Effect of Cellular Dislocation Structure on the Strength of Additively Manufactured 316L Stainless Steel

Additively manufactured (AM) 316L stainless steel (SS) often contains cellular dislocation structure which is a distinct microstructural feature compared with those fabricated traditionally, like casting and forging. The role of this unique cellular dislocation structure on the mechanical properties of the AM 316L SS needs to be determined to guide its further performance improvement. In this study, the effect of cellular dislocation structure on the strength of AM 316L SS was investigated via micro-mechanical compression test. Single crystalline micro-pillars were firstly prepared from both the as-built and annealed AM 316L SS bulk specimens, with and without cellular dislocation structure relatively. The results show a significant increase of the yield strength of the micro-pillars with the cellular dislocation structure. The micro-pillars containing cellular dislocation structure with different sizes and morphologies have been studied to evaluate the effect of cellular dislocation structure on the strength of AM 316L SS.

On the Development Concept for a New 718-Type Superalloy with Improved Temperature Capability

The superalloy 718 stands out for its excellent manufacturability and strength at ambient temperature. However, its application temperature is limited to about 650 °C due to the instability of the γ’’ precipitates. Here, we provide an in-depth account of an alloy development concept, allowing for the design of superalloys with 718-type properties, yet with a significantly improved microstructural stability. The article begins with a detailed discussion on how the microstructural and chemical composition must be altered to achieve this objective. Then, model alloys were used to explore and validate the outlined strategy. Finally, it is shown how these considerations ultimately led to a new 718-type superalloy with far more improved microstructural stability— namely, VDM Alloy 780. The introduction of a large amount of Co as a substitute for Fe (and partially Ni) is the most important element of our alloy development concept in terms of chemical composition. The most important microstructural feature is the introduction of low solvus temperature, high misfit γ´-strengthening, replacing γ´´-hardening.