Characterization and Modeling of Microstructure Development in Nickel-base Superalloy Welds Page: 2 of 9
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Introduction
Welding is important for economical reuse and
reclamation of used and failed nickel-base superalloy
blades, respectively [1]. Solidification and solid state
decomposition of y (Face Centered Cubic, FCC) phase into
y' (L1r-ordered) phase control the properties of these
welds. In previous publications, the microstructure
development in electron beam welds of PWA-1480 alloy
[2] and laser beam welds of CMSX-4 alloy [3] were
presented. These results showed that the weld cracking
in these alloys were associated with low melting point
eutectic at the dendrite boundaries [1, 2]. The eutectic-y'
precipitation was reduced at rapid weld cooling rates and
the partitioning between y-y' phase was found to be far
from equilibrium conditions [3, 4]. This observation was
related to diffusional growth of y' precipitate into y
phase. Subsequent to the above work, the precipitation
characteristics of y phase from y phase were evaluated
during continuous cooling conditions [5]. The results show
that the number density of y' precipitates increased with
an increase in cooling rate. However, the details of this
decomposition and also the fine-scale elemental
partitioning characteristics between y-y' were not
investigated. In this paper, the precipitation
characteristics of ' from y during continuous cooling
conditions were investigated with transmission electron
microscopy, and atom probe field ion microscopy. In
addition, thermodynamic and kinetic models were used to
describe microstructure development in Ni-base
superalloy welds.
Experiment
A directionally solidified Ni-base superalloy
CM247DS (Ni - 12.1 at. % Al - 9.12% Cr - 9.05% Co - 1.0%
Ti - 0.06% Nb - 0.37% Mo - 1.05% Ta - 3.06% W - 0.4% C)was used in this investigation. The alloy was subjected to
continuous cooling heat treatments in a Gleeble
thermomechanical simulator. The relative radius change
of the sample was monitored using dilatometry. The
heat treatments include 5-min hold at a solutionizing
temperature of 1300 C and continuous cooling at rates of
0.17 to 750Cs'. In addition, the samples were water
quenched from 1300*C.
An atom probe field-ion microscope [6] was used
to characterize the water-quenched sample. For APFIM,
small needles were machined from the sample. These
needles were electropolished using standard techniques.
The samples were imaged at a temperature range of 50 to
60 K and analyzed with a residual neon gas pressure of 3 x
10" Pa and a pulse fraction of 20% in the atom probe.
To evaluate the decomposition characteristics of
CM247DS alloy, thermodynamic calculations were
performed with ThermoCalc software [7]. The
calculations describe multi-component and multi-phase
thermodynamic equilibrium. The calculations used an 11-
element Ni-Fe database [8]. The alloying elements
considered in the calculations were Ni, Al, Nb, Ti, Cr, Co,
Mo, and C. The calculations ignored the effects of Ta and
W due to the limitation of the database.
Diffusion controlled kinetics of y phase
solidification, as well as, y precipitate growth into y
phase during continuous cooling was simulated with
DicTra software [9]. In these simulations,
thermodynamic local equilibrium at the phase
boundaries was calculated with the Ni-Fe database [8].
Moreover, the diffusion in y phase was ignored and as a
result the kinetics are controlled only by the diffusion in
the y phase.Babu et. al. 1 of 8
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Babu, S. S.; David, S. A.; Miller, M. K. & Vitek, J. M. Characterization and Modeling of Microstructure Development in Nickel-base Superalloy Welds, article, November 1, 1999; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc624821/m1/2/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.