitation at free surface x10-15 (c) Fe-Mo 5.0x10-15 (d) Fe-W Gibbs energy (J) -5.0x x x10-14 Precipitation in matrix Precipitation at free surface -2.0x Cluster size (nm) W.W. Sun et al, Metall. Mater. Trans. A, x x x10-14 Precipitation in matrix Precipitation at free surface -2.0x Cluster size (nm) 6

7 Advantages of W as healing agents in steels Be able to be tuned into supersaturation state D w > D Fe (diffusivity)* Fe-W R W 1.10 R Fe, R Fe2W 1.06 R Fe (atomic radius) BCC+Fe 2 W Less expansive than Au Lower neutron activation than Mo Widely used in most advanced creep-resistant steels as solid solution strengthening solute Gibbs energy (J) 5.0x x x x x10 *C.D. Versteylen et al, Phys. Rev. B, Fe-W Precipitation in matrix Precipitation at free surface Cluster size (nm) 7

8 Precipitation of Laves Fe 2 W at external surface in Fe-W alloys Surface precipitation at surface indentation damage (ageing at 600 o C for 140 h) Surface precipitation without damage (ageing at 700 o C for 30 min) 8

9 Creep tests of Fe-3.8 wt. % W 0.25 (a) σ = 140 MPa Strain t R = 236 h 0.75t R = 177 h 0.50t R = 118 h 0.25t R = 59 h Samples for synchrotron X-ray nano-tomography 0.05 Sample T ( o C) σ (MPa) t creep (h) Scanning resolutions (b) Time (h) σ = 160 MPa t R = 104 h 0.50t R = 52 h S t R = 236 h 100 nm & 30 nm S t R = 177 h 100 nm & 30 nm S t R = 118 h 100 nm & 30 nm S t R = 59 h 100 nm & 30 nm S t R = 104 h 100 nm & 30 nm S t R = 52 h 100 nm & 30 nm Strain Time (h) 9

10 Synchrotron X-Ray Tomography on Fe-4W alloy after creep tests Synchrotron ring Photons Sample Detector Projection images 33.6 kev ω 3D rendering 3D image with a nanometer (30 and 100 nm) resolution ESRF ID16A-NI nano-imaging beamline Exposure time: 1 s 1800 projections with pixels One measurement renders 3216 slices with pixels 3D rendering and visualization by FEI Avizo 8.1 Voxel size: 100 nm and 30 nm 10

11 (a) 0.25 t R (b) 0.50 t R (c) 0.75 t R (d) t R Side view Front view Top view 100 μm 100 μm 100 μm 100 μm cavity precipitate Creep at 550 o C and 140 MPa 11

12 Identification of cavities in different shapes using complexity (Ω 3 ), elongation (E) and flatness (F) (a) 0.25t R Sphere Equiaxed Rod Sheet Complex 0.2 μm 0.5 μm 1 μm 2 μm 10 μm (b) t R 0.5 μm 1 μm 1 μm 5 μm 10 μm Sphere + Equiaxed: form isolated; Rod + Sheet + Complex: more probably form by linking with neighbors H. Fang et al., Acta Mater. 121 (2016)

13 Movie showing shape classification of cavities 13

14 By shape classification and comparing cavity size to cavity spacing, we can identify isolated and linked cavities. Number density (µm -3 ) 1E-3 1E-4 1E-5 1E-6 Sphere Equiaxed Rod Total Sheet Isolated Complex Linked 1E-7 Total E-3 (a) Number density (c) Volume fraction (b) Number density (d) Volume fraction Linked cavities fulfill two criteria: 1) Shape is rod, sheet or complex 2) Major axes of cavity > λ cavity High number density of non-linked cavities Low volume fraction of non-linked cavities Volume fraction 1E-4 1E-5 Sphere 1E-6 Equiaxed Rod Total Sheet Isolated Complex Linked 1E-7 Total Creep time (h) Creep time (h) Coalescence of cavity increases by one magnitude at failure 14

15 (a) V = μm 3 V = μm (b) 3 (c) (d) FR = FR = V = μm 3 FR = V = μm 3 FR = Top view (e) (f) (g) (h) Front view (i) (j) (k) (l) Side view cavity precipitate Partial filling cavities in S3_140MPa_0.5t R 15

16 (a) V = μm 3 (b) V = μm 3 (c) V = μm 3 (d) FR = FR = FR = V = μm 3 FR = Top view (e) (f) (g) (h) Front view (i) (j) (k) (l) Side view cavity precipitate Partial filling cavities in S1_140MPa_t R 16

17 Filling ratio as a function of particle ( = cavity+precipitate) volume 1.0 (a) 0.25t R (b) 0.50t R 0.8 Filling ratio Isolated Linked Isolated Linked T = 550 o C σ = 140 MPa Resolution limit (c) 0.75t R (d) t R isolated cavities Filling ratio Isolated Linked Isolated Linked linked cavities Particle volume (µm 3 ) Particle volume (µm 3 ) Yellow region: filling ratio continuously increases until fully filled. Blue region: filling ratio increases to some extent and then drops down. 17

18 Filling ratio as a function of particle ( = cavity+precipitate) volume Filling ratio (a) 0.50t R Isolated Linked T = 550 o C σ = 160 MPa 0.2 Filling ratio (b) t R Isolated Linked Particle volume (µm 3 ) 18

19 Conclusions Creep cavities can be filled autonomously by precipitation of supersaturated solute in bcc iron matrix Self healing is most efficient in Fe-Au, but Fe-W provides a promising (and technologically more relevant) alternative for self healing alloys The filling behavior for isolated and linked cavities is distincly different X-ray nanotomography provides a unique insight in the damage formation and healing operation in these alloys 19

20 Aknowledgements TU Delft: PSI: ESRF: MPIE: Nikodem Szymanski, Shasha Zhang, Casper Versteylen, Youp van Goozen, Gijs Langelaan, Frans Tichelaar, Kees Kwakernaak, Hans Brouwer, Wim Sloof Joachim Kohlbrecher Peter Cloetens, Yang Yang Shanoob Balachandran Nair, Margarita Kuzmina, Michael Herbig, Dierk Raabe Funding: Collaboration: 20