Heterostructured materials with enhanced mechanical properties

Abstract

In the history of material science, a wide attempt to develop metallic materials with high strength-ductility combinations for high industrial suitability. However, the inevitable dilemma in strength and ductility is that the strengthening of materials results in a loss of ductility. Recently, Heterostructuring is an emerging method to alleviate or even overcome the trade-off dilemma, unavoidable in conventional materials. Among heterostructuring techniques, surface modification has attracted significant attention in academic and industrial fields because of its exceptional mechanical properties and procedural ease. Surface modification produces a gradient structure that is similar to natural structural materials such as seashells, bamboo, and wood. A hard coating layer protects the parts from external factors of wear/penetration, and the high strain-hardenability of the subsurface can withstand high deformation. This unique gradient microstructure in metallic materials assures excellent surface properties and shows an excellent strength-ductility combination which was unobtainable in homogeneous materials. Nonetheless, the unstable plastic deformation behavior of the hard coating may lead to poor ductility and formability of the material. Therefore, a new methodology for superior gradient structure is needed and was studied in the thesis. The main aims of the thesis consist of three goals. The first goal is that methodology to produce a strain-hardenable hard coating layer. A new strain-hardenable surface modification was developed via laser-cladding. Laser-cladding induces gradient structure due to thermal stress and sandwich structure consisted of coating/substrate layers. The hard coating with a large grain size shows high twinning activity. The enhanced twinning activity in the coating region increased strain hardening and generated dynamic mechanical incompatibility. The laser-cladding containing dynamically reinforce-\able heterogeneity introduced through texture-engineering was proposed. To increase the work hardening ability, the twinning favorable texture was fabricated. The textured laser-clad samples show superior strength and ductility combination. Metal additive manufacturing (AM) is an important technology to fabricate customized parts. Through AM, a maximum weight reduction strategy with topology optimization and near-net-shaped parts can be realized. Therefore, AM-processed materials with further superior mechanical properties are necessary to broaden the structural applicability. Improving the mechanical properties of AM parts can be realized through surface modification, especially in surface severe plastic deformation (SSPD). The second goal is obtaining AM parts with a superior gradient structure. A new strategy to obtain an improved gradient structure was proposed by combining annealing and ultrasonic nanocrystal surface modification (UNSM) to increase the efficiency of heterogeneity which meets the design criteria for heterostructures. The designed gradient structure induces predominant GNDs accumulation during deformation, which leads to significant HDI strengthening and strength-ductility synergy. The final goal of this thesis is to obtain superior tensile properties and formability properties. The usage of heterostructured materials is often limited in the industry because of poor formability. The high mechanical incompatibility between domains in heterostructured materials facilitates crack initiation and propagation during forming. To broaden the industrial applicability of heterostructured materials, it is necessary to balance their tensile properties and formability. The inverse-gradient structure was developed to be optimized for forming while maintaining superior tensile properties.