Influence of the base material on the mechanical behaviors of polycrystal-like meta-crystals

Architected lattice metamaterials offer extraordinary specific strength and stiffness that can be tailored through the architecture. Meta-crystals mimic crystalline strengthening features in crystalline alloys to obtain high strength and improved post-yield stability of lattice materials. This study investigates synergistic effects of the base material’s intrinsic crystalline microstructure and architected polycrystal-like architecture on the mechanical behavior of architected metamaterials. Four different polygrain-like meta-crystals were fabricated from 316L, Inconel 718 (IN718) and Ti6Al4V via laser powder bed fusion (L-PBF). While the elastic modulus of the meta-crystals did not vary significantly with the base material or the number of meta-grains, the strength of the meta-crystals showed strong increasing correlation with reducing the size of meta-grains. The differences between meta-crystals made by the three alloys were the most substantial in the post-yield behavior, where the 316L meta-crystals were the most stable while Ti6Al4V meta-crystals were the most erratic. The differences in the post-yield behavior were attributed to the base material’s ductility and intrinsic work-hardening. For all base materials, increasing the number of meta-grains improved the post-yield stability of meta-crystals. The tolerance to the processing defects also differed with the base material. Detrimental defects such as the high surface roughness on the downskin of the struts or the large, irregularly shaped pores near the surface of the struts led to early strut fracture in Ti6Al4V meta-crystals. In contrast, ductile IN718 was able to tolerate such defects, enabling the most significant synergistic strengthening across lengthscales to achieve architected materials of low relative density, but with a very high strength and an excellent energy absorption.

Tensile Deformation and Fracture Behavior of Nickel-Based Superalloy DZ951G

DZ951G is a novel developed nickel-based directional solidified superalloy with an incipient high melting point and low density. Compared with DZ417G superalloy, DZ951G superalloy has a higher ultimate tensile strength. At intermediate temperatures, the plasticity and strength were both markedly improved, and an obviously anomalous yield behavior could be observed where the yield strength reached its maximum at 760 °C. Below 600 °C, two competitive modes of dislocations shearing γ′ particles existed, in which one was the formation of stacking faults and another was a/2<101> dislocations shearing. At intermediate temperatures, a transitional phase between shearing γ′ particles and bypassing appeared, and the fracture translated from brittle fracture into ductile fracture. Exceeding 900 °C, bypassing of dislocations was operated under thermal activation. Moreover, short continuous stacking faults still existed at 760 °C. Finally, the various dislocation configurations were rationally illuminated and explained with the intrinsic connection of mechanical properties.

Yield Behavior of Tube Hydroforming and it’s Effecti on Hollow Component Manufacturing

In this paper, aiming at the mechanics of tube hydroforming, the plasticity method was adopted in analyzing the stress yield behavior of the process. The stress character and yield behavior during the different stage of tube hydroforming was given and discussed. And then, basing on the discussing result, the yield behavior calculating formula was given and derived. At the same time, combining with the formula, figure of yield behavior was given. Base on the yield behavior character the process analysis was discussed in the end.

Spreading Kinetics of Herschel-Bulkley Fluids Over Solid Substrates

The spreading kinetics of Herschel-Bulkley fluids on horizontal solid substrates were theoretically studied. The equations of film thickness were derived in both gravitational and capillary regimes. The dynamic contact angle for the capillary regime was also derived. Finally, a limiting result for the case of τ0 = 0 was obtained, which was compared with the known solution for validation. The results show that the yield behavior of the fluids had a significant impact on the spreading kinetics in both cases. Only when stress was larger than the yield stress, would substantial flow occur. The spreading zone was divided into two parts by the yield surface: sheared zone and yield zone, which was completely different from common Newtonian fluids or power-law fluids. The thickness of the yield zone mainly depended on yield stress and pressure gradient along the z-direction. According to the final evolution, both the film thickness and dynamic contact angle were affected not only by the power-law index but also by the yield behavior.