Fu-Gao Wei

Design of Laves phase strengthened ferritic heat resisting steels in the Fe-Cr-Nb(-Ni) system

Abstract A new class of heat resisting ferritic steels is investigated in the Fe–Cr–Nb and Fe–Cr–Nb–Ni systems, in which the major strengthener is the Fe2Nb Laves phase. Ferritic steels strengthened by Laves phase are expected to show excellent high temperature strength, while the brittleness of Laves phase may lower the toughness of the alloy. The α-Fe/Fe2Nb two-phase microstructure is selected to improve mechanical properties through changing volume fraction and morphology of Laves phase. Effects of Nb and Ni contents on the microstructure and mechanical properties of the alloys, fixed at 10 at.% Cr, have been systematically investigated. Mechanical properties were evaluated by tensile tests conducted at room temperature, 873 and 973 K. The tensile test revealed that the room temperature ductility decreases with increasing Nb content. The alloys with 1.0–1.5 at.% Nb are found to exhibit a good balance between room temperature ductility and high temperature strength.

Deformation of Co3AlC0.5 and a Co3AlC0.5-strengthened cobalt-based alloy

Abstract Plastic deformation features of Co3AlC0.5 and a Co3AlC0.5-strengthened cobalt-based alloy were studied by means of transmission electron microscopy. It was found that {111} slip system is activated and dissociation of superlattice dislocations occurs in Co3AlC0.5 in a manner similar to the L12 structure. However, in contrast to the L12 structure, motion of dislocations is hindered by the pre-existing thermally grown-in anti-phase boundaries formed by wrong site occupation of carbon atoms, a unique type of crystal defects in Co3AlC0.5. On the other hand, deformation of the α(Co) alloy containing fine Co3AlC0.5 precipitates is characterized by localized slip of 1/6 Shockley partial dislocations cutting through the fine Co3AlC0.5 particles, leaving behind a highly disordered band region.

Characterization of C11b/C40 lamellae in a MoSi2–15mol%TaSi2 alloy

The crystallography of the lamellar structure composed of the tetragonal C11b phase and the hexagonal C40 phase which is similar to the well-known TiAl/Ti3Al lamellae was investigated by means of transmission electron microscopy in a MoSi2–15mol%TaSi2 pseudo-binary alloy after homogenized at 1400 °C for 168 h. It was found that the orientation relationship between C11b and C40 phases obeys the coincidence of the equivalent hexagonal atomic planes for the two phases. Three distinct variants of this orientation relationship were identified by electron diffraction. 13<2110 Dislocations prevail in the C11b/C40 interfaces to accommodate the lattice misfit between the two phases. On the other hand, networks of <100] and <110] dislocations dominate the C11b/C11b boundaries. <100] and <110] dislocations are also present within C11b variant plates while 13<2110 and 13<2113 dislocations are frequently observed in C40 phase. Significance of the 13<2113 c-component dislocations for improvement of the plasticity of C40 phase has been discussed.

Orientation relationship between Nb and Nb5Si3 (D8l) phases in the eutectoid lamellar microstructure

The crystallography of a Nb/Nb5Si3 lamellar microstructure formed by Nb3Si decomposition has been investigated by transmission election microscopy. Upon solidification of a Nb–10 at.% Ti–20 at.% Si alloy, the high-temperature phase Nb3Si is retained at room temperature. During annealing at 1673 K, the Nb3Si phase transforms into the Nb and Nb5Si3 (D8l) phases with a lamellar microstructure through a eutectoid reaction. The orientation relationship between Nb and Nb5Si3 is determined by selected-area diffraction analyses as , and [133]Nb//[111]Nb5Si3. The present orientation relationship is explained by good atomic matching on the close-packed planes along the near close-packed directions.

Electron Diffraction Study on the Crystal Structure of a Ternary Intermetallic Compound Co 3 AIC x

Intermetallic compound Co{sub 3}AlC{sub x}, or called {kappa}-phase, has been reported to assume the E2{sub 1}, or Perovskite structure. In the present work its crystal structure is critically reinvestigated in the two-phase alloys containing {kappa}-phase in the Co primary solid solution matrix, {alpha}(Co), using conventional electron diffraction. It is shown that the crystal structure of {kappa}-phase is a derivative of E2{sub 1} being a cubic structure composed of eight E2{sub 1} sub-unit cells in a half of which the body center sites are not occupied by carbon atoms. As a result, its space group is Fm3m and the chemical formula should be Co{sub 3}AlC{sub 0.5}. It is also found that the lattice parameter of the phase is about twice as large as that of {alpha}(Co). Orientation relationship of the {kappa}-phase with the matrix {alpha}(Co) is found to be similar to the case for the {gamma}{prime} phase with the fcc {gamma} matrix in a Ni-base superalloy since three orthogonal axes of {kappa}-phase are parallel to those of {alpha}(Co). Lattice misfit under this orientation relationship between the two phases is found to be about 2.5%.