On the mechanism of cross slip in Ni 3 AI
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I.
INTRODUCTION
M A N Y LI2 compounds exhibit a unique combination of mechanical properties. A representative, commercially important L12 material is Ni3A1, which exhibits the following unusual properties at temperatures below about 700 ~ (1) the strength increases with increasing temperature; (2) the critical resolved shear stress (CRSS) on the {111} (1 I0) slip system is orientation-dependent; and (3) the CRSS in tension is different from the CRSS in compression. These trends have been quantitatively described using models based on the phenomenon of cube cross slip. The objectives of this paper are as follows: (1) to examine cyclic CRSS data of L12 alloys and high-~/ volume fraction (VI) superalloys; (2) to determine the implications of these data on the interpretation of the cross-slip event; and (3) to use these implications as a critical test of the available cross-slip models.
the CRSS of Ni3Ga crystals. They proposed that the crossslipped segments acted as pinning points along the dislocation line, thus strengthening the material. They showed that the rate of cross slip at constant temperature was a function of the shear stress resolved on the (010) plane in the direction of the Burgers vector. For crystals whose tensile axes are within the standard [001]-[011]-[111] stereographic triangle, this stress is T~o~o)tTo~].Since the rate of cube cross slip was also assumed to be thermally activated, the CRSS was described by an Arrhenius-type
equation :[31 ~-y = A exp / where ~'s = A = AH = V -r,.r = k = T = The TK
II.
CRSS M O D E L S B A S E D O N C R O S S SLIP
In the L12 structure, the antiphase boundary (APB) created by the glide of an a/2(110) dislocation has an antiphase boundary energy (APBE) which depends on the slip plane; the APB on {001} is lower than the APB on {111}. In Thus, screw dislocations gliding on {111} can reduce their energy by cross slipping to {001}. Kear and Wilsdorf t21 first proposed that this cross-slip event was responsible for the high degree of work hardening which they observed in ordered C u 3 A u . Takeuchi and Kuromoto t31 (TK) developed a quantitative model which described the temperature and orientation dependence of
WALTER W. MILLIGAN, formerly Graduate Student with the Mechanical Properties Research Laboratory, School of Materials Engineering, Georgia Institute of Technology, is Assistant Professor of Metallurgical Engineering with Michigan Technological University, Houghton, Michigan 49931. STEPHEN D. ANTOLOVICH, Director, Mechanical Properties Research Laboratory, and Director and Professor, School of Materials Engineering, is with the Georgia Institute of Technology, Atlanta, Georgia 30332-0245. Manuscript submitted March 24, 1989. METALLURGICAL TRANSACTIONS A
3kT
[11
CRSS; constant; activation energy for cross slip; activation volume; cross-slip stress, T(010)[i01]; Boltzmann constant; and absolute temperature.
model was
extended by
Pope and co-
w o r k e r s . 14,5,61 They showed that the CRSS of Ni3A1 al-
loys was strongly affected by the shear stress
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