younes sina, high temperature deformation ,creep, az91
DESCRIPTION
Younes Sina's presentation for Professor T.G.Nieh's class, High Temperature Deformation, Creep , AZ91 Alloy, Coble creep mechanismTRANSCRIPT
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