Materials Week '97: Wednesday AM 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 Wednesday morning, September 17.

GEORGE R. IRWIN SYMPOSIUM ON CLEAVAGE FRACTURE: Session V: Aluminides and Ceramics

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 Chairpersons: D.A. Koss, Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802; D.M. Dimiduk, Wright Laboratory, Wright-Patterson AFB, Dayton, OH 45433

OPENING REMARKS: Kwai S. Chan, Southwest Research Institute, San Antonio, TX 78238

8:30 am INVITED

FRACTURE AND FRACTURE TOUGHNESS OF DIRECTIONALLY SOLIDIFIED TiAl-BASE TWO-PHASE ALLOYS: N. Akiyama, S. Yokoshima, D.R. Johni, K. Kishida, H. Inui, and M. Yamaguchi, Dept. Materials Science & Engr., Kyoto University, Sakyo-ku, Kyoto 606-01, Japan

The lamellar microstructures in two-phase TiAl alloys are of special interest since the alloys in the lamellar form exhibit good fracture resistance at temperatures up to 800°C. We have introduced a new approach for studying the lamellar microstructures in two-phase TiAl alloys by producing polysynthetically twinned (PST) crystals with the lamellar orientation over the entire crystal by means of directional solidification techniques, and have been working to understand the fracture behavior of the lamellar structure in two-phase TiAl alloys at a fundamental level. We recently extended the fracture studies on PST crystals to ingots composed of columnar grains with the lamellar structure aligned parallel to the growth direction. In this paper, fracture surface morphology, toughness and toughening mechanisms of PST crystals and ingots composed of lamellar columnar grains are discussed as a function of microstructure characteristics, loading rate and environment.

9:00 am INVITED

DEFORMATION AND FRACTURE IN GAMMA TITANIUM ALUMINIDE ALLOYS UNDER MONOTONIC AND CYCLIC LOADING CONDITIONS: Young-Won Kim, UES, Inc., Dayton, OH 45432; Dennis M. Dimiduk, WL/MLLM, Wright Laboratory, WPAFB, OH 45433

Room and elevated temperature deformation and fracture behavior of gamma TiAl alloys was investigated for fine-grained duplex (DP) and large-grained fully-lamellar (FL) microstructures, under tensile and cyclic loading conditions. FL materials resulted in lower ductility and strength, but greater strength retention at high temperatures, and higher brittle-ductile-transition temperatures (BDTT) than DP structures. The Hall-Petch relationship with an unusually high constant exists for FL structures, resulting in strength levels for finer FL microstructures far exceeding those for DP structures. For both loading conditions at temperatures below the BDTT, the ductility was limited by the formation of strain incompatibility induced microcracks which grow to a critical size, comparable to the grain size, leading to cleavage fracture. At high temperatures, intergranular and interlamellar fracture became predominant, although transgranular cleavage fracture was persistent in FL materials. The high temperature deformation under high-cyclic loading was observed to take place by both glide as well as creep. The critical crack size for fatigue failure was related to the grain size, indicating that fine-grained FL materials are favored for enhanced fatigue resistance. In this paper, detailed, comparative analyses will be presented for the deformation and fracture characteristics of gamma TiAl alloys.

9:30 am

CLEAVAGE FRACTURE IN GAMMA-BASED TITANIUM ALUMINIDE INTERMETALLICS: W.O. Soboyejo, C. Mercer, and K. Lou; Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210-1179

The possible dislocation/microstructure interactions associated with the nucleation of cleavage fracture are elucidated for gamma alloys consisting of lamellar and equiaxed grains. Dislocation based models are thus proposed based on experimental (TEM) evidence of crack-tip deformation (by slip an/or twinning) prior to fracture by cleavage mechanisms. Cleavage fracture in duplex and lamellar gamma alloys is analyzed using classical initiation- and propagation-controlled fracture theories. The analyses and experimental evidence reveal the fundamental importance of plastic flow in the initiation of cleavage fracture. They also explain the Hall-Petch dependence of strength and fracture toughness that is observed in ductile alloys with controlled levels of Cr, Mn, and O.

9:50 am BREAK

10:00 am INVITED

FRACTURE IN B2 COMPOUNDS: P.R. Munroe, Materials Science and Engineering, University of New South Wales, Sydney, NSW205, Australia; and I. Baker, Thayer School of Engineering, Dartmouth College, Hanover, NY 03755

THIS PAPER HAS BEEN WITHDRAWN

10:30 am INVITED

MIXED-MODE CLEAVAGE FRACTURE OF AN IRON ALUMINIDE UNDER MONOTONIC LOADING: F. Robert Frasier, J.P. Hirth, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920

Polycrystalline 3-point bend specimens of an Fe₃Al alloy were K tested in a desiccated Ar atmosphere. Specimens were taken from a warm-rolled plate of nominal 6mm thickness and were fabricated in the L-T orientation. The measured K_lccwas found to be 31 MPa·. Fractographic examination of mode I specimens revealed that fracture had occurred by a complex mixture of mode I transgranular cleavage and mode III intergranular failure. No substantial evidence of ductile rupture was seen on the fracture surfaces. These results are unexpected for this alloy tested in a moisture-free atmosphere. A model for the state of stress responsible for these observations will be introduced, and the implications of these results will be discussed.

11:00 am

CLEAVAGE OF CERAMIC/MINERAL SINGLE CRYSTALS: R.C. Bradt, Dept. Met. & Mat. Eng., Univ. of Alabama, Tuscaloosa, AL 35487-0202

The criteria for cleavage planes in ceramic and mineral single crystals is reviewed from the traditional geometric perspective involving the structural elements of the various crystals with definitive cleavage planes. Crystal planes which exhibit near or quasi-cleavage are similarly addressed. Other historical criteria such as the bond density, the minimum elastic modulus, and a minimum surface energy are similarly considered, thus developing a structural sense as to why planes with a preferential character to cleave should exist within any crystal structure. Next, the rock salt structure is considered with respect to the {100} to {110} cleavage plane transition within that crystal structure. Finally, experimentally determined cleavage surface energies are compared with theoretically calculated surface energies in the alumina or sapphire structure to illustrate the minor role of plastic flow in these cleavages.

11:30 am

TENSILE FAILURE OF CERAMICS AT ELEVATED TEMPERATURES UNDER STATIC AND CYCLIC LOADING: K.J. Hsia, N. Dey, and D.F. Socie, Departments of Materials Science and Engineering, Theoretical and Applied Mechanics, and Mechanical Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801.

Failure of ceramics is usually brittle accompanied by little plastic deformation even at elevated temperatures. However, other micromechanisms such as flow of grain boundary phase may contribute to the overall fracture toughness. The present work studies experimentally the tensile failure of a ceramic containing a grain boundary viscous phase at 1000°C under static and cyclic loading. Under static loading, two failure mechanisms were observed: at high stresses, fracture is dictated by slow growth of a dominant crack along grain boundaries; whereas at low stresses, nucleation, growth and coalescence of multiple microcracks were observed. Fracture under cyclic loading was caused by growth of dominant crack. Interestingly, the lifetime under cyclic loading was one to two orders of magnitude longer (by about a factor of 30) than that under static loading for the same maximum stress. Extensive crack surface bridging by grain boundary viscous phase was observed, which is believed to be the major contribution to the strengthening under cyclic loading.

11:50 am

CLOSING REMARKS: Kwai S. Chan