It is a known fact that the creep of SLM processed IN 718 is much lesser than as-cast samples. Want to know the factors causing this reduced creep in SLM processed Inconel 718. Is it dislocation movement or the presence of laves phase or micro-segregation or small grains or the large residual stress that play a major role in this creep reduction?
Dear Roshan,
The mechanisms that are generally proposed for creep depend on the values of stress σ and temperature T:
High stress σ:
- High temperature T: diffusion-controlled (generation and) motion of dislocations in the matrix (lattice diffusion); grain boundary sliding (GBS)
- Low temperature T: diffusion-controlled (generation and) motion of dislocations along dislocation cores; grain boundary sliding (GBS)
Low stress σ:
- High temperature T: diffusional creep: flow of vacancies (and counter flow of atoms) through the lattice from grain boundaries under tension to those under compression (Nabarro-Herring creep)
- Low temperature T: diffusional creep along grain boundaries (Coble creep)
Hope this is helpful,
Jose
[1] Srivastava, V., Williams, J. P., McNee, K. R., Greenwood, G. W., & Jones, H. (2004). Low stress creep behaviour of 7075 high strength aluminium alloy. Materials Science and Engineering A-Structural Materials Properties Microstructure and Processing, 382(1-2), 50–56. doi:10.1016/j.msea.2004.04.047
[2] Evans, R. W., & Wilshire, B. (1996). Constitutive Laws for High-Temperature Creep and Creep Fracture. In A. S. Krausz & K. Krausz (Eds.), Unified Constitutive Laws of Plastic Deformation (Vol. 1, pp. 107–152). San Diego, CA, USA: Academic Press, Inc.
Thank you Jose Ignacio Rojas Gregorio .
Some books on creep use steady-state creep rate over stress and try to give out the big picture. This explanation, is well-structured regarding the stress and the temperature.
How would the creep be explained in superalloys produced by SLM process, where it has meta-stable microstructure and small, elongated grains having a large amount of residual stress?
The microstructure comparison between as-cast and as-printed samples should be done to answer this question. It is well known that actually three creep mechanisms responsible for a creep: dislocations movement, diffusion and grain boundary sliding. The last one is typical for small grained metallic alloys without grain boundary phases, while the material is at at extremely high temperatures. It seems, that this mechanism non-typical for Inconel 718. I would consider only dislocation movement and diffusivity.
Alexander Katz-Demyanetz : Thank you for the answer. The typical as-processed SLM microstructure of Inconel 718 contains elongated small grains, columnar interdendritic regions within the grains with Laves phase segregation and high dislocation density (residual stress).
The temperature at which the samples are tested never exceeds 750 C (0.6 Tm). Would these two mechanisms prevail in these temperatures and microstructures?
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