High resolution study of ordering reactions in gold-chromium alloys
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The ordering reactions in Au4Cr and Au3Cr alloys have been investigated using high r e s o lution dark field and lattice imaging techniques. In all cases the ordered structures can be described in t e r m s of compositional modulations as occur during spinodal decomposition. In alloys quenched from above the critical temperature sro microdomains of size 10 to 154 have been resolved with structures represented by modulations in composition along (420). After aging the Au4Cr alloy below T c the D l a lro phase appears as ordered domains of size 50 to 100A showing composition modulations with wavelength of 5d(531). These domains grow in size up to 500A becoming finally the Dla fully ordered structure. A new observation has been mdde in the AusCr alloy, v i z that the lro structure is nonstoichiometric L1 o with Xtoo~l = 4/~. The transformation from sro in this alloy occurs with the formation of a transitional ordered phase, (similar to D02~), which can be described in terms of composition modulations along [001], with wavelength equal to the c parameter (8/k) of the D022 structure. Lattice imaging of the two variants of superlattice planes in ordered Au4Cr enabled the atomic arrangement in the area of translational and rotational antiphase boundaries to be determined. G o L D - c h r o m i u m alloys show a wide range of solid solubility of chromium in gold, v i z . up to 43 at. pct Cr at 1150~ Disordered fcc alloys near the composition Au4Cr form the ordered Dla structure on o r d e r ing below the critical temperature ~ 330~ As can be seen in Fig. 1(a), the Dla structure has tetragonal s y m m e t r y with chromium atoms occupying every fifth {420} and {620} plane of the original fcc solid solution. Among other structures, which may appear in this system are the tetragonal DOz= and L1 o superlattices, which are schematically shown in Fig. l(b) and l(c) (see Refs. I, 3). Alloys containing up to 25 at. pct Cr show the existence of short range order at temperatures close to and above T c. The nature of sro in these alloys is similar to that existing in Ni~Mo and AUaV and has been the subject of a number of investigations. I-4 Alloys quenched from temperatures above T c show diffuse {1890} superlattice maxima in the diffraction pattern~ which are probably caused by the presence of imperfectly ordered DO~2 and Dla microdomains, s Okamoto and Thomas 2 proposed a model of sro based on composition modulations along the (210) directions analogous to the spinodal approach to o r d e r - d i s o r d e r transformations due to Cook and de Fontaine. 5 The presence of sinusoidal composition modulations of wavelength close to 4 (d4zo) explains the absence of {110} and {210} superlattice spots and the elongated shape of the superlattice maxima by the presence of weak satellites near the superlattice positions. The diffuse scattering maxima corresponding to sro are observed at different r e c i p r o c a l lattice positions to those of the long range ordered phase existing below Tc. The mechanism of the transformation from short to
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