SOLID SOLUTION METAL ALLOYS

Transcription

1 SOLID SOLUTION METAL ALLOYS Synergy Effects vs. Segregation Phenomena D. Manova, J. Lutz, S. Mändl, H. Neumann 1

2 Table of Content Motivation Alloys vs. Pure Elements or Intermetallic Compounds Introduction Diffusion & Phase Formation During Ion Nitriding of Bulk Alloys Experimental Results Nitriding of Stainless Steel Thin Films Discussion Phase Stability in the System Fe-Cr(-Ni)-N Outlook 2

3 Motivation Number of valence electrons determines physical properties: hardness, cohesion energy, melting point Similar observance for steel alloys, however additional prefactor instead of pure additive behaviour: pitting Resistance Equivalent number PRE = %Cr %Mo + 16 %N (for 1.5 at.%) Attention: Additional metallurgical expertise relies on precipitate engineering Nitrogen insertion into steel and CoCr alloys: simple factor analysis sufficient to explain effects or additional synergies between alloying elements and nitrogen? J. Häglund. Grimvall, T. Jarlborg, A. Fernández-Guillermet, Phys. Rev. B 43 (1991), 144 3

4 Motivation Alloy composition (in wt.%): Co Ni Cr W Fe Mo CoCr alloys L bal HS bal MP35N bal. 5 SY21 66 bal. 5 Stainless Steel bal. 316Ti bal. 2 Austenitic fcc structure Large grain size of 1 25 µm Low carbon alloys: very small amount of TiC/WC precipitates 4

5 Introduction Stainless steel (34) CoCr (HS188) Intensity [a.u.] (111) (2) (22) (311) (222) fcc-fe fcc-crn Angle 2θ [ ] 58 C 555 C 435 C 37 C 3 C 23 C untreated Intensity [a.u.] (111) (2). (22) (311) (222) fcc-cocr fcc-crn Angle 2θ [ ] Well known facts: Base materials: fcc-phase Nitrogen insertion: expanded fcc-phase at lower temperatures Development of CrN precipitates at higher temperatures Standard assumption: no CrN no corrosion, CrN corrosion 58 C 555 C 435 C 37 C 3 C 23 C untreated 5

6 Introduction 35 3 Layer Thickness (nm) PIII LEI 32 C 4 C v s (1-8 cm/s) D (1-12 cm 2 /s) New: detailed investigation of diffusion at higher temperatures For 58 C: CoCr: slower than t 1/2 after 3 minutes, SS: faster than t 1/2 after 1.5 hours; delay corresponding to surface oxide must be corrected For 3 4 C: perfect agreement when sputter removal is included Temperature dependence of (i) sputter yield or (ii) diffusivity? Time (min) Layer Thickness (µm) Layer Thickness (µm) PIII: 58 C CoCr HS188 SS Time 1/2 (min 1/2 ) PIII, CoCr HS188: High temperature - 58 C Low temperature - 45 C Time 1/2 (min 1/2 ) 6

7 Introduction Hypothesis: decomposition of base material is function of temperature and time + diffusivity depends strongly on phase composition Complete decomposition: ~ 3% CrN + 7% Fe-Ni-alloy (ferrite or austenite?) ~ 2% CrN + 8% Co-Ni-Mo-alloy (fcc or hcp?) Percolation theory: Diffusion only in interconnected FeNi and CoNiMo-system No major diffusion in isolated CrN-clusters; blocking of diffusion pathway by isolated clusters not enough to explain data for CoCr; wrong sign of effect for SS 7

8 Experiment in-situ spectral ellipsometer RF plasma source 4.68 MHz gas supply PIII Experiment Substrates: stainless steel thin films deposited on Si QMS pumping system sample high voltage pulse generator Thin Film Deposition by Ion Beam Sputtering (IBS) Pre-Heating: external heating up to temperature of 35 C or 45 C PIII: 1 kv high voltage pulses, 15 µs pulse length, 1 min. process time, no additional heating during process, temperature maintained only by ion heating Sputtering: Ar + ions, energy 1 kev, Target: stainless steel 34 Substrate: Si (1) Film thickness: 5 nm. Analysis: elemental depth distribution, phase formation, SIMS, XRD, TEM 8

9 Experiment Nitrogen Concentration (at.%) nm Layer Thickness (PIII, 45 C) Depth (nm) 45 C, PIII 35 C, PIII non implanted (PIII, 35 C) as deposited Thermally activated diffusion of nitrogen in steel thin films Nitriding of complete layer at 45 C At 35 C, nitrided zone of 75 nm, additionally diffusion tail with 2 4 at.% nitrogen Column width of about 7 nm corresponds to an effective grain boundary area of 5% Faster diffusion along grain boundaries 9

10 Experiment 35 C: CrN precipitation along grain boundaries Below surface region, undisturbed ferritic structure CrN precipitates of 4 9 nm diameter with characteristic distance of 2.39 Å between {111} CrN planes visible along grain boundaries 5 nm 5 nm D. Manova, T. Höche, S. Mändl, H. Neumann, Nucl. Instrum. Meth. B (29), doi:1.116/j.nimb

11 Experiment 45 C: CrN precipitation inside grains upper part lower part 2 nm Si substrate artificial surface from TEM preparation 5 nm 5 nm D. Manova, T. Höche, S. Mändl, H. Neumann, Nucl. Instrum. Meth. B (29), doi:1.116/j.nimb Layered structure with expanded ferritic grains in lower part and decomposed structure in upper part Demarcation line 7 nm from interface Lower 7 nm: CrN precipitates only at grain boundaries Upper 33 nm: CrN inside original grains 11

12 Transformation Time (s) Discussion h SS + Nitrogen SS + Carbon E a ~ 1.5 ev Thin Film Temperature (K) 1, 1,1 1,2 1,3 1,4 1,5 1,6 1/T (1 3 /K) T. Bell, Key Eng. Mater. 373/374 (28) h Formation of CrN (respective CrC x ) shows thermal activation E a corresponding to atomic diffusion or reorganisation processes Value for thin films much lower Influence of grain size or voids encountered during deposition? 12

13 Phase Formation ~ 5 at.% T. Christiansen, M.A.J. Somers, Metall. Mater. Trans. 37A (26) 675 M. Hättestrand, P. Larsson, G. Chai, J.-O. Nilsson, J. Odqvist, Mater. Sci. Eng. A 499 (29) 489. Decomposition of expanded austenite into CrN + ferrite follows from thermodynamic stability, however always remaining austenite phase (depending on temperature and alloy composition) Miscibility gap between Fe and Cr (α + α ) should promote this effect with nucleation pathway at present Cr concentrations 13

14 Outlook 35 3 Layer Thickness (µm) C CoCr HS188 SS !? Time 1/2 (min 1/2 ) Increase in nitrogen diffusivity after austenite-ferrite transformation in accordance with literature No literature data for CoMo or CoNi alloys: comparison with pure elements necessary D.L. Williamson, O. Öztürk, R. Wei, P.J. Wilbur Surf. Coat. Technol. 65 (1994)

15 N Conc. (at.%) N Conc. (at.%) Outlook Depth (nm) Co Time (s) 5 kv, 33 C, 1 h 1 kv, 45 C, 4 min. 3 kv, 58 C, 1.25 h 15 kv, 65 C, 1 h 15 kv, 7 C, 3 min. Mo Depth (1 17 cm -2 ) N conc. (a.u.) 3 C 35 C 4 C 45 C N Conc. (at.%) Depth (nm) W 3 C 35 C 4 C 45 C Time (s) 2 Ni 37 C 415 C 52 C 6 C 66 C N Conc. (at.%) Depth (nm) Cr Depth (nm) 3 C 4 C 42 C 435 C Co (hcp): metastable, Co x N, decomposition > 35 C Ni (fcc): meta-stable Ni 4 N, decom-position > 415 C Mo: slow diffusion W: no diffusion Cr: normal diffusion expectation Co+Cr: normal diff. Co-Cr: no diff. Why do we have accelerated diffusion in CoCr alloys at all? 15

16 Acknowledgments Jürgen Gerlach Thomas Höche 16

17 Fe-Cr Phase Diagram 17