Tailoring plasticity mechanisms in compositionally graded hierarchical steels fabricated using additive manufacturing

Abstract

While there exists in nature abundant examples of materials with site-specific gradients in microstructures and properties, engineers and designers have traditionally used monolithic materials with discrete properties. Now, however, additive manufacturing (AM) offers the possibility of creating structures that mimic some aspects of nature. One example that has attracted attention in the recent years is the hierarchical structure in bamboo. The hierarchical architecture in bamboo is characterized by spatial gradients in properties and microstructures and is well suited to accommodate and survive complex stress states, severe mechanical forces, and large deformations. While AM has been used routinely to fabricate functionally graded materials, this study distinguishes itself by leveraging AM and physical metallurgy concepts to trigger cascading deformation in a single sample. Specifically, we have been successful in using AM to fabricate steel with unique spatial hierarchies in structure and property to emulate the structure and deformation mechanisms in natural materials. This study shows an improvement in the strength and ductility of the nature-inspired “hierarchical steel” compared with conventional cast stainless steels. In situ characterization proves that this improvement is due to the sequential activation of multiple deformation mechanisms namely twinning, transformation-induced plasticity, and dislocation-based plasticity. While significantly higher strengths can be achieved by refining the chemical and processing technique, this study sets the stage to achieve the paradigm of using AM to fabricate structures which emulate the flexibility in mechanical properties of natural materials and are able to adapt to in-service conditions.

Thermodynamics of Composition Graded Thermoelastic Solids

“…It is worth observing the derivation of such equation does not follow either from experimental results or from kinetic modeling because both are lacking in the literature, but it is a theoretical postulate based on the assumption that changes in strain and temperature necessarily induce variation in the composition. In order to justify such an assumption, one should note that variable composition is related to the internal microstructure, such as, for instance, the presence of a grain structure, of dislocations, or of multiple metastable phases, leading to multiple deformation mechanisms which can be manipulated by controlling the local chemistry [29]. Then, to our view, time-dependent deformation and heating of such material necessarily lead to changes in time of microstructure, i.e., to a time dependency of the stoichiometric variable.…”

Part Tailoring in Metal-Additive Manufacturing: A Step towards Functionally Graded Customized Stainless-Steel Components Using Laser Powder Bed Fusion

“…Zou et al successfully demonstrated that by controlling the process parameters it is possible to control the grain orientation of parts printed using LPBF, thereby achieving tailored magnetic properties in Ni-Fe-based soft magnets [14]. Sridharan et al successfully demonstrated that using MAM with steel, it is possible to fabricate components with pre-defined spatial hierarchies that emulate natural bio-inspired structures that provide components with multi-functionality [15].…”