Powder metallurgy (PM) processes for porous copper and alloys have seen some commercial successes, but PM methods have the disadvantage of relatively low porosity or strength that is compromised by stress-concentrating interparticle bonds. To increase porosity without compromising scalability, a Cu-CuO metal matrix composite powder was utilized to produce additional microscale porosity within the particles by oxide reduction. These Cu-CuO powders were pressed at 1, 2, or 3 GPa, and made porous at 600, 800, or 1000 °C to investigate the effects of pressing and sintering parameters on the overall strength and density. It was found that the formation of porosity is weakly dependent on compaction pressure (maximum 6% difference from 1 GPa to 3 GPa), while the final porosity varied by ~16% overall (~40% for 1 GPa and 600 °C to 24% for 3 GPa and 1000 °C). The strength of the porous Cu was highest after being reduced at 600 °C but also exhibited some flaking at the edges at high strain. The 1 GPa, 600 °C samples have a higher specific strength than wrought Cu annealed at the same temperature, as was demonstrated under uniaxial quasi-static compression as well as split Hopkinson pressure bar impact.

Powder-based methods that are used to make porous metals are relatively simple and scalable, and porosity can be controlled by interparticle spacing as well as the addition of a sacrificial template. A relatively new process based on reducing oxides in a metal matrix has been demonstrated to produce microscale porosity within individual powder particles and thereby may be used to enhance other powder metal techniques. Templating methods require relatively large quantities of powder, but oxide-reduction feedstock powders have only been produced by small-batch ball milling processes (e.g., 10 s of grams). Planetary ball milling is capable of processing larger quantities of powder (e.g., 100 s of grams) but has significantly different milling characteristics. To successfully apply this technique, it was systematically studied in terms of composition, milling conditions, and the addition of stearic acid to control powder size and morphology along with final porosity. It was found that by controlling basic parameters, such as oxide levels and milling time, a relatively high porosity (25%) and powder percentage (99%) can be achieved in Cu-2 mol% CuO with only 0.035 wt% stearic acid and only 90 min of milling.

In this work, an Internal State Variable (ISV) elastoviscoplasticity-damage (EVP-D) constitutive model is developed to account for the thermomechanical material behavior of 316 stainless steel (316 SS). This is an important material system as it is used in nuclear reactors where high-temperature low cycle fatigue (LCF) and creep-fatigue (CF) behavior are critical to performance lifetime. In this study, an intergranular cavitation damage equation was added to a pre-existing ISV EVPD model that captures the structure-property relationship of creep damage leading to a decrease in cycles to failure. The cavity damage model describes a static damage evolution driven mainly by internal vacancy diffusion to grain boundaries under stress over time. This new cavitation damage ISV was incorporated with consideration of kinematics, thermodynamics, and kinetics in the continuum ISV theory framework. Using this novel ISV EVPD model with/without various holding times at each cycle, the results show good agreement with the experimental stress-strain and stress-life data of 316 SS. With the current advances, this ISV constitutive model may be applied for complex thermomechanical boundary value problems, such as nuclear reactors, particularly when discussing on high-temperature applications. Such studies will offer insights for improving structural integrity and optimizing safety procedures in a timelier manner.

An Internal State Variable (ISV) elastoviscoplasticity-damage (EVPD) constitutive model is extended by adding an intergranular cavitation equation to help simulate damage in low cycle fatigue (LCF) and creep-fatigue (CF). The updated model used a 316 stainless steel (316 SS) alloy tested at elevated temperatures to exemplify its efficacy. Specific intergranular effects were added to bridge the gap where past ISV models failed to capture the synergistic or competitive coupling of creep damage during tensile strain hold periods. The intergranular cavity damage model, which includes the kinematics, thermodynamics, and kinetics, describes a static damage progression driven mainly by internal vacancy diffusion to grain boundaries under stress over time. Using this extended ISV EVPD model with/without various holding times at each cycle, the results show good agreement with the experimental stress-strain and stress-life data of 316 SS, demonstrating better predictive capabilities over standard uncoupled linear damage rules under extended hold times. The resulting constitutive framework serves as a foundation for analyzing complex thermomechanical boundary value problems and evaluating the structural integrity of metallic components subjected to elevated temperature cyclic loading.