100 Years and Still Cracking: A Griffith Fracture Symposium: Fracture in Complex Alloys
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Megan Cordill, Erich Schmid Institute of Materials Science; William Gerberich, University of Minnesota; David Bahr, Purdue University; Christopher Schuh, Northwestern University; Daniel Kiener, Montanuniverstität Leoben; Neville Moody; Nathan Mara, University of Minnesota; Erica Lilleodden, Fraunhofer Insitute for Microstructure of Materials and Systems (IMWS)
Monday 8:30 AM
March 15, 2021
Room: RM 47
Location: TMS2021 Virtual
Session Chair: David Bahr, Purdue University
8:30 AM
Introductory Comments: 100 Years and Still Cracking: A Griffith Fracture Symposium: Megan Cordill1; 1Erich Schmid Institute of Materials Science
Introductory Comments
8:35 AM Invited
Designing Ductility in BCC High Entropy Alloys?: Eleanor Mak1; Binglun Yin1; William Curtin1; 1Epfl Sti Igm Lammm
Some refractory BCC high-entropy alloys (HEA) can show exceptional high temperature strengths but are very brittle at RT. To understand trends in ductility versus composition, we propose a criterion for RT ductility based on intrinsic ductility. We postulate that ductility is limited by fracture, and that fracture occurs when a sharp crack tip is unable to blunt by dislocation emission prior to Griffith cleavage. That is, the critical stress intensity KIe for emission is larger than that for cleavage KIc. Using fracture mechanics, first-principles theory, and experiments Nb, V, Ta, Mo, and W, we identify a critical ratio for KIe/KIc below which alloys are ductile. Applied to a range of HEAs using calibrated properties from interatomic potentials, our criterion is in broad agreement with experimental trends and enables the identification of ductile composition ranges. Coupled with recent theory for strength, strong and ductile compositions can be identified.
9:15 AM
2,000 Years and Still Getting Dull: Mechanisms of Blade Chipping: Gianluca Roscioli1; S. Mohadeseh Taheri-Mousavi1; Cemal Tasan1; 1Massachusetts Institute of Technology
In order to achieve high hardness and wear resistance at a reasonable cost, blade steels are often engineered to have martensitic microstructures with high carbide content. Yet these materials become practically unusable upon interacting with materials more than 50 times softer. Considering a shaving process in detail, we have experimentally demonstrated that the stress imparted by the hair being cut is sufficient, under specific circumstances, to produce ductile fracture in the steel and chip the blade prior to appreciable wear. Using 3D parametric finite element simulations, we have shown that failure occurs when the combination of three separate factors occurs: (i) large mode III component in the stress, (ii) particular position of the hair relative to an asperity along the sharp edge, and (iii) presence of spatial variation of the heterogeneous lath martensite structure along the sharp edge. The change in failure mechanisms with increasing corrosion severity is also investigated.
9:35 AM
A Length-scale Independent Phase-Field Model for Quantitative Prediction of Ductile Fracture: William Huber1; Mohsen Asle Zaeem1; 1Colorado School of Mines
The phase-field approach for fracture in materials based on the Griffith theory, which features a diffusive approximation of the crack, has been recently extended to study ductile fracture. However, the existing models for ductile fracture show a severe dependence on the choice of the gradient length scale regularization parameter. The consequence of this effect is a diminished capability in quantitative prediction of the actual mechanical response of a material. To address this issue, we propose an improved phase-field model which considers an elastic strain energy threshold for softening through including the material properties related to fracture in the so-called degradation function. The extension to ductile fracture is made based on a plastic strain dependence on the damage threshold. The capabilities and the accuracy of this model are shown with numerical examples which are verified by analogous experiments.
9:55 AM
Quantitative Phase-Field Modeling of Crack Propagation in Multi-Phase Material Based on Griffith’s Fracture Theory: Arezoo Emdadi1; Mohsen Asle Zaeem2; 1Missouri University of Science and Technology; 2Colorado School of Mines
A quantitative multi-phase-field model based on the regularized formulation of Griffith’s fracture theory is developed to predict crack propagation in homogeneous and heterogeneous brittle materials. The accuracy and transferability of the model are demonstrated by simulating crack propagation and benchmarking against fracture experiments of concrete in the form of fracture of L-shaped plates and wedge splitting tests, and four-point bending of ZrB2-based laminates and fibrous monolithic composites. Additionally, the effective fracture toughness of ZrB2-based ceramics with different engineered microarchitectures are numerically evaluated and validated by experimental results. To demonstrate the capability of the proposed model for study the fracture of polycrystalline systems, intergranular and transgranular crack propagation in ZrB2 bicrystal and polycrystalline systems under tensile loading are studied in detail. The developed multi-phase-field model can quantitatively provide guidelines for designing microscale architectures and properties of different phases to promote crack deflection and increase the fracture toughness in multi-phase brittle materials.
10:15 AM Invited
On the Fracture of Multi-element Metallic Alloys: Bernd Gludovatz1; Robert Ritchie2; 1UNSW Sydney; 2Lawrence Berkeley National Laboratory
There are two classes of advanced metallic alloys of current interest with compositions which consist of multiple elements, namely bulk-metallic glasses and high-entropy alloys, the former alloys naturally being amorphous, the latter fully polycrystalline and often primarily single phase. Specific alloys in both categories display combinations of strength and toughness that represent some of the best on record, although the mechanisms underlying this behavior can be quite different. We will examine these toughening mechanisms underlying such exceptional damage-tolerance. In addition, we explore the highly variable nature of the measured toughness of bulk-metallic glasses, in terms of the fact that nonlinear-elastic fracture mechanics may be inappropriate for unambiguous measurement due to insufficient strain hardening, and the presence of unstable states of local order in the amorphous state that can have a profound effect on the nucleation of shear-bands, which represents the fundamental essence of plasticity, and hence toughness, in metallic glasses.
10:55 AM
On the Transition from Shear Banding to Fracture in Metals: In Situ Analysis of Plastic Flow and Deformation Fields: Shwetabh Yadav1; Harshit Chawla1; Dinakar Sagapuram1; 1Texas A&M University
Using direct high-speed imaging, we study a mode of failure in high-rate deformation of metals where localized plastic flow inside shear bands acts as a precursor to fracture. A 2D cutting configuration is used to impose large strain deformation under controlled rates of shear. A range of outcomes in the material behavior — homogeneous flow, highly localized shear band flow, partial fracture along the band, to complete material separation — is demonstrated using the same geometry by simply varying the shearing rate. Shear band formation, inhomogeneous strain field development, and subsequent dynamics of material separation and fracture are quantitatively characterized using particle image velocimetry. It is shown that a critical stress criterion governs the first transition from homogeneous flow to shear banding, while the evidence suggests localized flow to fracture transition follows a strain-based rule. Implications of plastic instabilities and flow transitions for material removal processes will be also discussed.
11:15 AM
Probing Small-scale Fracture and Plasticity in Quasicrystals and High-entropy Alloys: Yu Zou1; 1University of Toronto
In the first part of the presentation, we show that typically brittle quasicrystals can exhibit remarkable ductility of over 50% strains and high strengths of ∼4.5 GPa at room temperature and sub-micrometer scales. In contrast to the generally accepted dominant deformation mechanism in quasicrystals—dislocation climb, our observation suggests that dislocation glide may govern plasticity under high-stress and low-temperature conditions. In the second part of the presentation, we present that the fracture properties of high-entropy alloys (HEAs). Most refractory HEAs are brittle and suffer from limited formability at ambient temperature. Here, using in situ micro-cantilever tests, we show that the fracture toughness of a bi-crystal HEA is one order of magnitude lower than that of single crystalline ones. In addition, we have documented and described aspects of mud-cracking specific to Cr-containing electrodeposited HEA alloys made from trivalent Cr electrolytes.