Materials Week '97: Tuesday 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 Tuesday afternoon, September 16.

GEORGE R. IRWIN SYMPOSIUM ON CLEAVAGE FRACTURE: Session IV: Fracture Mechanisms

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: A.W. Thompson, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720; A.R. Rosenfield, Rosenfield and Rosenfield, Columbus, OH 43212

2:00 pm INVITED

OXIDE FILM EFFECTS ON CLEAVAGE: W.W. Gerberich, University of Minnesota, Minneapolis, MN 55455; N.R. Moody, Sandia National Laboratories, Livermore, CA; M.D. Kriese, University of Minnesota, MN

When cleavage nucleates from crack tips containing oxide layers or when time-dependent, subcritical crack growth occurs by a brittle cleavage or intergranular process, what is the role of an oxide film? The mechanism of stress corrosion cracking by a film rupture process has been postulated for some time. In addition, film-induced cleavage has been proposed more recently by Sieradzki and Newman. While either or both of these may exist, is there yet an even more general effect which must be considered? That is, either along with these mechanisms or in some cases in lieu of them, must we consider how oxide films affect dislocation emission which then becomes involved in shielding effects on cleavage or time-dependent failure? What we do know is that oxide films greatly affect the yield load occurring under a sharp indenter. Might this not also affect yield behavior at a crack tip since both represent sharp stress concentrators? For example, indentation induced yield excursions can vary by an order of magnitude in Fe-3wt%Si and tungsten with different oxide films. Plans for sorting this out are underway using companion experiments undergoing cleavage as well as indentation. Results on NiAl single crystals will be reported.

2:30 pm INVITED

SURFACE FILM SOFTENING AS A PROBLE OF CLEAVAGE FRACTURE: R. Gibala, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136

We have examined experimental data on the phenomenon of surface film softening of quasi-brittle metals and intermetallics in the context of cleavage fracture and its interaction with the intrinsic plasticity of these materials. Surface film softening is manifested as increased plasticity and, in many instances, as reduced flow stresses of a substrate material due to enhanced dislocation generation associated with film-substrate constraint under applied stress. Nominally ductile refractory metals such as Nb and Ta can be substantially ductilized by surface films to the point of exhibiting appreciable free strain. More brittle metals, such as Mo or W exhibit film-enhanced plasticity, some evidence of free strain, and associated changes in fracture mechanisms. Intermetallic alloys and compounds, e.g. NiAl, FeAl, and MoSi₂, can exhibit film-enhanced plasticity but usually without significant changes in fracture behavior.

3:00 pm

SHEAR-INDUCED CLEAVAGE FRACTURE: Kwai S. Chan, Southwest Research Institute, San Antonio, TX 78238

Experimental evidence shows that shear-induced cleavage fracture is prominent in many intermetallic alloys and in-situ composites, as well as in other brittle materials. In materials with a plastically deformable matrix, the characteristics of fracture generally include planar slip, slipband decohesion, and cleavage crack formation. In contrast, shear cracks with wing-tip cleavage cracks dominate in plastically nondeformable materials. In this article, the phenomenon of shear-induced cleavage fracture is revisited. Theoretical analyses of the driving force for the fracture process are summarized to illustrate the salient features of shear-induced cleavage fracture and to derive the critical condition for crack instability. Applications of the brittle fracture theory to treating shear-induced cleavage in TiAl alloys, Nb-Cr-Ti alloys, rock salt, alumina scale on overlay coating, and thermal barrier coating are presented with experimental verifications.

3:20 pm BREAK

3:30 pm

CLEAVAGE MECHANISM IN VANADIUM ALLOYS: G.R. Odette, E. Donahue, G.E. Lucas, Department of Mechanical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106-5070

The effect specimen geometry, loading rate and irradiation on the ductile-to-brittle transition in a V-4Ti-4Cr alloy were evaluated and modeled. Confocal microscopy-fracture reconstruction and SEM were used to characterize the sequence-of-events leading to cleavage, as well as the CTOD at fracture initiation. This alloy undergoes normal stress-controlled transgranular cleavage below a transition temperature that depends primarily on the tensile properties and constraint. Thus an equivalent yield stress model is in good agreement with observed effects of loading rate and irradiation hardening. Predicted effects of specimen geometry based on a critical stress-area criteria and FEM simulations of crack tip fields were also found to be in agreement with experiment. Some interesting characteristics of the fracture process are also described.

3:50 pm

A NEW TOOL FOR TOPOGRAPHIC AND CRYSTALLOGRAPHIC ANALYSIS OF CLEAVAGE FRACTURE SURFACES: C.O.A. Semprimoschnig, O. Kolednik, R. Pippan, Erich-Schmid-Institut für Festkörperphysik der Österreichischen Akademie der Wissenschaften, A-8700 Leoben, Austria

A new experimental technique is presented for the combined topographic and crystallographic analysis of cleavage fracture surfaces. The technique combines an automatic surface reconstruction system and the electron back scatter diffraction technique. The automatic surface reconstruction system uses stereo-image pairs (digital fractographs) taken in the scanning electron microscope to determine the 3-dimensional shape of a fracture surface region. The orientation of cleavage planes or lines on these planes with respect to a certain specimen coordinate system can be interpolated. The electron backscatter diffraction technique provides the crystallographic orientation of single grains or subgrains on the fracture surface with respect to the same coordinate system. The combination of both techniques enables us to measure crystallographic indices of cleavage planes as well as indices of directions on the planes. The technique can be also applied to detect a possible plastic deformation during the propagation of a cleavage crack. Different examples for the application of the new technique are presented.

4:10 pm

A FRACTOGRAPHIC ANALYSIS OF THE BRITTLE TRANSGRANULAR FRACTURE OF THE AUSTENITIC PHASE OF HIGH NITROGEN STAINLESS STEELS: J. Ivan Dickson, Ecole Polytechnique, Département de métallurgie et génie des matériaux, CP 6079, Montréal, Québec H3C 3A7, Canada; Jean-Bernard Vogt, Université Physique, B,t. C6, 59655 Villeneuve d'Ascq, France

The study consists of a crystallographic analysis of the brittle fracture planes of a Cr Mn austenitic s.s.(0.9%N) and of the austenitic phase of a / duplex s.s.(0.6%N). After fracture at low temperature, evidence of pure cleavage on {111} and {100} is pointed out according to the presence of the large and flat primary facets. The most frequently observed {111} facets contain numerous slip lines suggesting an important role of plasticity in the nucleation of cleavage cracks. "Non pure" cleavage (appearance of river lines on primary facets) and alternative cracking on {111} microfacets resulting in a {110} average orientation are also reported. Hydrogen embrittlement leads to cleavage on {100} planes for the a.s.s with a restricted contribution of plasticity. In the d.s.s., cleavage occurs on {100} planes after fracture at room temperature in the phase without any other orientations.

4:30 pm

BRITTLE FRACTURE IN HIGH-STRENGTH AUSTENITIC STAINLESS STEELS: Yu I. Chumljakov, I.V. Kireeva, E.I. Litvinova, V.I. Kirillov, and N.S. Surikova; Siberian Physical-Technical Institute, Revolution sq. 1, 634050, Tomsk, Russia

On austenitic stainless steels single crystals with stacking fault energy _st =0.02 J/m² the role of high deforming stress level reached by hardening with nitrogen, hydrogen, precipitate nitride particles has been investigated. Friction level, deformation mechanism-slip/twining have been shown to be essential factors for brittle fracture of high-strength crystals, and "ductile-brittle" transition. 1) Regardless of the certain way of achieving high strength and crystal orientation, there are critical values of deforming stresses _cr>=G/100 (G-shear modulus), under which at temperature below Tcr=300K brittle fracture by cleavage occurs on {111} planes. At T>_cr in high strength fcc crystals "ductile-brittle" transition is observed, which is typical for bcc crystals. 2) Decrease of deforming stress level _cr<G/100 leads to appearance of orientational dependence of fracture mechanism-in <111> twining at T<300K and brittle fracture, in <100>-slip and ductile fracture. 3) The role of hydrogen in fracture of high strength crystals has been investigated.

4:50 pm

INFLUENCE OF TiN PARTICLES AND MICROSTRUCTURE ON CLEAVAGE FRACTURE IN SIMULATED HAZs: L.P. Zhang, C.L. Davis, and M. Strangwood, School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom

Thermally stable TiN particles can effectively pin austenite grain boundaries in HAZs, improving toughness, but can act as cleavage initators. The effects of prior austenite grain size and matrix microstructure on the cleavage fracture of simulated HAZs in steel with two Ti levels (0.045 and 0.1 wt% have been investigated using two peak temperatures (T_p) and three cooling times (t_8/5) using a Gleeble 1500TCS. Coarse (1-6µm) and fine (35-400 nm) TiN particles were identified, whose mean size increase with increasing Ti content at constant number density. Higher T_p (1350(C vs. 1100°C) for the same cooling time gave austenite grain growth and decreased toughness significantly. However, increasing cooling time from 8 to 90s changed the matrix microstructure from bainite to ferrite-pearlite with little variation in toughness, although further investigation is required at lower Ti levels.