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1. Effect of Post Processing Heat Treatment Routes on Microstructure and Mechanical Property Evolution of Haynes 282 Ni-Based Superalloy Fabricated with Selective Laser Melting (SLM)

   https://doi.org/10.3390/met10050629  

   Deshpande, Anagh ; Deb Nath, Subrata ; Atre, Sundar ; Hsu, Keng ( May 2020 , Metals)

   Selective laser melting (SLM) is one of the most widely used additive manufacturing technologies. Fabricating nickel-based superalloys with SLM has garnered significant interest from the industry and the research community alike due to the excellent high temperature properties and thermal stability exhibited by the alloys. Haynes-282 alloy, a γ′-phase strengthened Ni-based superalloy, has shown good high temperature mechanical properties comparable to alloys like R-41, Waspaloy, and 263 alloy but with better fabricability. A study and comparison of the effect of different heat-treatment routes on microstructure and mechanical property evolution of Haynes-282 fabricated with SLM is lacking in the literature. Hence,more »in this manuscript, a thorough investigation of microstructure and mechanical properties after a three-step heat treatment and hot isostatic pressing (HIP) has been conducted. In-situ heat-treatment experiments were conducted in a transmission electron microscopy (TEM) to study γ′ precipitate evolution. γ′ precipitation was found to start at 950 °C during in-situ heat-treatment. Insights from the in-situ heat-treatment were used to decide the aging heat-treatment for the alloy. The three-step heat-treatment was found to increase yield strength (YS) and ultimate tensile strength (UTS). HIP process enabled γ′ precipitation and recrystallization of grains of the as-printed samples in one single step.« less
2. Nanoscale precipitates as sustainable dislocation sources for enhanced ductility and high strength

   https://doi.org/10.1073/pnas.1914615117  

   Peng, Shenyou ; Wei, Yujie ; Gao, Huajian ( March 2020 , Proceedings of the National Academy of Sciences)

   Traditionally, precipitates in a material are thought to serve as obstacles to dislocation glide and cause hardening of the material. This conventional wisdom, however, fails to explain recent discoveries of ultrahigh-strength and large-ductility materials with a high density of nanoscale precipitates, as obstacles to dislocation glide often lead to high stress concentration and even microcracks, a cause of progressive strain localization and the origin of the strength–ductility conflict. Here we reveal that nanoprecipitates provide a unique type of sustainable dislocation sources at sufficiently high stress, and that a dense dispersion of nanoprecipitates simultaneously serve as dislocation sources and obstacles, leadingmore »to a sustainable and self-hardening deformation mechanism for enhanced ductility and high strength. The condition to achieve sustainable dislocation nucleation from a nanoprecipitate is governed by the lattice mismatch between the precipitate and matrix, with stress comparable to the recently reported high strength in metals with large amount of nanoscale precipitates. It is also shown that the combination of Orowan’s precipitate hardening model and our critical condition for dislocation nucleation at a nanoprecipitate immediately provides a criterion to select precipitate size and spacing in material design. The findings reported here thus may help establish a foundation for strength–ductility optimization through densely dispersed nanoprecipitates in multiple-element alloy systems.« less
3. Adjustment of the Mechanical Properties of Mg2Nd and Mg2Yb by Optimizing Their Microstructures

   https://doi.org/10.3390/met11030377  

   Schmidt, Jonas ; Beyerlein, Irene J. ; Knezevic, Marko ; Reimers, Walter ( March 2021 , Metals)

   The deformation behavior of the extruded magnesium alloys Mg2Nd and Mg2Yb was investigated at room temperature. By using in situ energy-dispersive synchrotron X-ray diffraction compression and tensile tests, accompanied by Elasto-Plastic Self-Consistent (EPSC) modeling, the differences in the active deformation systems were analyzed. Both alloying elements change and weaken the extrusion texture and form precipitates during extrusion and subsequent heat treatments relative to common Mg alloys. By varying the extrusion parameters and subsequent heat treatment, the strengths and ductility can be adjusted over a wide range while still maintaining a strength differential effect (SDE) of close to zero. Remarkably, themore »compressive and tensile yield strengths are similar and there is no mechanical anisotropy when comparing tensile and compressive deformation, which is desirable for industrial applications. Uncommon for Mg alloys, Mg2Nd shows a low tensile twinning activity during compression tests. We show that heat treatments promote the nucleation and growth of precipitates and increase the yield strengths isotopically up to 200 MPa. The anisotropy of the yield strength is reduced to a minimum and elongations to failure of about 0.2 are still achieved. At lower strengths, elongations to failure of up to 0.41 are reached. In the Mg2Yb alloy, adjusting the extrusion parameters enhances the rare-earth texture and reduces the grain size. Excessive deformation twinning is, however, observed, but despite this the SDE is still minimized.« less
4. Investigating the Effect of Microbial Activity and Chemical Concentrations on the Mineralogy and Morphology of Ureolytic Bio-Cementation

   https://doi.org/10.1061/9780784482834.010  

   Burdalski, Robert J. ; Gomez, Michael G. ( February 2020 , GeoCongress 2020)

   Numerous laboratory studies in the past decade have demonstrated the ability of microbially induced calcite precipitation (MICP), a bio-mediated soil improvement method, to favorably transform a soil’s engineering properties including increased shear strength and stiffness with reductions in hydraulic conductivity and porosity. Despite significant advances in treatment application techniques and characterization of post-treatment engineering properties, relationships between biogeochemical conditions during precipitation and post-treatment material properties have remained poorly understood. Bacterial augmentation, stimulation, and cementation treatments can vary dramatically in their chemical constituents, concentrations, and ratios between researchers, with specific formulas oftentimes perpetuating despite limited understanding of their engineering implications. Inmore »this study, small-scale batch experiments were used to systematically investigate how biogeochemical conditions during precipitate synthesis may influence resulting bio-cementation and related material engineering behaviors. Aqueous solution chemistry was monitored in time to better understand the relationship between the kinetics of ureolysis and calcium carbonate precipitation, and resulting precipitates. Following all experiments, precipitates were evaluated using x-ray diffraction and scanning electron microscopy to characterize mineralogy and morphology. Results obtained from these investigations are expected to help identify the primary chemical and biological factors during synthesis that may control bio-cementation material properties and« less
5. An as-cast high-entropy alloy with remarkable mechanical properties strengthened by nanometer precipitates

   https://doi.org/10.1039/C9NR08338C  

   Qin, Gang ; Chen, Ruirun ; Liaw, Peter K. ; Gao, Yanfei ; Wang, Liang ; Su, Yanqing ; Ding, Hongsheng ; Guo, Jingjie ; Li, Xiaoqing ( February 2020 , Nanoscale)

   High-entropy alloys (HEAs) with good ductility and high strength are usually prepared by a combination of forging and heat-treatment processes. In comparison, the as-cast HEAs typically do not reach strengths similar to those of HEAs produced by the forging and heat-treatment processes. Here we report a novel equiatomic-ratio CoCrCuMnNi HEA prepared by vacuum arc melting. We observe that this HEA has excellent mechanical properties, i.e. , a yield strength of 458 MPa, and an ultimate tensile strength of 742 MPa with an elongation of 40%. Many nanometer precipitates (5–50 nm in size) and domains (5–10 nm in size) are foundmore »in the inter-dendrite and dendrite zones of the produced HEA, which is the key factor for its excellent mechanical properties. The enthalpy of mixing between Cu and Mn, Cr, Co, or Ni is higher than those of mixing between any two of Cr, Co, Ni and Mn, which leads to the separation of Cu from the CoCrCuMnNi HEA. Furthermore, we reveal the nanoscale-precipitate-phase-forming mechanism in the proposed HEA.« less