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A new metric was developed to quantify the impact of surface-connected defects and internal pores of different morphologies, namely irregular lack of fusion (LoF) pores and spherical keyhole pores, on the mechanical properties and fracture location of AlSi10Mg tensile samples fabricated using laser powder bed fusion additive manufacturing. As defect volume alone has been shown to be insufficient to predict fracture location, the proposed defect impact metric (DIM) incorporates contributions from additional defect features, including proximity to the surface, interaction with neighboring defects, morphology, and reduction in load-bearing cross-sectional area to better assess a defect’s propensity for corresponding to fracture location. The fracture location of keyhole samples was captured by large surface-connected defects with numerous neighboring defects and resulted in increased losses in load-bearing area. In contrast, LoF samples fractured at regions with either large surface-connected defects or large internal pores with many defects in close proximity, high curvatures, and large projected areas. The proposed DIM outperformed existing defect-based frameworks in identifying fracture locations in both LoF and keyhole samples by incorporating surface roughness, defect projected area, and interactions between defects based on distance, volume, and configuration. Additionally, the maximum DIM value within the fracture range was more strongly correlated to strength and ductility than porosity or defect size for LoF samples, demonstrating the potential of the DIM to non-destructively assess the effects of defects on mechanical behavior. The broader applicability of the DIM framework was demonstrated in its ability to capture fracture in both PBF-LB AlSi10Mg and Alloy 718. 

Additively manufactured metals often contain pores, which limit the strength and ductility of resulting components. In this study, a ductile fracture model was developed to describe the effect of pore size, in terms of absolute and relative metrics, on fracture strain under uniaxial tension. The model approximates lack of fusion (LoF) pores as penny-shaped cracks, and damage accumulation was based on the J-integral and secondary Q parameter. The model was calibrated with Ti-6Al-4V samples with intentionally introduced pores fabricated by laser powder bed fusion (PBF-LB) additive manufacturing (AM) in as-built and heat-treated conditions. The model captures the experimentally observed size effect, where for a given pore area fraction, larger samples fracture at smaller strains. By identifying the critical pore size for a single, isolated pore for either load or displacement-controlled applications, the model developed in this study is a crucial step to developing a comprehensive fracture model for establishing confidence in the structural capability of pore-containing additively manufactured components. 

This study investigates the effects of pores on the mechanical properties of metals produced by additive manufacturing, which can limit strength and ductility. This research aims to both measure and model the rate of crack growth emanating from these pores in additively manufactured Ti-6Al-4 V fabricated with laser powder bed fusion. Uniaxial tensile samples containing intentionally embedded penny-shaped pores were mechanically tested to failure, and loading was interrupted by a series of unload steps to measure the stiffness degradation with load. The factors contributing to reduction in stiffness, namely (1) elastic and plastic changes to geometry, (2) the effect of plastic deformation on modulus, and (3) crack growth, were deconvoluted through finite element modeling, and the crack size was estimated at each unloading step. The stiffness-based method was able to detect stable crack growth in samples with large pores (1.6% to 11% of the cross-sectional area). Crack growth as a function of strain was fit to a model where the crack driving force was based on equivalent strain and a model where the crack driving force was based on energy release rate. Significant crack growth occurred only after the onset of necking in samples containing small pores, while samples containing large pores experienced continuous crack growth with strain. 

Ultra-high temperature ceramics (UHTCs) are refractory transition-metal carbides, nitrides, and borides with the highest melting temperatures known materials, making them prime candidates for applications in aerospace and hypersonic vehicles. Of the UHTCs, tantalum carbide (TaC) and hafnium carbide (HfC) feature the highest melting temperatures. We investigated the binderless consolidation of HfC/TaC powder blends using Field Assisted Sintering Technology (FAST). Powders consisting of 90/10, 50/50, and 10/90 vol% HfC:TaC were sintered to high densities (>94 %). Bulk and nanomechanical, chemical, and microstructural characterization revealed substantially greater strength, hardness, and stiffness for ternary alloys. Mechanical properties correlated with physiochemical analysis indicated trace oxygen phases, solid-solution strengthening, and nonstoichiometric carbon were the key mechanisms driving the peak property enhancement of the 50 vol% solid-solution sample, despite lower densities. This study provides insight into optimizing the compositional design of HfC-TaC alloys by balancing influences from solid solution strengthening and the thermodynamic effects of oxygen/carbon stoichiometry. 

Molybdenum and its alloys are of interest for applications with extreme thermomechanical requirements such as nuclear energy systems, electronics, aerospace vehicles, and hypersonic vehicles. In the present study, pure molybdenum and samples with added hafnium carbide (HfC) grain refiners were produced using field assisted sintering technology (FAST). The molybdenum and HfC reacted with oxygen to produce MoO2 and HfO2, and increased HfC content from 1 wt% to 5 wt% decreased grain size while the microhardness correspondingly increased. Room temperature three-point bending tests were conducted, and finite element modeling was used to define HfC-dependent bilinear material models. The presence of oxygen most severely affected pure molybdenum, which exhibited little strength and limited ductility, whereas for samples with added HfC, HfO2 was present, resulting in increased toughness hypothesized to be due to microcrack toughening. The samples with 1 wt% added HfC had the greatest energy absorption capability. 

This study investigates the disparate impact of internal pores on the fracture behavior of two metal alloys fabricated via laser powder bed fusion (L-PBF) additive manufacturing (AM)—316L stainless steel and Ti-6Al-4V. Data from mechanical tests over a range of stress states for dense samples and those with intentionally introduced penny-shaped pores of various diameters were used to contrast the combined impact of pore size and stress state on the fracture behavior of these two materials. The fracture data were used to calibrate and compare multiple fracture models (Mohr-Coulomb, Hosford-Coulomb, and maximum stress criteria), with results compared in equivalent stress (versus stress triaxiality and Lode angle) space, as well as in their conversions to equivalent strain space. For L-PBF 316L, the strain-based fracture models captured the stress state dependent failure behavior up to the largest pore size studied (2400 µm diameter, 16% cross-sectional area of gauge region), while for L-PBF Ti-6Al-4V, the stress-based fracture models better captured the change in failure behavior with pore size up to the largest pore size studied. This difference can be attributed to the relatively high ductility of 316L stainless steel, for which all samples underwent significant plastic deformation prior to failure, contrasted with the relatively low ductility of Ti-6Al-4V, for which, with increasing pore size, the displacement to failure was dominated by elastic deformation.