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Carbide formation in alloy 718 during electron-beam solid freeform fabrication

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Abstract

In electron-beam solid freeform fabrication (EBSFF), material in wire form is fed into a melt pool maintained on the surface of the part by the electron beam, and controlled rapid movement of the part deposits metal selectively in a layer-by-layer fashion. Solidification occurs at a high rate, forming a fine dispersion of primary carbides in Alloy 718. Growth of the carbide particles has been modeled, assuming diffusion control and a linear change in the driving force for precipitation with temperature. The model predicts the maximum carbide size as a function of EBSFF operating parameters and alloy composition. For the material and conditions used experimentally in this work, the model predicts a maximum diameter of approximately 1.0 µm. Extraction-replica transmission electron microscopy of EBSFF samples identified carbides in the 300 to 600 nm range, consistent with a population having the predicted maximum size. Control material from a conventional vacuum-arc remelted ingot was also examined, and platelike carbides up to 40 µm in length were noted. This is an indication of the potential of EBSFF to refine the carbide morphology and size distribution.

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Abbreviations

e 1 :

L → γ+NbC eutectic reaction point

e 2 :

L → γ+Laves eutectic reaction point

e 3 :

L+γ+NbC → L+γ+Laves class II reaction point

C:

carbon content (weight fraction)

m :

slope of the liquidus (weight fraction/°C)

X :

cooling rate (°C/s)

t :

time (s)

r :

carbide particle radius

ρ :

density (g/cm3)

M :

atomic mass (g/mol)

D :

diffusion coefficient (m2/s)

R :

radial coordinate

S :

temperature range for carbide formation

a :

fatigue-crack length (mm)

N :

number of cycles to failure

α :

pre-exponential fatigue constant

β :

exponential fatigue constant

L :

liquid phase

NbC:

carbide phase

max:

maximum

faceted:

crystallographic fatigue regime

striated:

grain-independent fatigue regime

transition:

transition between fatigue regimes

critical:

fast-fracture regime

i :

interface

0:

initial

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Author notes

  1. J. E. Matz (Graduate Student Materials Technologist)

    Present address: the Department of Materials Science and Engineering, Massachusetts Institute of Technology, 02139, Cambridge, MA

Authors and Affiliations

  1. Pratt and Whitney Aircraft Engines, 06457, Middletown, CT

    J. E. Matz (Graduate Student Materials Technologist)

  2. the Department of Materials Science and Engineering, Massachusetts Institute of Technology, 02139, Cambridge, MA

    T. W. Eagar (Thomas Lord Professor of Materials Engineering and Engineering Systems)

Authors
  1. J. E. Matz
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  2. T. W. Eagar
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Matz, J.E., Eagar, T.W. Carbide formation in alloy 718 during electron-beam solid freeform fabrication. Metall Mater Trans A 33, 2559–2567 (2002). https://doi.org/10.1007/s11661-002-0376-y

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  • DOI: https://doi.org/10.1007/s11661-002-0376-y

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