On Coble creep in Mg–9Al–1Zn alloy with ultrafine-grained

microstructure

W.J. Kim

December 2007

Younes SinaThe University of Tennessee, Knoxville Professor T.G.Nieh

Magnesium alloy designations

AZ91C-T6

2 digits

Quantity of principal alloys

9 = 9% Al

1 = 1% Zn

1 letter

Distinguishing code

C = Third alloy of this type

2 letters

Principal 2 alloy elements

A = aluminum

Z = zinc

Temper designation

-T6 = solution treated and artificially aged

• A - Aluminum• E - Rare earths• H - Thorium• K - Zirconium• M - Manganese• Q - Silver• T - Tin• Z - Zinc

• F - As fabricated• O - Annealed• H10 and H11 - slightly strain hard• H23, H24, H26 - strain hardened and partially

annealed• T4 - artificially aged• T6 - Solution treat, aged• T8 - Solution treat, cold work, aged

Oil Pan housing

Oil Pan- Honda

Key Lock housing

Door- Lupo

Radiator support

Magnesium alloys are the lightest metallic structural materials and are, hence, very attractive in applications such as automotive, railway and

aerospace industries.

Material Method of grain size reduction

Grain size nStress

exponent

T(K) Q

Nano Mg-5%Al Mechanical alloying

45 nm 1.5 283-323

Pure Mg Mechanical milling

20 nm 1.65 300

Pure Cu <100 1 Qc=Q g b

Ultra fine AZ91 ECAP 0.7 μ 2(GBS)1(Diff.)

423-523 Qc=Q gbQc=Q g b

Pure Mg >100 nm 2(GBS) Qc=Q g b

Pure Cu >100 nm

Nano grain= 1-100 nmUltra fine=100-500 nm

Deformation mechanism of Mg, Cu, Mg alloys

Correct valueD0=7.79*10-3 m2s-1

D0=5*10-12 m2s-1

DMM for pure Mg

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

Hydro-Mechanical Extrusion

Twist hydro-extrusionECAP : 2-turn square pass ECAP : 3-turn round ECAP : 3-turn square pass

Recovery, Rex. & Grain growth during rolling

SPD

SEM images on (a) the weakly deformed front part of the sampleafter 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=1.74μHigh density dislocationL=3.6 for β-Mg17Al12 Many precipitate (0.1-0.5 μ)

d=1.4L

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

α-Mg

β-Mg17Al12

A true stress–true strain curve of the HRDSRed AZ91 from the SRC test at 553 K.

Prestrain

n= |T =[(ln (έ2/έ1)] / [ln (σss2/σss1)]

ss = C n exp (- Q diffusion / RT)

Constants

Stress dependence

Temperature dependence

.

The SRC results as double-log plots of strain rate vs. stress at different temperatures for the HRDSRed AZ91 and the EX AZ91

493

573 523

553

553

473

493

T

?

n=d ln έ /d ln σ

? Grain growth @ 573, Large decrease in volume fraction of β-Mg17Al12

.α 1/d2 and 3

T(K) D (micron)

493-523 2.1-2.5

553 3.5

573 5.7

Grain coarsening

σ EX=σ HRDSR σ EX<σ HRDSR

Strain rate compensated by Dgb vs. /E for the HRDSRed AZ91 and the EX AZ91.

Dgb=D pipe=D0 exp (-Qgb/k T) D0=7.79*10-3 m2s-1

Qgb=92 kJmol-1

553

553573

493

493

523

a. Dynamic recovery occurs during forming, and is followed by static recovery subsequent to forming

b. The grains recrystallize after deformationc. Dynamic recrystallization (recrystallization during deformation) takes place and is

followed by static recrystallization

Deformation mechanism maps for Mg alloys at several temperatures in the range 300–573 K

HRDSR AZ91

EX AZ91

N=1

N=1

N=7

N=2N=2

Comparison between the experimental data of the HRDSRed AZ91 and the values predicted by the three creep models at 553 K

GBS

Coble Creep

Dislocation climb creep

HRDSRed AZ91

Coble

GBSD p-slip creep(Dislocation Climb Creep)

d=3.5μ

έ=10-8 s-1

έ=10-6 s-1

έ=10-4 s-1

έ=10-4 s-1

έ=10-6 s-1

έ=10-8 s-1

Mg

Cu

Critical grain size for copper is significantly smaller

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 densityThe same research for other Mg alloys

Thank you

Kurdistan, Iran