Cleavage-like fracture along slip planes in Fe-18Cr-3Ni-13Mn-0.37N austenitic stainless steel at liquid helium temperatu
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0.5
Fe-25Mn-5A1RT 5Ni-0.3C 233K 173 K 77 K Fe-30Mn-5A1RT 0.3C-0.1Nb 233 K 173 K 77 K
0.4 .!
j,- i"
0.3
D,- S
~L..~I~ ~ 0.2
77K
~
.
.
173K
RT233 K
N 0.18 0.16 0.17 0.15 0.23 0.23 0.21 0.19
S,c (Pct)*
e.., (Pct)**
18 20 25 41 23 23 24 46
19 21 26 39 19 19 21 43
*Calculated true uniform elongation **Measured true uniform elongation
0.I
o
M 0 0.18 0.33 0.63 0 0 0.14 0.59
o'.l
olz
o13
0'.4
True Strain ( s Fig. 3--Variation of strain hardening exponents with true strain at RT, 233 K, 173 K, and 77 K.
where K is a material constant. The constant M is related to the degree of the strain-induced phase transformation at various temperatures. The value of M at RT was zero because no strain-induced phases were formed. In this case, Eq. [4] reduced to the empirical true stress-strain relation by Hollomon: I4~ o" = K s "
where K = material constant n = strain hardening exponent The above Hollomon's relation is not applicable when the strain hardening exponent varies with strain as in the cases tested at 233 K, 173 K, and 77 K in this work. The slopes of the lines in Figure 3 increased with the decreased test temperature as a result of the increased amounts of the strain-induced phases during the tests. The flow behavior of this alloy at the lower temperatures can be well described by Eq. [4]. In order to obtain high fracture toughness and good formability, it is desirable to maximize the extent of uniform elongation prior to the onset of localized necking. By assuming that necking starts at the maximum engineering stress (UTS), the following necking criterion is obtained for the cases in which the strain-induced phase is forming: N e, - - 1-M
[5]
where e, = the amount of true uniform elongation; N, M were defined in Eq. [4]. The extent of true uniform elongations at lower temperatures can be calculated from Eq. [5] after determining the values of M and N. In Figure 3, the strain hardening exponents varied about linearly with the strain. The values of M and N could be determined by the least square method assuming the linear relationship between strain hardening exponents and true strain. The calculated and measured values of true uniform elongations at various temperatures for this alloy are summarized in Table I. The results for the previ1626--VOLUME 19A, JUNE 1988
ously reported cryogenic Fe-30Mn-5A1-0.3C-0.1Nb steel [a] are also included in Table I. Table I shows that the calculated values of the true uniform elongation are in good agreement with the measured values at various temperatures in these high-Mn cryogenic alloys. It is proposed that Eqs. [4] and [5] are the generalized true stress-plastic strain relation and its necking criterion for metallic materials which have either a constant n or varying n with strain.
This work was supported by Korea Science and Engineering Foundation (KOSEF), Ministry of Science and Technology, Republic of Korea. The authors express thanks to Dr. Jae. K. Han of NBS for discussions on this paper.
REFERENCES 1. Y.G. Kim, Y. S. Park, and J. K. Han: Metall. Tran
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