Secondary carbide precipitation in a directionally solified cobalt-base superalloy

Abstract

A directionally solidified cobalt-base alloy, DZ40M, was solidified with a columnar grained austenitic matrix with a ͗001͘ preferential orientation and primary carbides of chromium-rich M 7 C 3 and MC at grain boundaries and interdendritically. Secondary carbides in DZ40M alloy are chromium-rich M 23 C 6 and tungsten-rich M 6 C. The M 23 C 6 carbide has a cube-cube orientation relationship with the austenitic matrix. Initial precipitation of secondary carbide, M 23 C 6 , occurred on dislocations in the austenitic matrix of the as-cast DZ40M alloy during cooling. Aging treatment (100 to 1000 hours at 850 ЊC) produced a profusive precipitation of the M 23 C 6 carbide mainly around the primary carbides. In the interior of grains, M 23 C 6 precipitated preferentially on dislocations and stacking faults. Subsequently, M 23 C 6 grew into laths near the primary carbides and coalesced into chains. The precipitation behavior of M 23 C 6 can be explained by the following reaction: 23M ϩ 6C → M 23 C 6 . The primary carbides are a carbon reservoir for the precipitation of M 23 C 6 . The M 6 C carbide was found only on the surface of the primary M 7 C 3 carbide adjacent to tungsten-rich MC in the aged condition. The precipitation of the tungsten-rich M 6 C is attributed to the tungsten segregation, which resulted from decomposition of the tungsten-rich MC and good lattice match between M 6 C and M 7 C 3 . The inhomogeneity of secondary precipitation is due to the uneven distribution of alloying elements.

Rejuvenation of degraded first stage gas turbine nozzle by heat treatment

“…On the other hand, this decomposition (M 6 C → M 23 C 6 ) led to precipitation of M 23 C 6 in the matrix. The precipitation of the secondary M 23 C 6 occurs profusely in cobalt-base superalloys with more than 5% chromium from the older X-40 to the newer MM-509 alloy [10,11]. Fig.…”

Failure analysis of stellite dies for brass extrusion: The effect of aging

“…The black-coarse M 7 C 3 carbides of the unused stellite are not any more present in the microstructure. Also the light-gray areas of cobalt with a fine dispersion of M 7 C 3 , which were situated between the Co-␥ dendrites, have been replaced by a continuous network of elongated carbides, presumably grown at some preferential crystallographic orientations [17][18][19][20][21][22][23][24], Fig. 6b.…”

“…Since M 7 C 3 is mainly a chromium carbide, the surrounding matrix should be oversaturated with Cr and as a result the Cr/C ratio will become high. This favors the formation of M 23 C 6 [17] according to reaction (2) [18][19][20][21]:…”

“…The reactions (1) and (2) creates rich in tungsten areas, simply because the chromium content in the cobalt matrix is reduced by reaction (2). In these areas, precipitation of -carbide occurs according to reaction (3) [22][23][24]:…”

Sliding wear studies of microwave clad versus unclad surface of stainless steel 304

“…Hence, it is very much essential to modify the surface of the components to improve the service life of the component. Wear, corrosion and tear of the component surface are the main reason for the failure of the engineering materials [1][2][3][4]. It is comprehensively not verbalized in potential wear applications.…”