Materials Week '97: Monday PM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Monday afternoon, September 15.

GEORGE R. IRWIN SYMPOSIUM ON CLEAVAGE FRACTURE: Session II: Dislocation and Atomistic Theories

Sponsored by: SMD Mechanical Metallurgy Committee, MSCTS Flow & Fracture and Computer Simulation Committees

Program Organizer: Kwai S. Chan, Southwest Research Institute, San Antonio, TX 78238

Room: 211

Session Chairs: V. Vitek, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104; David L. Davidson, Southwest Research Institute, San Antonio, TX 78238

2:00 pm INVITED

KINETICS OF THE CRACK-TIP-GOVERNED BRITTLE TO DUCTILE TRANSITIONS IN INTRINSICALLY BRITTLE SOLIDS: A.S. Argon, G. Xu, Massachusetts Institute of Technology, Cambridge, MA 02139; M. Ortiz, California Institute of Technology, Pasadena, CA 91125

Several activation configurations of dislocation embryos emanating from tips of (001) cleavage cracks in alpha iron at the verge of propagating, have been analyzed in detail by the variational boundary integral method as central elements of the rate controlling process of nucleation governed fracture transitions from brittle cleavage to tough forms, as is expected to be the case for BCC transition metals. The configurations include those on inclined planes, oblique planes and crack tip cleavage ledges. Surface ledge production resistance is found to have a very strong embrittling effect. Only nucleation of dislocation embryos on oblique planes near a free surface and at crack tip cleavage ledges are found to be energetically feasible to explain brittle-to-ductile transition temperatures in alpha iron, in the experimentally observed ranges.

2:30 pm INVITED

MODELLING CRACK TIP PLASTIC ZONES AND BRITTLE-DUCTILE TRANSITIONS: P.B. Hirsch, S.G. Roberts, Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK

Models based on crack tip shielding by arrays of interacting dislocations on a single slip plane, representing the plastic zone, have been developed to explain the sharp and gradual brittle-ductile transitions in different materials. Sharp transitions occur when the density of crack tip sources is small, and the transition temperature then depends sensitively on the source density. Gradual transitions occur when the source density is large. For both types of transition the strain-rate dependence is controlled by dislocation velocity (which is related to yield stress). The application of this model to experiments on the brittle ductile transition of a number of materials will be described. Extensions of the model to cases, such as steels, where the fracture event may occur ahead of the main crack will also be discussed.

3:00 pm INVITED

A NEW MODEL OF THE BRITTLE-TO-DUCTILE TRANSITIONS BASED ON A COLLECTIVE DISLOCATION GENERATION INSTABILITY: Robert H. Folk, II, Steven M. Labovitz, and David P. Pope, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104

We have proposed a model of the brittle-to-ductile (BDT) transition based on a new cooperative mechanism of dislocation generation. In the existing models, the BDT is assumed to be either controlled by the nucleation of a single dislocation or the mobility of a group of dislocations. The effects of temperature enter these models only via thermally activated generation or motion of dislocations. In contrast, the model advanced in this work suggests that the BDT corresponds to a combined thermal-and-stress induced cooperative instability of a large number of dislocation loops. The analysis is carried out in the framework of statistical mechanics and is closely related to the well-known Kosterlitz-Thouless type dislocation instability in two dimensions. The new model not only identifies the specific role of the crack tip in the BDT but also suggests that a sudden onset of plasticity at a certain temperature is possible in crack-free crystals.

3:30 pm BREAK

3:40 pm INVITED

INSIDE THE CRACK TIP: Michael Marder, Department of Physics and Center for Nonlinear Dynamics, The University of Texas at Austin, Austin, TX 78712

This talk will discuss experimental, numerical, and analytical studies of cleavage fracture, with special attention directed towards consequences at the macroscopic scale of atomic detail. I will show that a range of crack velocities is forbidden in brittle materials at low temperatures because of atomic effects, and that the ultimate speed of cracks is limited by dynamical instabilities. These claims will be illustrated in experiments on amorphous and crystalline materials, explained by exact solution of idealized models, and backed up by computer simulations designed to allow comparison of large and small scales.

4:10 pm

ATOMISTIC SIMULATIONS OF FRACTURE: Diana Farkas, Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

Embedded atom interaction potentials are used to simulate the atomistic aspects of the fracture process. Simulations are presented for the behavior of cracks in pure metals and intermetallics, near the Griffith condition. The materials considered include Fe, Cu, Ni as well as Fe, Ni, Co and Ti aluminides. The work focuses on the comparative study of fracture behavior in the different materials. The role of the atomic relaxation at the crack tip and of lattice trapping phenomena is analyzed.

4:30 pm

EFFECT OF IMPURITIES ON CLEAVAGE FRACTURE: H, C, B, AND S IN Ni₃Al: Nicholas Kioussis and G. Lu, Department of Physics, California State Univ. Northridge, Northridge, CA 91330-8268; M. Ciftan, US Army Research Office, Research Triangle Park, NC 27709; A. Gonis, Lawrence Livermore National Laboratory, Livermore, CA 94550

The Ll₂ intermetallic Ni₃Al exhibits unique mechanical properties that make it attractive for high temperature structural applications. Understanding the origins in the electronic structure of the impurity-induced strengthening or impurity-induced environmental embrittlement in Ni₃Al is of practical importance. The effects of hydrogen, carbon, boron, and sulfur impurities on the ideal cleavage fracture properties of Ni₃Al under tensile stress are investigated using total-energy full-potential electronic structure calculations with a repeated slab arrangement of atoms simulating an isolated cleavage plane. Results for the stress-strain relationship, cleavage energies, ideal yield stress and strains with and without impurities are presented, and the electronic mechanism underlying the contrasting effects of impurities on the ideal cleavage of Ni₃Al is elucidated. *Supported by US Army Research under contract No. DAAH04-93-G-0427.

8:00 pm GEORGE R. IRWIN SYMPOSIUM BANQUET LECTURE: INVITED

GEORGE R. IRWIN: THE MAN WHO SHOWED ENGINEERS HOW TO PUT FRACTURE TO WORK: H.P. Rossmanith, Institute of Mechanics, Technical University Vienna, International Society for Technology, Law & Technical Insurance, ISTLI Secretariat General Vienna, Wiedner Hauptstr. 8-10/325,A-1040 Vienna, Austria

The lecture presents a short biography of the Father of Fracture Mechanics whose professional life has been entirely devoted to the development and application of the field of fracture research. First, the general scientific and engineering setting of the time of his conversion from "journalism" to engineering will be illuminated. His appointment to the Naval Research Laboratory and the early work on armor followed by the path-breaking early papers on the basics of fracture mechanics and ensuing pilgrimage of scholars from all over the world to the NRL and, later, to Lehigh University and the University of Maryland. Some of the key developments in fracture mechanics and the decisive contributions by Professor George R. Irwin will be highlighted in this presentation. The sheer incomprehensible task of being an outstanding scientist and engineer, a very much sought after consultant for more than 50 years, an exemplary and beloved husband, father and head of a large family, a helpful and advisory professional colleague, a partner in sports and leisure and a superb teacher and promoter of the young and students in engineering, can only be achieved and flourish in a family setting created and tended by his charming and wonderful wife Georgia and his extremely helpful and caretaking children. The presentation will close with a few bonmots collected by the friends of the jubilant.