Two isomorphous borides extracted from different superalloys possess a double boride structure, involving metal atoms of varying size. The structure is tetragonal with c/a = 0.54 and belongs to space group P4/mbm. It consists of larger metal atoms located at the vertices of directly stacked 3 ~ . 4.3.4 semi-regular-tessellation layers with smaller metal atoms at the centers of square prisms and boron at the centers of triangular prisms. Boron atoms occur in isolated bonded pairs.
ExperimentalA new boride structure has been found in alloys ttS-88 and Incoloy 901, each with around 0.1% boron addition. The borides were extracted electrolytically from unaged specimens and examined by X-ray powder analysis. Small amounts of TiC were also present in both extracts. Incoloy 901 without boron does not produce this structure.
The work of many investigators has strongly indicated that sigma phase is characterized by a certain averaged electronic structure. The characteristic was first thought to be the ``electron vacancy'' concentration in the d band, but it is now appreciated that the density of electrons outside the argon shells may also be a valid criterion. The latter characteristic is used here with the application of H. Jones' theory. The result shows a definite correlation between the homogeneity ranges and the x-ray diffraction patterns for sigma phase and several other complicated structures which occur in alloys of transition metals.
A diffraction phenomenon that is rare for X-rays but very common for 50- to 100-kv electrons is a principal source of difference between X-ray and electron diffraction intensities. Workers at our laboratory first became aware of this effect and its extent after examining electron diffraction patterns of cuprite (Cu2O) in a Phillips EM 100A electron microscope at an accelerating potential of 100 kv. The powder pattern (Fig. 1) contains every possible reflection from a primitive cubic lattice. The strongest lines are those of a face-centered cubic lattice—that is, the Miller indices are all even or all odd. Lines having mixed indices with even sums are of intermediate intensity, while those having one odd and two even indices are the weakest set. There are only a few spots on each Debye ring of the latter set.
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