Athermal Solid Solution Hardening in Tantalum
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I.
INTRODUCTION
R E C E N T L Y a new athermal solid solution hardening (SSH) mechanism has been proposed by the authors. 1 This mechanism is specific for bcc metals at temperatures where the magnitude of the operating fraction of the Peierls stress is still important. The threefold symmetry and especially the polarity of (111) screw dislocations in bcc metals is qualitatively taken into account in the proposed model. The athermal SSH Aty~ is suggested to be of the form A cG = K e 2 C
[1]
with C the solute atomic fraction, K a constant, and ea the atomic size misfit parameter ea = ( l / a ) (Aa/AC)
[21
The linear athermal SSH dominated by the atomic size misfit effect (Eq. [1]) is explained in terms of interactions between, on the one hand, long-range hydrostatic stress fields around groups of solute atoms having a different atomic size and, on the other hand, dilatation and compression fields associated with constrictions and kinks in straight (111) screw dislocations. This model has been supported in the same paper by the results of Vickers hardness measurements on substitutional binary and ternary Ta-base alloys, the alloying elements being Re, Mo, W, Hf, Zr, and Nb. However, it has been suggested that hardness measurements at room temperature are not indicative for athermal solid solution hardening in tantalum, because of the influence of thermal activation. 2 Therefore, the effect of these substitutional solutes on the athermal component of the flow stress of tantalum has been investigated and is described below. A comparison is made between SSH studied by means of hardness measurements on the one hand and dip-tests (incremental unload technique3) on the other hand. The results are compared with the above described binary SSH model and with the authors' phenomenological model for multicomponent SSH. 4
L.A. GYPEN, formerly with the Departement Metaalkunde, Katholieke Universiteit Leuven, Belgium, is now with the Departemen Mesin, Institute of Technology Bandung, Bandung, Indonesia. A. DERUYTTERE is Professor of Metallurgy, Departement Metaalkunde, Katholieke Universiteit Leuven, Leuven, Belgium. Manuscript submitted July 8, 1981. METALLURGICAL TRANSACTIONS A
The latter model starts from the assumption that the binary athermal SSH Ao-~, is given by an equation of the form Ao'~, = R , C q
(i = 1 . . . v )
[3]
where Ci is the solute atomic fraction, R, the strengthening factor of solute i, and q the concentration exponent. Eq. [3] is in agreement with the generally accepted athermal solid solution theories mentioned, e . g . , in References 1 and 5. The total athermal SSH, when v dilute solute distributions are simultaneously present in the base metal without interacting with each other, is, according to the above mentioned model for multicomponent SSH, given by: morl.~ = (~,Ao"p.ll/q)q
[4]
This model has been extended to both extrinsic 4 and intrinsic6 solid solution softening and has been shown to be in agreement with many experimental data on both fcc and bcc solid solutions. 1,6,7 II.
EXPERIMENTA
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