This document discusses Coble creep in an Mg–9Al–1Zn alloy with an ultrafine-grained microstructure produced via high-ratio differential speed rolling. It summarizes the deformation mechanisms of Mg and Mg alloys including dislocation slip, twinning, and grain boundary sliding. It presents experimental data on the strain rate and temperature dependence of creep in the alloy and compares the results to predictive creep models including Coble, grain boundary sliding, and dislocation climb creep models.

1. On Coble creep in Mg–9Al–1Zn alloy with ultrafine-grained microstructure W.J. Kim December 2007 Professor T.G.Nieh The University of Tennessee, Knoxville Younes Sina

3. E - Rare earths

4. H - Thorium

5. K - Zirconium

6. M - Manganese

7. Q - Silver

8. T - Tin

9. Z - Zinc

10. F - As fabricated

11. O - Annealed

12. H10 and H11 - slightly strain hard

13. H23, H24, H26 - strain hardened and partially annealed

14. T4 - artificially aged

15. T6 - Solution treat, aged

17. Deformation mechanism of Mg, Cu, Mg alloys Nano grain= 1-100 nm Ultra fine=100-500 nm

18. DMM for pure Mg Correct value D0=7.79*10-3 m2s-1 D0=5*10-12 m2s-1

21. process concept for forward extrusion of magnesium alloys, prior (left) and during (right).

22. Twist hydro-extrusion ECAP : 2-turn square pass ECAP : 3-turn square pass ECAP : 3-turn round Hydro-Mechanical Extrusion

23. Recovery, Rex. & Grain growth during rolling SPD

24. SEM images on (a) the weakly deformed front part of the sample after four passes and (b) the severely strained internal part of the specimen after eight asses. TEM micrograph of the HRDSRed AZ91 L=0.8 , d Ex=6.2 μ & d HRDSR=1.74μ High density dislocation L=3.6 for β-Mg17Al12 Many precipitate (0.1-0.5 μ) d=1.4L

25. Hot-extruded AZ91 alloy with an initial grain size of 1.7 m.

26. α-Mg β-Mg17Al12

27. Prestrain A true stress–true strain curve of the HRDSRed AZ91 from the SRC test at 553 K. |T=[(ln (έ2/έ1)] / [ln (σss2/σss1)] n=

28. . ss= C n exp (- Q diffusion/ RT) Temperature dependence Stress dependence Constants

29. T 493 553 473 553 493 523 ? 573 The SRC results as double-log plots of strain rate vs. stress at different temperatures for the HRDSRed AZ91 and the EX AZ91 n=d ln έ /d ln σ

30. ? Grain growth @ 573, Large decrease in volume fraction of β-Mg17Al12 .α 1/d2 and 3

31. Grain coarsening

32. σEX=σHRDSR σEX<σHRDSR

33. 493 523 553 493 553 573 Strain rate compensated by Dgb vs. /E for the HRDSRed AZ91 and the EX AZ91. D0=7.79*10-3 m2s-1 Qgb=92 kJmol-1 Dgb=D pipe=D0 exp (-Qgb/k T)

35. Dynamic recovery occurs during forming, and is followed by static recovery subsequent to forming b. The grains recrystallize after deformation c. Dynamic recrystallization (recrystallization during deformation) takes place and is followed by static recrystallization

36. N=1 N=7 EX AZ91 N=1 HRDSR AZ91 N=2 N=2 Deformation mechanism maps for Mg alloys at several temperatures in the range 300–573 K

38. HRDSRed AZ91 GBS Coble Creep Dislocation climb creep Comparison between the experimental data of the HRDSRed AZ91 and the values predicted by the three creep models at 553 K

39. Coble GBS D p-slip creep(Dislocation Climb Creep) d=3.5μ

40. έ=10-8 s-1 έ=10-6 s-1 έ=10-4 s-1 έ=10-8 s-1 Mg έ=10-6 s-1 έ=10-4 s-1 Cu Critical grain size for copper is significantly smaller

41. Future works: Comparison with ECAP and other SPD methods (microstructure developed after SPD depends on #of passes) Evaluation of grain homogenousity (check if there is a significant growth during deformation) Measuring of dislocation density The same research for other Mg alloys

42. Thank you Kurdistan, Iran