Kinetics of aging in an Fe-12Ni-6Mn maraging alloy
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Communications
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60(
o ~: ssc
Kinetics of Aging in an Fe-12Ni-6Mn Maraging Alloy
soc
s
350
Floreen and Decker I have shown that in 18 pct Ni maraging steels, the early stages of hardening may be represented by the empirical relationship: 11o = A H
H, -
25C
I
. . . . . . . .
I
1
10
100
TIME IN HOURS
[1]
(Kt)"
:
300
Fig. 2--Age-hardening curves for alloy.
where H, H0 K n
= = = =
hardness at time t initial hardness constant constant
In the early stages of aging when k t < 1 this reduces to
During this period, hardening is due to coherency strains. There exists various theoretical expressions to describe the hardening due to these strains; 5 in general they have the form:
During investigation of an Fe-12Ni-6Mn maraging alloy 2'3 a similar behavior was found to occur (see Figure 1). This alloy consists of lath martensite in the air cooled condition and shows pronounced hardening on aging in the temperature range 350 ~ to 500 ~ (Figure 2). Hardening is due to precipitation of 0 NiMn on dislocations (Figure 3). The empirical relationship may be explained as follows: One can assume that the reaction kinetics relating fraction transformed, y, to the aging time t is given by the JohnsonMehl equation: 4 y = 1 - exp[-(k/)"]
20
!
Increase in shear stress, AT
where L e R f
[2] Ageing
10
[3]
y = [kt] m
~
= = = =
[4]
= L~qR'ff ~ A H
constant misfit strain radius of particle y = fraction transformed
Time in m i n u t e s
50
100
200
500
I
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I
I
10OO I
400 3OO
425"C n= O. 43
o
400"C n= 0 . 4 6
375"C n =O.50
350"C n= O.51
> 200 "iF O I o
L 1-
I
100
80 6O
c L
.c_
t_
40
30
26
IC
i
O.1
i
i
i
i
i
i
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i
i
i
i
i
1
,
,
, I
_1
,
,
,
,
,
10 Ageing
Time
in
,
,
100
hours
Fig. 1 --Variation of initial hardness increase AH with aging time, t, in an Fe-12Ni-6Mn maraging alloy.
D.R. SQUIRES is Deputy Head, Huntington School, York, England. E. A. WILSON is a Senior Lecturer, Department of Metallurgy, Sheffield City Polytechnic, Pond Street, Sheffield S1 1WB, England. Manuscript submitted October 30, 1983. METALLURGICALTRANSACTIONS A
For one of these expressions, 6 r = 1/2 and s = 1/2; hence we can say: Increase in hardness, AH
~ Ar
~ R ' / 2 y 1/z
[5]
VOLUME 15A, OCTOBER 1984--1947
Finally, the activation energy for aging 129 - 9 kJ/mol, determined from the rate constant k, is in agreement with the activation energy determined from the time to a specific hardness level, 128 --- 17 kJ/mol (Figure 4). This activation energy is rather low for diffusion of substitutional elements in u-Fe, - 2 6 0 kJ/mol, 9 but is in good agreement with the values of 210 to 105 kJ/mol with increasing values of AH reported by Floreen and Decker. 1 In this case, the low activation energy values have been attributed to the high dislocation density giving rise to pipe diffusion, 1 particularly when precipitates as in this case are nucleated on dislocations. Although other reasons hav
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