Stress–Strain Properties and Gas Permeability Evolution of Hybrid Fiber Engineered Cementitious Composites in the Process of Compression

Polyvinyl alcohol (PVA)-steel hybrid fiber reinforced engineered cementitious composites (ECC) characterized by optimal combination of high strength and high ductility were developed recently. These composites exhibit even tighter crack width than normal ECC, showing great potential for lower permeability in cracked state, and consequently improving the durability of ECC structures. In addition, the wide variety of promising applications in underground or hydraulic structures calls for knowledge on the mechanical behavior and corresponding permeability properties of strained ECC under multiaxial stress, as they are essential for structural analysis and durability design. Experimental investigations into the compressive properties and the in-situ gas permeability of PVA-steel hybrid fiber ECC were performed in this study, with special focus on the impact of additional steel fiber content and confining pressure. The test results show that the presence of a low confinement level allows ECC to attain a substantial improvement on compressive behavior but impairs the enhancement efficiency of additional steel fiber. The permeability evolution of strained ECC corresponds to the variation of radial strains, both of which experience little change below the threshold stress but a rapid increase beyond the peak axial strain. Apart from exhibiting low permeability at relatively small strains in the pre-peak stage, ECC can also exhibit low permeability at higher levels of compressive strain up to 2.0%. However, unlike the case in tensile loading, impermeability of cracked ECC in compression would be weakened by additional steel fibers, especially in the post-peak stage. The present research is expected to provide insight into performance-based durability design of structures made of or strengthened with ECC

Effects of Thermal Exposure on the Microstructure and Mechanical Properties of a Ti–48Al–3Nb–1.5Ta Alloy via Powder Hot Isostatic Pressing

Research on how thermal exposure affects the microstructure and mechanical properties of the Ti–48Al–3Nb–1.5Ta (at. %) alloy, which is prepared via powder hot isostatic pressing (P–HIP), is essential since this low-density alloy shows promise for use in high-temperature applications, particularly for aero-engines, which require long-term stable service. In this study, a P–HIP Ti–48Al–3Nb–1.5Ta (at. %) alloy was exposed to high temperatures for long durations. The phase, microstructure and mechanical properties of the P–HIP Ti–48Al–3Nb–1.5Ta alloy after thermal exposure under different conditions were analyzed using XRD, SEM, EBSD, EPMA, TEM, nanomechanical testing and tensile testing. The surface scale is composed of oxides and nitrides, primarily Al2O3, TiO2, and TiN, among which Al2O3 is preferentially generated and then covered by rapidly growing TiO2 as the thermal exposure duration increases. The nitrides appear later than the oxides and exist between the oxides and the substrate. With increasing exposure temperature and duration, the surface scale becomes more continuous, TiO2 particles grow larger, and the oxide layer thickens or even falls off. The addition of Ta and Nb can improve the oxidation resistance because Ta5+ and Nb5+ replace Ti4+ in the rutile lattice and weaken O diffusion. Compared with the P–HIP Ti–48Al–3Nb–1.5Ta alloy, after thermal exposure, the grain size does not increase significantly, and the γ phase increases slightly (by less than 3%) with the decomposition of the α2 phase. With increasing thermal exposure duration, the γ phase exhibits discontinuous coarsening (DC). Compared with the P–HIP Ti–48Al–3Nb–1.5Ta alloy, the hardness increases by about 2 GPa, the tensile strength increases by more than 50 MPa, and the fracture strain decreases by about 0.1% after thermal exposure. When the depth extends from the edge of the thermally exposed specimens, the hardness decreases overall