Session: 12-12-02: Modeling of the Fracture, Failure, and Fatigue in Solids

Paper Number: 148552

148552 - Anomalous Crack Growth Resistance in Atomically Layered Ternary Carbides 

A family of atomically layered ternary carbides and nitrides, known as MAX phases, exhibits some of the best attributes of metals and ceramics. Like ceramics, they are lightweight, elastically stiff, thermodynamically stable, and refractory; and like metals, they are damage-tolerant, pseudo-ductile, and machinable. The unique set of properties of MAX phases is generally associated with their layered hexagonal crystal structure, which combines strong intralayer and weak interlayer atomic bonds. Mechanical tests on bulk polycrystalline MAX phases have shown that, unlike their counterpart binary carbides (MX), MAX phases undergo basal slip, cleavage, buckling of layers, and kink-banding, which endows these materials with unconventional damage tolerance. Micromechanical single-crystal level tests have shown that grain-level deformation and failure mechanisms of MAX phases depend on both the crystallographic orientation and deformation constraint of the grains.

Building on these recent works, we further analyze the crack growth resistance of these materials using small-scale notched cantilever specimens and crystal plasticity finite element analyses. The small-scale notched cantilever specimens used in this work introduce mode-I type loading at the notch tip. Intuitively, under mode-I loading, the notch opens and acts as a stress concentrator, leading to crack growth and eventual fracture in an isotropic elastic-plastic material. However, this work demonstrates that the effect of a notch in atomically layered MAX phases, characterized by strong intralayer and weak interlayer atomic bonds, is not always intuitive. Our experimental results reveal that despite the imposed loading promoting mode-I notch opening in all tests, the initiation and propagation of cracks depend on the crystallographic orientation.

Specifically, we conducted tests using specimens with the basal planes oriented parallel, perpendicular, or at an angle close to 45 degrees relative to the notch. The results show that when the basal plane is parallel to the notch, the mode-I loading promotes unstable crack growth parallel to the notch. When the basal plane is normal to the notch, crack growth is delayed, and it only partially grows normal to the notch. However, when the basal plane is inclined with respect to the notch, no crack growth occurs, and the cantilever specimen undergoes plastic deformation due to crystallographic slip.

The crystal plasticity finite element simulations, focusing on correlating the effects of crystallographic orientation and crack growth response of these materials, help rationalize the experimental observations. In particular, they show that even though the driving force for the crack to grow parallel to the notch is roughly the same for the scenarios where the basal planes are parallel or normal to the notch, the difference in the fracture toughness for these two orientations likely leads to the observed crack growth responses. Similarly, they show that when the basal plane is inclined with respect to the notch, the onset of easy crystallographic slip completely suppresses the driving force for crack growth. Thus, no fracture occurs for this orientation despite the presence of mode-I type loading at the notch tip.

Presenting Author: Ankit Srivastava Texas A&M University

Presenting Author Biography: Ankit Srivastava is an Associate Professor in the Department of Materials Science and Engineering at Texas A&M University.

Authors:

Ankit Srivastava Texas A&M University