Performing and evaluating of creep tests

Transcription

1 Performing and evaluating of creep tests W. Blum 2, J. Dvorak 1, P. Kral 1, P. Eisenlohr 3, V. Sklenicka 1 1 CEITEC IPM, Institute of Physics of Materials, Academy of Sciences of the Czech Republic, CZ Brno, Czech Republic 2 Inst. f. Werkstoffwissenschaften, University of Erlangen-Nurnberg, Martensstr. 5,D-91058,Erlangen, Germany 3 Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824, USA 13th International Conference on Creep and Fracture of Engineering Materials and Structures May 31 June 4, 2015, Toulouse, France

2 Introduction

3 ε 0.30 pure Al ([2] is common Determination of noise free creep rate with 0.20 tion of time SmooMuDS smooth. Ho 0.10 disadvantag (smoothing of multi-parameter data series) able to char 0.00 crostructure quence of di t / s with strain 3 Examples 10-2 with time t. occur at wi 3.1 Tensile creep of Aluminum time windo Figure 2(a) shows the result of a tensile this hinders 10-4 creep test at constant load performed on di erent tes pure Al ([2], available at ResearchGate). It tageous to p is common to display the strain as function of time t ( creep curve ). The curve is ing stress expressed b smooth. However, the -t curves have two problems of disadvantages. First, t is not a good variable to characterize the evolution of the mi- In order widely di e crostructure 10-8 that is proceeding as a consequence of dislocation being directly related the -t curv t / s ε ing this in with strain inel, and only indirectly via termining with time70 t. Second, tests at di erent loads measured creep curve without/with noise points leads occur at widely di ering rates so that the reduction Fig. 2(b). time window 50 for each test di ers greatly; large. This Eisenlohr P (2006) SmooMuDS: flexible this hinders smoothing direct graphical of multivalued comparison data of series: di erent tests. It is therefore most advantageous to plot the deformation resistance, during the resulting fro 30 t ε ε / s -1

4 First topic Step loading test (full information about deformation proces) Example: compression creep of CG Cu at constant stress

5 Stepwise compressive loading of CG Cu at 573 K ε total / s = σ tech / MPa t/s CREEP RATE ε. [s -1 ] MPa 133 MPa STRAIN ε strain-time curves evaluated for rate = 10-4 s -1

6 Resulting stress-strain curve of CG Cu at 573 K Evolution of strength with strain at contsant rate in compression and tension

7 Second topic Deformation at constant load or force Case 1 first: fracture after quasistationarity Example: creep of mc Cu at constant load => stress increases during uniform tension as σ=σ 0.exp(ε)

8 Creep rate-stress curves of microcrystalline Cu at 473 K 10-2 σ / MPa Cu tension F = cst 10-4 Case 1 tensile fracture after quasi-stationarity 10-8 strain scale! ε = = σ / G Tension (8 ECAP passes)

9 Creep rate-stress curves of microcrystalline Cu at 473 K 10-2 σ / MPa 8Cu quasi-stationary 8Cu tension F = cst tensile fracture after quasi-stationarity + quasi-stationary 10-8 ε = 0.5 ε = σ / G

10 Creep rate-stress curves of microcrystalline Cu at 473 K 10-2 ε ε min σ / MPa 8Cu quasi-stationary 8Cu tension cst F 8Cu compression tensile fracture after quasi-stationarity + compression data (8 ECAP passes) 10-8 ε = σ / G

11 Second topic Deformation at constant load or force Case 2 now: fracture before quasi-stationarity Example: creep of CG Cu at constant load => stress increases during uniform tension as σ=σ 0.exp(ε)

12 Creep rate-stress curves of CG Cu at 573 K σ / 573 K ε = tensile fracture before 10-4 quasi-stationarity CG Cu 573 K tension CG vs. mc state: - no merging of curves - ductility goes down σ / G

13 Creep rate-stress curves of CG Cu at 573 K σ / 573 K ε = σ 0.2% tensile fracture before 10-4 quasi-stationarity CG Cu 573 K tension +yield stress σ 0.2% => Creep before/above yield stress (Blum et al. Evidence of change in strength control in progress) σ / G

14 Creep rate-stress curves of CG Cu at 573 K σ / 573 K ε = 0.5 σ0.2% 0.2% tensile fracture before 10-4 quasi-stationarity 10-8 power law n = 9 Qc=89 kj/mol Qc=152 kj/mol (Raj and Langdon) 9 CG Cu 573 K tension Explanation? σ / G

15 Creep rate-stress curves of CG Cu at 573 K σ / 573 K ε = 0.5 σ 0.2% 0.2% tensile fracture before quasi-stationarity CG Cu 573 K tension σ / G Qc= 89 kj/mol (initial value) Q c =180 kj/mol (this work after fitting) Qc=152 kj/mol (Raj and Langdon)

16 Creep rate-stress curves of CG Cu at 573 K σ / 573 K ε = % tensile fracture before 10-4 quasi-stationarity Cu-P tension [1] (better ductility) 9 CG Cu 573 K tension from 673 K Cu-P σ / G Cu-P tension: [1] Sandström R, Andersson HC The effect of phosphorus on creep in copper, J Nucl Mater., 372,(2008), 66 75

17 Creep rate-stress curves of CG Cu at 573 K σ / 573 K ε = % tensile fracture before quasi-stationarity 10-4 explanation of minimal 10-8 creep rate + compression (no fracture influence) CG Cu 573 K compression tension from 673 K Cu-P => good agreement with Cu-P σ / G

18 Creep rate-stress curves of CG Cu at 573 K σ / 573 K ε = % qs tensile fracture before quasi-stationarity 10-4 explanation of minimal creep rate quasi-stationary fit with sinh law with fixed misorientation of strain induced lab CG Cu 573 K compression tension from 673 K Cu-P σ / G

19 Creep rate-stress curves of CG Cu at 573 K σ / 573 K ε = % qs tensile fracture before quasi-stationarity 10-4 explanation of minimal creep rate θ= ( σ/mpa) CG Cu 573 K compression tension explanation of σ 9 law in terms of misorientation (quasi-stationary curve with variable misorientation) from 673 K Cu-P => new way of understanding power laws σ / G Blum et al., A nine power law of creep and its interpretation in terms of deformation and fracture-in progress

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21 Creep rate as structure sensor: DRX Cu (573 K/50 MPa) 10-4 σ / MPa pcu 573 K ε tot ECAP: - change of strength - mc grains (DRX) - HAGB - high ductility 0 = CG Cu 1-8 = N ECAP passes

22 Creep rate as structure sensor: DRX (573 K/50 MPa) σ / MPa Cu higher ductility 10-7 pcu 573 K ε tot

23 Creep rate as structure sensor: DRX (573 K/50 MPa) σ / MPa 2=p Cu - further enhanced ductility - indication of inflection point 10-7 pcu 573 K ε tot

24 Creep rate as structure sensor: DRX (573 K/50 MPa) σ / MPa 2=p Cu - two relative minima 10-7 pcu 573 K ε tot

25 Creep rate as structure sensor: DRX (573 K/50 MPa) σ / MPa 2=p /12Cu -highest ductility -two minima 10-7 pcu 573 K ε tot

26 Creep rate as structure sensor: DRX (573 K/50 MPa) σ / MPa microcrystalline 8Cu recrystallizes 10-5 DRX 8 8Cu, T=573K, σ=50 MPa pcu 573 K ε tot DRX Hardening from 1. to 2: -grain coarsening (DRX) -transition from hab to lab control of deformation

27 Creep rate as structure sensor: DRX (573 K) DRX DRX in situ test ε tot = µm x 44 µm

28 Creep rate as structure sensor: DRX (573 K) DRX DRX ε tot = µm x 44 µm

29 Creep rate as structure sensor: DRX (573 K) DRX DRX ε tot = µm x 44 µm

30 Creep rate as structure sensor: DRX (573 K) DRX DRX ε tot = µm x 44 µm

31 final parts: STRESS REDUCTIONS TESTS small stress reductions => activation volume for glide large stress reductions => forward recovery flow in hard regions example: Cu-Zr (673 K)

32 Stress reduction tests in conventional creep machines instantaneous response to perturbation = R = σ/σ0 total strain σ0 = 272 MPa K 0.15 microcrystalline Cu-Zr 0.18 => nonlinear elastic behavior E eff (σ) (machine + specimen) for elimination of elastic strains and stress-strain rate relation at constant structure in small strain interval (strain is total strain, inelastic strain is nearly constant) time / s 1600

33 stress reduction tests in conventional creep machines σ0 = 272 MPa instantaneous response to perturbation 0.98 = R = σ/σ nonlinear elastic behavior E eff (σ) (machine + specimen) 0.67 total strain E eff / MPa E eff = σ/ ε K 0.15 microcrystalline Cu-Zr time / s σ / MPa

34 stress reduction tests in conventional creep machines instantaneous response to perturbation σ0 = 272 MPa 0.98 = R = σ/σ instantaneous rates, linear plot + total strain K 0.15 microcrystalline Cu-Zr time / s σ / MPa

35 stress reduction tests in conventional creep machines instantaneous response to perturbation = R = σ/σ instantaneous rates, log plot σ0 = 272 MPa total strain K 0.15 microcrystalline Cu-Zr time / s σ / MPa

36 stress reduction tests in conventional creep machines instantaneous response to perturbation activation volume V 10-4 thermally activated glide in material with average dislocation spacing bg/σ: 10-5 Vσ / Gb 3 1 here: = σ / MPa

37 stress reduction tests in conventional creep machines instantaneous response to perturbation recovery strain, first backward,... forward then 272 MPa ε eps inel ε eps inel 54 MPa t [s] t / s t / s t [s]

38 stress reduction tests in conventional creep machines instantaneous response to perturbation

39 stress reduction tests in conventional creep machines instantaneous response to perturbation recovery strain rate σ / MPa Dashed line: well known forward recovery strain rate line for many pure materials from single crystal (LiF) to nanocrystal (nc Ni)

40 stress reduction tests in conventional creep machines 10-4 instantaneous response to perturbation dotted: quasi-stationary line if n = quasi-stationary n = composite model: internal back stresses in soft regions (grains) forward recovery flow in hard regions (boundary) σ / MPa

41 Summary Creep machines are optimal to investigate all aspects of inelastic deformation as function of stress e.g.: stress-strain curves at constant inelastic strain rate strain rate-strain curves at constant stress/force monitoring of structure changes (DRX, structural coarsening) kinetics of work hardening strain (activation volume,..) kinetics of recovery strain (back flow in the soft and forward flow in hard regions due to dislocations and boundaries)

42 Thank you very much for your attention IPM AS CR, v.v.i. Creep laboratory CEITEC creep laboratory