A coating with a duplex layer structure, outer β-NiAl and inner σ-Re-Cr-Ni layers, was formed on the third generation Nb-5Mo-15W-16Si- 5Hf-5C alloy by using successively Re-pack cementation, electroplating of Re-Ni film, and Cr/Al pack cementation. The duplex layer coating changed during high temperature oxidation to form a coating with a four layer structure: an outermost Ni2Al3, an outer Ni, an inner σ-Re-Cr-Ni, and an innermost χ-Re-Nb. The Re-pack cementation was carried in an alumina crucible where the specimen was buried in Re metal powder, in vacuum at 1573K to form a Re film, and then a Re-Ni film was electroplated onto the Re-pack treated alloy. The coated alloy formed a protective α-Al2O3 scale when oxidized at 1573K in air for 14.4ks. It was concluded that the σ-Re-Cr-Ni phase can act as a diffusion-barrier against both inward Al diffusion and outward diffusion of alloying elements from the alloy substrate to the β-NiAl.
The effects of coatings on the deformation and oxidation behavior of Ti-50Al alloy were investigated at 1173 K in air at a constant loading of 30 MPa. The coating was formed by a two-step Cr/Al diffusion treatment and consisted of an outermost TiAl2 layer, an outer Al-rich γ layer, an intermediate γ, Laves and β mixed layer, and a Cr diffusion zone. Tensile tests were also carried out with sole Cr or Al coated TiAl and uncoated TiAl at 1173 K for comparison purposes. The oxide scales formed on the uncoated TiAl and the sole Cr coated specimens were a mixture of TiO2 and Al2O3, which displayed extensive exfoliation. Both the two-step Cr/Al coated TiAl and the sole Al coated specimens formed a protective Al2O3 layer and little oxide exfoliation was observed here. Significant cracks were observed in the sole Al coated TiAl, while no cracks were observed in the sole Cr coated TiAl; the two-step Cr/Al coated TiAl showed a number of cracks in the coatings. Low creep rates in the two-step Cr/Al coated TiAl could be due to the Laves phase with a hexagonal C14 structure in the intermediate, γ, β and Laves phase mixture, and the high creep rates of the sole Cr coated TiAl may originate in the major β phase component with a B2 structure in the γ, β, and Laves phase mixture.
Concentration profiles of nickel in the (Co, Ni)O scales grown on the Co-low Ni alloys (0.45, 2.46 and 7.23 wt%Ni) over the temperature range from 1000 to 1300°C were determined by means of EPMA and then analyzed according to the analytical equation after Wagner and the differential equations after Dalvi and Coates.By making use of both computation methods, the concentration profiles similar to the observed ones were obtained, but an agreement between them was found to be insufficient in the inner part of the scale. The values of P(DNi⁄DCo) and n(PO21⁄n) were determined as a result of the present analytical method and found to be within the range reported in the literature. These values of P and n depended on all parameters, in particular, strongly on ξ′ (NiO content at the alloy-scale interface) and k′(=k⁄DCo0), respectively. In contrast with the presentation by Dalvi and Coates, both P- and n-values changed with NiO content in the oxide.The mass balance equations at both scale interfaces were used to judge the validity of the numerically obtained solutions. The resulting solutions were unsatisfied with the mass balance equation at the scale-gas interface. Therefore, it was concluded that the perfect solutions could not be obtained by using the equations by Dalvi and Coates. This result may be ascribed to the changes of the P- and n-values with NiO content in the oxide.On the other hand, the equation by Wagner is suitable for an approximate estimation of cation distribution and cation diffusivities in oxide scales because of its simplified form.
Formation of Cr-metallized layer on the surface of Si3N4 ceramic by the vapor-diffusion method was studied. The metallization was carried out in a quartz capsule at 1273K for up to 72ks. As a vapor source, pure Cr powder was used. The kinetics of layer growth was parabolic with the rate constant of 1.0×10-14m2·s-1 at 1273K. The metallized layer was composed of two lyers; Cr2N and CrN at the surface layer and Cr2Si at the inner layer as identified by the X-ray diffraction analysis. The hardness of the metallized layer increased gradually from Hv 668 at surface to Hv 1179 at inner part.
A new method has been proposed for the estimation of the distribution of the three-dimensional grain diameter in a polycrystalline material from the measured distribution of the two-dimensional grain diameter on the cross section of the material. This method has been applied to the estimation of the three-dimensional grain diameter distribution of SUS310S stainless steel during the grain growth. The three-dimensional and the two-dimensional distributions have been compared.The three-dimensional grain diameter distribution had the larger average value and the smaller variation coefficient, compared with the two-dimensional grain diameter distribution. The activation energy for the grain growth was larger for the analysis of the three-dimensional average grain diameters than for the two-dimensional analysis, and the value was close to those for the grain boundary diffusion of Fe, Cr and Ni.
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)