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.

MICROSTRUCTURE PROPERTY RELATIONSHIPS IN / TITANIUM ALLOYS: Session II: Fatigue and Creep

Session Chairperson: Blair London, Materials Engineering Department, California Polytechnic State University, San Luis Obispo, CA 93407

THE INFLUENCE OF STRAIN RATE ON THE STRUCTURE/PROPERTY BEHAVIOR OF Ti AND Ti-ALLOYS: George T. (Rusty) Gray III, Los Alamos National Laboratory, Materials Research and Processing Science, Los Alamos, NM 87545

The high-strain-rate stress-strain response of titanium alloys is receiving continued attention related to design considerations for crash-worthiness and foreign-object damage in aerospace systems, ballistic and armor applications, high-rate forming, and high-rate machining. Interest in building more physically-based constitutive models to describe these processes utilizing Ti-alloys requires a knowledge of the coincident influence of temperature, strain rate, texture, and microstructure on the high-strain-rate mechanical response of Ti and Ti-alloys. While numerous experimental studies have probed the low-rate mechanical behavior of Ti and Ti-alloys, considerably less is known about the systematic effects of alloy content, interstitial content, microstructure, temperature, and texture on the high-strain-rate constitutive and fracture response of Ti-based alloys. In this paper the influence of strain rate, with emphasis on the high-rate regime, on the structure/property response of Ti and Ti-alloys will be reviewed. Substructure evolution in Ti and Ti-alloys as a function of strain rate will be discussed including the role of deformation twinning on plastic flow and fracture under high-rate, impact, and shock-loading conditions.

9:10 am

HIGH CYCLE FATIGUE PROPERTIES OF Ti-6Al-4V ALLOYS WITH EQUIAXED MICROSTRUCTURE: Svetlana G. Ivanova, Frederick S. Cohen, Materials and Mechanics Engineering, Pratt & Whitney, 400 Main Street, East Hartford, CT 01608; Ronald R. Biederman, Richard D. Sisson, Jr., Materials Science and Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609-2280

The high cycle fatigue properties were experimentally investigated for both forged and cross-rolled Ti-6Al-4V with equiaxed microstructure. The cross-rolled material exhibited significant crystallographic texture while the forged material showed little texture. Constant fatigue life diagrams for both cross-rolled and forged materials lie below Goodman linear relationship. The HCF strength is similar for both cross-rolled and forged materials at stress ratios between R=0.1 and R=0.8. However, in reverse loading (R=-1) cross-rolled material has approximately 10% higher HCF. The effect of texture on HCF strength will be presented and discussed. SEM investigation of fracture surfaces revealed that failure always initiated at the surface for both forged and rolled materials at all loading conditions, including at tensile mean stresses (up to R=0.8 stress ratios).

9:30 am

MICROSTRUCTURE AND FATIGUE PROPERTIES OF b-CEZ: J.O. Peters, G. Lütjering, Technical University Hamburg-Harburg, 21071 Hamburg, FRG

The influence of the microstructure on the fatigue properties of the high-strength -CEZ alloy was investigated by comparing three distinctly different microstructures: a lamellar (-annealed), a bi-modal (+-processed), and a necklace microstructure (processed through the -transus). The fatigue tests were performed in vacuum and in air. Comparing the fatigue properties of the three microstructures at the same yield stress level of 1200 MPa, the bi-modal microstructure showed the best HCF and LCF properties. Concerning fatigue crack propagation, small surface cracks (microcracks) exhibited the slowest propagation rate in the bi-modal microstructures, with the tendency that the lamellar microstructure had a slightly higher resistance against macrocrack propagation. The results of fatigue properties are explained by differences in the b-grain sizes and by differences in the crack front profiles of the three microstructures.

9:50 am

FATIGUE CRACK GROWTH BEHAVIOR OF TITANIUM ALLOYS UNDER GASEOUS ENVIRONMENT AT HIGH TEMPERATURE: C. Sarrazin-Baudoux, Y. Chabanne, J. Petit, Laboratoire de Mcanique et de Physique des Matriaux, URA CNRS 863-ENSMA Site du Futuroscope, Chasseneuil du Poitou, B.P. 109, 86960 FUTUROSCOPE CEDEX

The deleterious influence of ambient air on the fatigue crack growth resistance of most of the metallic alloys is now well established at room temperature and has been clearly related to the presence of moisture in the surrounding environment. At higher temperatures the respective role of water vapor and oxygen is more disputed, and these species act differently according to the alloy. This paper aims to put in light general trends about microstructure (and composition) on the fatigue behavior of a selection of titanium alloys (Ti-6246, Ti-6242, IMI 834) tested at temperature levels extending from room temperature to 500°C in ambient air and high vacuum. From data performed under environments with controlled amounts of water vapor and oxygen, including high vacuum and with closure correction, critical conditions (partial pressure, frequency, load ratio, mean load, maximum load...) for the occurrence of water vapor assisted corrosion-fatigue, creep-fatigue and stress corrosion are explored and supported by microfractographic observations.

10:10 am BREAK

10:30 am

THE INFLUENCE OF THE MICROSTRUCTURE ON THE DWELL-TIME FATIGUE PROPERTIES OF Ti-6242: R. Faber, M.E. Kassner, Department of Mechanical Engineering, Oregon State University, Corvallis, OR 97331; Y. Kosaka, J.E. Kosin, B. Bristow, S.H. Reichman, OREMET Titanium, Albany, OR 97321

An increasingly important property of Ti-6242 is favorable dwell-time fatigue life. This study investigated the influence of the microstructural features associated with changes in the annealing temperature below the beta transus on the ambient temperature dwell-time fatigue life. Determination of the cycles (of complete unloading) to failure were performed for a variety of sustained stress levels for each set of specimens annealed at various temperatures below T. These results will be discussed along with comparisons to the conventional fatigue properties of the alloy.

10:50 am

CREEP BEHAVIOR OF Ti-6Al-2Sn-4Zr-2Mo: R.W. Hayes, Metals Technology Inc., 19801 Nordhoff Street, Northridge, CA 91324; E. Landers , B. London, Materials Engineering Department, California Polytechnic State University, San Luis Obispo, CA 93407

The creep behavior of the Ti alloy Ti-6Al-2Sn-4Zr-2Mo has been studied over the temperature range 510 to 538°C at initial applied stress levels ranging from 345 to 414 MPa. Two microstructures, a lath type and a fine equiaxed type, were studied. Both stress increase and stress reduction creep experiments were performed. From the stress increase experiments, the activation energies and the stress exponents for the minimum strain rates were obtained. The values of the activation energies and the stress exponents for both microstructures are consistent with a dislocation motion controlled recovery process which describes structure evolution under the present creep conditions. In addition to the minimum strain rates, the primary transient creep of the equiaxed and lath microstructures was also studied. An activation energy based upon the primary transient time suggests that recovery processes may be important in primary creep of this alloy. A brief discussion of the transient creep response following a stress reduction will also be presented.

11:10 am

ORIENTATION EFFECTS AND THE ROLE OF / INTERFACES IN ROOM-TEMPERATURE CREEP BEHAVIOR OF TITANIUM ALLOYS: S. Suri, T. Neeraj, V. Babu, G.S. Daehn, M.J. Mills, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210; D.-H. Hou, TEM Laboratories, Micron Technology, Inc., Boise, ID 83706

Room temperature creep at low stresses in two phase /Ti alloys has been widely reported in the literature. However, little fundamental understanding of the mechanisms associated with this creep behavior in the / alloys exists. In this study, single colony crystals of a near -Ti alloy have been grown using a float zone technique. The colony crystals have been oriented so the different prismatic slip systems have the highest resolved shear stress. Significant anisotropy in creep behavior has been observed. An attempt has been made to explain the anisotropy by investigating the micromechanisms of slip transfer across the / interface using scanning and transmission electron microscopy. The role of interfacial dislocations in the creep behavior has also been studied. These results will be presented and discussed. This work is supported by the Air Force Office of Scientific Research, Dr. Charles H. Ward, Project Monitor.

11:30 am

PHENOMENOLOGICAL DESCRIPTION OF CREEP BEHAVIOR IN AND / TITANIUM ALLOYS USING CONSTANT STRAIN RATE TESTS: T. Neeraj, S. Suri, B. Viswanathan, G.S. Daehn, M.J. Mills, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210; D.-H. Hou, TEM Laboratories, Micron Technology, Inc., Boise, ID 83706

Titanium alloys exhibit creep at low homologous temperatures and stress levels below the yield strength. In this work an attempt has been made to correlate constant strain rate tests and creep response of these alloys. At low homologous temperatures, creep curves in Ti alloys follow a power law: =At^a. An analytical solution describing the creep strains from constant strain rate tests data has been developed. Constant strain rate and creep tests have been performed on both single phase and two phase / alloys. The analytical solution has been used to predict the creep curves for several model binary Ti-Al alloy systems and Ti-6242 will be presented and discussed.