1997 TMS Annual Meeting: Monday Abstracts

PHYSICAL METALLURGY: FUNDAMENTALS AND APPLICATIONS (General Abstract Session)

Room: 232B

Session Chairperson: Warren M. Garrison, Dept. of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

8:30 am

EFFECT OF COLD WORKING ON THE OMEGA PHASE IN BETA TITANIUM ALLOYS: M.Saqib, N. Stefansson, R. Srinivasan, I. Weiss, Department of Mechanical and Materials Engineering, Wright State University, Dayton, OH 45435

High specific strength, excellent corrosion resistance, good cold formability and age hardening response are some of the attractive properties of the metastable titanium alloys. The effect of cold deformation on the stability of phase present in metastable alloys is not clearly understood. In this study, two titanium alloys: TIMETAL 21S and TIMETAL LCB were fast cooled from above the -transus temperatures and subsequently cold rolled to total reductions ranging from 5 to 35%. The fast cooled microstructure consisted of phase with dispersion of athermal precipitates. Selected area electron diffraction patterns from the as-cooled and the cold rolled specimens revealed the presence of diffuse intensity maxima at 1/3<112> and 2/3<111> locations, characteristic of the presence of phase. The average integrated intensities of the diffuse maxima from the phase and the sharp diffraction spots from the phase were measured using microdensitometer. The intensity ratio; (diffuse intensity) /(diffraction spots intensity) was used as a measure of the relative volume fraction of the phase in the phase matrix. The volume fraction of the phase was found to decrease linearly with an increase in the amount of cold deformation both alloys.

8:50 am

GRAIN BOUNDARY NIOBIUM-RICH CARBIDES IN INCONEL 718*: Ming Gao, Robert P. Wei, Dept. of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015

In a previous study, niobium was identified as the principal embrittling element for oxygen-enhanced crack growth in nickel based alloys, such as Inconel 718. Niobium was believed to come principally from the oxidation and decomposition of niobium-rich carbides at the grain boundaries. To better establish their type density and distribution, g.b. carbides in Inconel 718 that were exposed by crack growth at 700°C or by hydrogen embrittlement were characterized by SEM and EDS. The carbides were also analyzed by high-resolution AEM. Two types of carbides were identified; those associated with the primary NbC, and those (secondary carbides) that formed during heat treatment. The results will be presented and discussed in relation to environmentally assisted crack growth. Research supported by the National Science Foundation, Division of Materials Research, under Grant DMR-9102093.

9:10 am

THE INFLUENCE OF CARBON LEVEL ON SULFIDE PARTICLE DISTRIBUTIONS IN TITANIUM MODIFIED STEELS OF THE AF1410 TYPE: Luana E. Iorio, Warren M. Garrison, Jr., Dept. of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

Earlier studies showed that small titanium additions to the 0.10 wt. % carbon steel HY180 lead to the gettering of sulfur as particles of Ti2CS. The Charpy impact energy of HY180 when sulfur is gettered as MnS is about 160J but increases to about 270J when sulfur is gettered as Ti2CS. The improvement in toughness is attributed to the particles of Ti2CS being more resistant to void nucleation than particles of MnS. The effects of small titanium additions on sulfide and other second phasae particle distributions and the toughness of AF1410 steel at carbon levels of 0.16 wt% and 020 wt% are being investigated. The results indicate that titanium additions lead to a substantial improvement in toughness at a carbon level of 0.16 but the improvement in toughness is less at a carbon level of 0.16 but the improvement in toughness is much less at a carbon level of 0.2. The objective of the work reported here is identification of sulfide type and measurement of sulfide size in these two higher carbon steels. Because of the large number of possible sulfides of titanium the major emphasis has been on identifying sulfide type from transmission electron microscopy diffraction results.

9:30 pm

HIGH TEMPERATURE PHASE STABILITY OF THE YTTRIA STABILIZED t' PHASE ZIRCONIAS: James S. Smith, Fu-Jen Pan, Jan-Fong Jue, Materials and Systems Research Inc., 1473 S. Pioneer Rd., Salt Lake City, UT 84104; Kevin T. Zysk, AEDC/DOT, 1099 Avenue C, Arnold Air Force Base, TN 37389-9011

Yttria doped zirconias containing 3 to 8 mole% stabilizer were fabricated by (i) sintering at 1450°C and (ii) heat treating at 2100°C for 15 minutes with a gas fired furnace to obtain (i) regular tetragonal (t) and cubic (c) phase, and (ii) largegrained tetragonal (t') and c phase materials, respectively. The materials were annealed at temperatures between 1100°C and 2000°C in air for various times. The effect of annealing time and temperature as well as cooling rate on the phase stability of the t' phase zirconia will be discussed. A working T-T-T diagram will be proposed. High temperature bending strength and compressive strength data will be presented. The possibility of precipitation strengthening in the annealed samples will also be addressed.

9:50 am BREAK

10:00 am

PHASE STABILITY IN NbCr2-BASED LAVES PHASE ALLOYS: J.H. Zhu, P.K. Liaw, Department of Materials Science and Engineering The University of Tennessee, Knoxville, TN 37996; C.T. Liu Metals and Ceramics Division Oak Ridge National laboratory Oak Ridge, TN 37831-6115

Phase stability in NbCr2-based transition-metal Laves phases is studied in this paper, using data from binary X-Cr, Nb-X, and ternary Nb-CrX phase diagrams. It was shown that when the atomic size ratios are kept identical, the average electron concentration factor (e/a = the average number of electrons per atom outside the closed shells of the component atoms) is the determinate factor in controlling the phase stability of NbCr2-based transition-metal Laves phases. The e/a ratios for different Laves phase structures were determined as follows: with e/a < 5.76, the C15 structure is stabilized; at an e/a range of 5.88-7.53, the C14 structure is stabilized; with e/a > 7.65, the C15 structure was stabilized again. A further increase in the electron concentration factor (e/a > 8) leads to the disordering of the alloy. The electron concentration effect on the phase stability of transition-metal A3B intermetallic compounds and Mg-based Laves phases is also reviewed and compared wit'n the present observations in transition-metal Laves phases. Finally, experimental results on Cr-Nb-Cu ternary system were also presented and explained in light of the e/a ratio correlation with phase stability.

10:20 am

THE MARTENSITIC TRANSFORMATION CURVES FOR SHAPE MEMORY ALLOYS: H.Y. Yu, Naval Research Laboratory, Mechanics of Materials Branch, Code 6380, Washington, D.C. 20375-5343

The mechanical behavior of shape memory alloys is closely related to the martensitic phase transformation. A martensitic transformation is considered to be a first order solid state structural transformation that is diffusionless and whose initial phase maintains metastability. Conventional models, such as the "exponential model" and the "cosine model" do not describe the martensitic transformation satisfactorily. In this study, the martensitic transformation curves for thermoelasaticity and pseudoelasticitiy are obtained by using the principle of energy balance. The results are in good agreement with experimental results for steels and nitinol alloys.

10:40 am

RULE OF ADDITIVITY IN PHASE TRANSFORMATIONS: David T. Wu, Dept. of Mechanical Engineering, Yale University, P.O. Box 208284, New Haven, CT 06520

The rule of additivity is used to predict CCT diagrams from TTT diagrams. Avrami showed that a nucleation and growth reaction is additive if it is isokinetic. Later Cahn proposed site-saturation as another condition for additivity to hold. This talk presents more general conditions under which the rule of additivity is valid.

11:00 am

REVISITING THE EFFECT OF CURVATURE ON CHEMICAL POTENTIAL FOR BINARY SYSTEMS: N. Kulkarni, R T. DeHoff, 253A Rhines Hall, Depas1ment of Materials Science and Engineering, University of Florida, Gainesville, F1-32611

The effect of curvature on the chemical potential for binary systems, at constant temperature and pressure, is usually given in the literature by the Gibbs-Thomson equation. Many forms of this equation have appeared in the literature, that can be shown to be incorrect and in violation of the condition for chemical equilibrium for a two-phase system. A simple self-consistent procedure to derive the Gibbs-Thomson equation for binary systems is provided. The resultant equation for the chemical potential shift is independent of the solution model and has an inverse dependence on the width of the two-phase field.

11:20 am

METRIC AND TOPOLOGICAL CONTRIBUTIONS TO THE RATE OF CHANGE OF BOUNDARY LENGTH IN TWO DIMENSIONAL GRAIN GROWTH: R.T. DeHoff, Department of Materials Science Engineering, 253A Rhines Hall, University of Florida, Gainesville, Fl-32611

The boundary in a two dimensional grain structure decreases its length during grain growth. Four distinct processes may contribute to this length change: 1) The smooth motion of the grain edges toward their centers of curvature; 2) Migration of cell corners resulting from the edge motion; 3) The annihilation of small grains (topological process b); 4) The switching process (topological process II). Using von Neumann's assumption that the local velocity is proportional to the local boundary curvature and geometrically general kinematic equations, the contribution of each of these four processes is assessed The result is a kinetic equation in which the metric and topological contributions to the rate of grain growth in two dimensions are made explicit.