1997 TMS Annual Meeting: Wednesday Abstracts

FUNDAMENTALS OF GAMMA TITANIUM ALUMINIDES: Session V: Microstructure/Property Relationships--Fatigue, Fracture, and Damage Modeling

Session Chairpersons: Robert O. Ritchie, Dept. of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720; Kwai S. Chan, Southwest Research Institute, San Antonio, TX 78228-0510

8:25 am OPENING REMARKS

8:30 pm INVITED

FATIGUE AND FRACTURE OF TiAl PST CRYSTALS: Y. Umakoshi, Department of Materials Science and Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565 Japan

Orientation and temperature dependences of cyclic hardening, fatigue life and fracture mode of TiAl PST crystals were investigated. Deformation twins and vein-like structure containing high density of ordered dislocations were formed below 500°C depending on the type of ordered domain in the phase. The vein-like structure was responsible for strong cyclic hardening, while deformation twins were formed at the initial stage of fatigue and did not contribute the hardening. Formation of twins and vein-like structure was suppressed at 700°C and fatigue life rapidly decreased showing cyclic softening. Deformation twins produced extrusions on the specimen surface and played an important role for crack initiation and fatigue failure. Effect of additional third elements such as V and Nb on the cyclic hardening and fatigue life will also be presented.

9:00 am

FUNDAMENTAL ASPECTS OF FATIGUE FRACTURE IN TiAl ALLOYS: Kwai S. Chan, Southwest Research Institute, San Antonio, TX 78238; D.S. Shih, McDonnel Douglas Aerospace, St. Louis, MO 63166

The fatigue mechanisms in a TiAl alloy heat-treated to the lamellar and equiaxed microstructures were studied to determine the effects of microstructure on the initiation of microcracks and their subsequent growth into large cracks. The results indicated microcracks initiated at grain/colony boundaries and at slip bands. Most microcracks were arrested after nucleation, but a few grew at K below the large crack thresholds. The populations of non-propagating and propagating cracks varied with life fractions. Ligaments in the wake of a fatigue crack were prone to fatigue failure. The destruction of the crack-wake ligaments resulted in lower fracture resistance in materials under cyclic loading than those under monotonic loading. *Supported by the AFWL Materials Directorate through Contract No. F33615-92-C-5951.

9:20 am

MICROMECHANICS OF FATIGUE AND FRACTURE IN LAMELLAR TiAl: Bimal K. Kad, Robert J. Asaro, Department of Applied Mechanics & Engineering Science, University of California-San Diego, La Jolla, CA 92093

Finite element based numerical procedures, incorporating physically based crystal plasticity models, are employed to study the evolution of non-uniform deformation, under monotonic and fully reversed (R=-1) cyclic loadings, in lamellar TiAl microstructures. The impetus for such efforts is to gather fundamental insight into microstructure sensitive deformation mechanisms, and to extract additional information, not obtainable from traditional mechanical property measurements. Such an effort is particularly desirable to help track various aspects of plastic anisotropy of specific layers, and microconstituents as implicit in polycrystalline aggregates. Computational efforts are directed to address constant strain, as well as constant stress amplitude loading schemes in the low cycle fatigue regime. Irreversible deformation twinning effects are fully accounted for in the fatigue maximum of 100 cycles. We will present several examples of experimentally observed, and numerically computed results, to identify hot spots for strain localization in monotonic and fully reversed loadings, and prescribe microstructural remedies to alleviate such effects.

9:40 am

MICROCRACK NUCLEATION AND PROPAGATION IN TiAl: Zhe Jin, George T. Gray III, Los Alamos National Laboratory, Los Alamos, NM 87545

Microcrack nucleation and propagation in a PST-TiAl crystal and a duplex -TiAl alloy at high and quasi-static strain rates and temperatures from 196°C to 1200°C was investigated under compression loading. In PST crystals, two microcracking habit planes, the {110} planes for translamellar cracking and the (111) interface plane for interlamellar cracking, were observed when the <110> directions in the lamellar interfaces were perpendicular to the loading direction. However, three microcracking habit planes were observed when the <321> directions in the lamellar interfaces were perpendicular to the loading direction. Most microcracks were found to nucleate at/near the lamellar interfaces and propagate into the laths along their habit planes to form translamellar cracks. In the fine-grained duplex microstructure, both grain interior microcracks and grain boundary microcracks were observed. The grain interior microcracks occurred primarily within the equiaxed grains and appeared to be formed by shear displacements along the maximum shear planes. An attempt to correlate the microcrack formation with the crystal deformation modes and crystal orientations is made and the microcrack nucleation and propagation mechanisms in TiAl are analyzed.

10:00 am BREAK

10:10 am INVITED

DAMAGE MODELING OF GAMMA TITANIUM ALUMINIDES: M.J. Pfuff, B.U. Wittkowsky, GKSS-Forschungszentrum Geesthacht GmbH and SFB 371, Institut für Werkstofforschung, Max-Planck-Str., D-211502 Geesthacht, Germany

A common feature of Gamma Titanium Aluminides is the heterogeneous character of their microstructure. This heterogeneity causes failure processes to be progressive, with a continuous transition between a diffused and a localized mode of damage, the relative contribution of both modes to failure being determined by the degree of randomness of microstructural heterogeneity. At room temperature the failure is preceded by the formation of microcracks which control the strain to failure either by microcrack coalescing or by the nucleation of a critical-size crack. The size and local distributions of microcracks reflect the heterogeneity of microstructure, and depend on stress state, size and geometry of the specimens. The results of model calculations are presented, which are based on mesoscale lattice of beams models used in statistical physics in order to treat the relationships between microcracking and failure. A comparison of the numerical results to the experimental findings emphasizes the significant role microstructural heterogeneity plays during the failure process of Titanium Aluminides.

10:40 am

LARGE-SCALE MOLECULAR DYNAMICS SIMULATIONS OF THREE-DIMENSIONAL FRACTURE IN A -TiAl ALLOY: S.J. Zhou, D.M. Beazley, P.S. Lomdahl, B.L. Holian, Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545

We have carried out new large-scale molecular-dynamics (MD) simulations of fracture in a titanium-aluminum alloy, using the methods we developed in previous MD simulations, where we studied dislocation emission from a crack in a three-dimensional thin film of embedded- atom-method copper, comprising 35 million atoms. In that first study, we observed a variety of dislocation emission modes in a sample that was sufficiently thick that we could be sure that the results did not depend on the thickness. The sequence of dislocation emission in the crack blunting process strongly depends on the crystallographic orientation of the crack front and differs strikingly from anything previously conjectured. This finding is essential to establish a precise dislocation emission criterion (i.e. intrinsic ductility criterion). We will report on similar calculations for the TiAl system, with emphasis on the crack-tip plasticity and interaction between a crack and lamellar interfaces.

11:00 am

ATOMISTIC SIMULATIONS OF FRACTURE IN TiAl: Julia Panova, Diana Farkas, Department of Materials Science and Engineering, Virginia Polytechnic Institute, Blacksburg, VA 24061

Atomistic simulations of fracture in L1o TiAl were carried out using EAM interatomic potentials and molecular statics. We studied the behaviour of semi-infinite cracks under mode I loading in different crack-tip orientations. From the [1-10](111) crack tips we observed extensive dislocation emission, involving the formation of several planar faults, such as APB, CSF and SISF. We found that the edge dislocations with 1/2[110] Burgers vector that were emitted from [001](110) cracks were very compact and mobile. No planar fault formation was involved in the dislocation emission from this crack geometry. Cracks with [001](100) orientation were observed to cleave near the Griffith value of loading in a purely brittle manner.

11:20 pm

ON THE CORRELATION BETWEEN MECHANISMS OF PLASTICITY AND FRACTURE IN Ti-48 AT.% Al ALLOYS: J.M.K. Wiezorek, P.M. DeLuca, M.J. Mills, H.L. Fraser, Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43210

The technologically most relevant two-phase (-2) TiAl compounds contain a significant volume fraction of morphologically lamellar grains. Interestingly, a change in the predominant fracture mode has been observed from transgranular at room temperature to intergranular at elevated temperature for both equiaxed and duplex microstructures. It has previously been suggested that the segregation of alloying additions and/or impurities to grain boundaries during exposure at elevated temperature could be responsible for this change in fracture characteristics. However, the origin for the observed fracture mode change has not been ascertained to date. In this study binary Ti48at.%Al alloys of equiaxed, duplex and fully lamellar microstructure have been deformed in tension to various degrees of strain up to fracture, at both room and elevated temperature. The active fracture modes have been identified by SEM fractography. Moreover, the deformation modes active in these two phase TiAl compounds during tensile loading have been determined by TEM. Thin foils for the TEM characterization have been obtained from cuts perpendicular and parallel to the load axis at various locations in the gage sections as close as 10 µm from the actual fracture surface. The results of these investigations are discussed specifically with respect to the origin of the reported fracture mode changes, and the mechanisms fundamentally underlying the plasticity and fracture of these (-2) TiAl compounds. This work has been supported by a grant from the National Science Foundation with Dr. Bruce MacDonald as program manager.

11:40 am

FINITE ELEMENT ANALYSIS OF GRAIN BOUNDARY CAVITATION IN FULLY LAMELLAR TITANIUM ALUMINIDE INTERMETALLIC ALLOY: Anirban Chakrabort, James C. Earthman, Materials Science and Engineering, University of California, Irvine CA 92697

Creep constrained grain boundary cavitation in fully lamellar form of titanium aluminide intermetallic alloy has been studied using finite element techniques. Two different forms of fully lamellar models are considered. Cavitation is modeled in the presence of grain boundary sliding for the case of straight former grain boundaries and cavitation without grain boundary sliding is considered for fully lamellar microstructure with serrated former grain boundaries. Effect of interaction of cavitating facets on rupture life has been studied. A comparison between the fully lamellar forms and a dual phase equiaxed microstructure having the same phase ratio (2/) is also made to examine the relative susceptibility of these microstructures to high temperature damage.