Fracture Behaviour of an Advanced High Strength Multilayer Composite Consisting of Carbide-free Bainitic Steel and High Mn TWIP Steel

Abstract

It is well known that within materials science and engineering, the advancement of steels is subject to the conflicting objectives of achieving high strength, energy absorption, and ductility within a single material. Multilayer metal composites (MLMCs), combining multiple advanced high strength steels (AHSSs), are promising candidates for designing materials that can achieve these mechanical property combinations which are unattainable by monolithic steels. However, the mechanical behaviour and corresponding properties of MLMCs are challenging to predict, due to the number of variables within the design space of the composite. Variables such as alloy design, number, thickness, configuration of layers, and interfacial bonding strength, all impact the potential mechanical properties. Accordingly, this work addressed the fracture behaviour of a multilayer AHSS composite, consisting of carbide-free bainitic (CFB) steel and high Mn twinning-induced plasticity (TWIP) steel, in both sequential deformation and co-deformation of layers to determine the potential advantages of a multilayer structure. In tensile deformation, a balanced combination of high strength (ultimate tensile strength (UTS) of 1290 MPa) and high ductility (total elongation (TE) of 23%) was achieved with a sandwich structure configuration consisting of two outer layers of the TWIP steel and an inner core layer of the CFB steel. The composite consisted of equal volume fractions of each constituent steel. The TE achieved by this structure exceeds that which previous studies would predict, which suggest that the elongation of a composite is controlled by the elongation limits of the monolithic hard layer (which in the case of the CFB steel is 13%). In the sandwich configuration, the soft outer layers contributed to increased ductility of the composite by inhibiting crack formation in the hard layer and exerting a compressive stress on the inner CFB core. The increased compression caused the CFB to yield at a lower stress (than it would in monolithic conditions), allowing it to plastically deform further, and the composite to have a greater total elongation. This was attributed to the strong interfacial bond, which enabled the layers to co-deform without any delamination. A bilayer composite consisting of the same volume fractions (as the sandwich configuration), demonstrated the same UTS, but a total elongation of 13%. The reduced ductility is a result of smaller compressive forces on the CFB, as well as, crack formation in the CFB at the 13% elongation (the TE of monolithic CFB), which led to immediate fracture of the sample. In tensile deformation with a pre-existing crack (double-edge notched tension (DENT)), the bilayer composite exhibited a high essential work of fracture (EWF)/cracking resistance. In the sandwich configuration, the outer TWIP layers exerted a compressive stress on the inner CFB core, which was possible due to the strong interfacial bond. This compressive stress and the thin layer configuration caused the CFB core to fracture in a ductile manner. The impact energy absorption of the sample was investigated by Charpy impact testing, and the procedure of crack propagation analyzed by three-point bending. High energy absorption was achieved with a notch positioned in the TWIP layer, in which the composite exceeded the energy absorption of either monolithic steel. The sample absorbed the energy through plastic deformation of the two layers, as the interface prevented crack formation in the CFB layer. When the notch was positioned in the CFB layer, the impact energy absorption was nearly equal to that of the monolithic TWIP steel. In this configuration, the composite absorbed the energy through dissipation of the propagating crack along the interface, causing delamination and subsequent bending of the TWIP layer. In assessing the experimental results in this work, it was determined that in both deformation conditions (sequential and co-deformation), the composite is sensitive to the layer configuration. To produce an optimal and balanced combination of mechanical properties (strength, energy absorption, and ductility), it is critical to inhibit or at minimum, delay crack initiation within the CFB (hard steel) layer. Overall, this research shows that the experimental multilayer composite is promising for developing an AHSS structure that can demonstrate properties unattainable by monolithic steels.