Characterizing Deformation Mechanisms in Ni 3 Ge-Fe 3 Ge Intermetallic Alloys

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nisms operate by impeding dislocation motion, another approach may also be used, namely altering the structure of the dislocation itself, rather than the matrix through which it must move. Slight changes in the dislocation core can have a powerful effect on the yield strength, by intrinsically impeding the motion of the dislocation. In order to characterize these mechanisms, the relationship between chemistry, structure and properties must be well understood. In intermetallic systems, the anomalous increase of yield strength with increasing temperature is known to depend on the core geometry of the dissociated superdislocation and may be expected to be highly dependent on alloy chemistry [2]. The Ni3Ge-Fe3 Ge system has been chosen to study the effect of alloy chemistry on the mechanical behavior of LI 2 intermetallics. Although the majority of L1 2 intermetallics exhibit the YSA, there are several examples of alloys that show a normal decrease of yield strength with increasing temperature, e.g. Pt 3Al [3]. The pseudo-binary system (NixFe1 .l)3Ge, in which Ge content is held constant at 25 at%, lends itself as a model system for this investigation because it exhibits complete solid solubility as Fe is substituted for Ni across the composition range. Ni3Ge, Fe3Ge and all intermediate alloys have the L 12 crystal structure, but Ni3Ge exhibits the YSA [4, 5], whereas Fe3Ge shows more normal behavior [5, 6]. The change in mechanical behavior is believed to result from a transition in the defor-

mation mechanisms. The dissociation of a superdislocation in an L12 intermetallic is ultimately responsible for the mechanical behavior. Because the L1 2 structure is ordered, the Burgers vector of a superdislocation is

a< 110>, with twice the magnitude of an a/2< 110> ordinary dislocation in a face-centered cubic metal. The high energy associated with a superdislocation may be reduced by dissociation into two a/2< 110> superpartials that bound an anti-phase boundary (APB). Further dissociation of each superpartial may occur, resulting in a pair of Shockley partial dislocations that bound a complex stacking fault (CSF). KK10.8.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society

In Ni3Ge-Fe3 Ge, variations in these planar fault energies, which modify the core structure of the superdislocation, can have a profound effect on mobility. For example, changes in APB energy originally noted by Flinn [7], and subsequently modified to include the effects of anisotropic elasticity [8, 9], provide an energetic driving force for cross-slip locking by the formation of Kear-Wilsdorf (KW) locks [10]. Moreover, alloying additions of boron in Ni3A1 have been shown to increase the CSF energy and consequently increase the yield strength through enhanced cross-slip locking [11]. The current study is part of a collaborative effort to compare experimental measurements and theoretical predictions of fault energies. EXPERIMENTAL PROCEDURES

In order to span the composition range between Ni 3Ge and Fe3Ge, the following 6 alloys we