Electron diffraction study of short-range-order diffuse scattering from disordered binary alloys

Abstract

A qualitative study was made by electron diffraction of the short-range-order diffuse scattering from disordered Au-Pd, e phase Au-Zn, and e phase Ag-Mg alloys. Twofold and fourfold splittings of diffuse maxima are observed at 100, 110 and equivalent positions on the diffraction patterns with the incident beam parallel to [001], in the composition ranges from about 28 to 62 at.% Pd for Au-Pd, from about 7 to 24 at.% Zn for Au-Zn and from about 11 to 28 at.% Mg for Ag-Mg systems. The separation of the split diffuse maxima increases monotonically with Pd, Zn or Mg content. The results can be interpreted with the Fermi-surface-imaging concept proposed by Krivoglaz. The variation with composition of the Fermi wave vector parallel to the [110] directions is discussed with the nearly-free-electron and the rigidband models. The latter model is also applied to the experimental results for disordered Cu-Pd and Cu-Pt alloys reported by the present authors [Ohshima & Watanabe (1973), Acta Cryst. A29, 520-526].

X-ray diffraction study of short-range-ordered structure in a disordered Ag–15·0 at.% Mg alloy

“…A fourfold splitting of the diffuse scattering at the 110 position and the diffuse streaks parallel to the h and k axes through h 10 and 1 kO can be clearly seen. The pattern is qualitatively in good agreement with the electron diffraction pattern ( in good agreement with 0.147 (5) given by electron diffraction observation (Ohshima & Watanabe, 1977).…”

“…Since the Fermi sphere is truncated along this direction, the Fermi wave number k~ along the (110) directions was taken to be k~= tkF where t is the truncation factor representing the flatness of the Fermi surface. For the present alloy composition (Ohshima & Watanabe, 1977) it is estimated to be 0.955. However, the interaction potential along the [ 110] direction falls off much faster than (rtm,)-i.…”

“…The quantity 2kFao is related to the electron/atom ratio as 4.911(e/a) ~/3. In the present case e/a is evaluated (Ohshima & Watanabe, 1977) as well as the (111) necks which also exist in this alloy, so that the potentials along and close to the (110) and (111) considered to deviate from the sphere. The pair-interaction potentials along these directions should therefore be excluded from the comparison.…”

“…As for the a-phase Ag-Mg alloy which also forms f.c.c, structure for compositions up to 28 at.% Mg (Hansen & Anderko, 1958), conspicuous diffuse rods are observed on the diffraction pattern parallel to the h and k axes through hl0, h30,.., and lk0, 3k0,..., besides the split diffuse maxima (Ohshima & Watanabe, 1977). A typical electron diffraction pattern from a disordered Ag-19.0at.% Mg alloy is shown in Fig.…”

“…Characteristic fourfold splittings of diffuse scattering have been observed at the 100, 110 and equivalent positions on the electron and X-ray diffraction patterns from disordered or quenched states of several binary alloys with a face-centered cubic structure [Cu-Au: Raether (1954); Sato, Watanabe & O~awa (F~ 1986 International Union of Crystallography (1962); Marcinkowski & Zwell (1963); Watanabe & Fisher (1965); Moss (1969); Hashimoto & Ogawa (1970) ;Yamagishi, Hashimoto & Iwasaki (1982); Bessi~re, Lefebvre & Calvayrac (1983) ;Ohshima, Harada & Moss (1986); Cu-AI: Scattergood, Moss & Bever (1970); Cu-Pd: Watanabe (1959) ;Ohshima, Watanabe & Harada (1976); Cu-Pd and Cu-Pt: Ohshima & Watanabe (1973); Ag-Mg, Au-Pd and Au-Zn: Ohshima & Watanabe (1977); Au-Pd: Lin, Spruiell & Williams (1970)]. The split diffuse maxima correspond to the intersections of slightly curved diffuse streaks around the 100 and 110 positions and their separation varies monotonically with increasing alloy content.…”