1997 TMS Annual Meeting: Monday Abstracts

MECHANICAL BEHAVIOR (General Abstract Session)

Session Chairperson: George T. Gray, III, Los Alamos National Laboratory, Los Alamos, NM 87545

8:30 am

INFLUENCE OF TEXTURE AND STRESS STATE ON THE CONSTITUTIVE RESPONSE OF AN AUTOMOTIVE STEEL: Carl M. Cady*, Shuh-Rong Chen, George T. Gray III, Los Alamos National Laboratory (LANL), MST-5 MS-G755, Los Alamos, NM 87545; John F. Bingert, David A. Korzekwa, LANL, MST-6 MS-G770, Los Alamos NM 87545

The relationships between the stress state, texture, revalue (Lankford parameter), and yield surface for a DQSK mild sheet steel have been analyzed. The strain rate and temperature sensitivity of the flow stress and the insensitivity of the strain hardening rate indicate that thermal activation over a Peierls barrier is the rate controlling mechanism for deformation in DQSK. The stress/strain response for DQSK steel is shown to be strongly influenced by strain rate and temperature which is consistent with this controlling mechanism. A calculated yield surface, using the Quadratic Hill criterion, is shown to produce an accurate correlation with the experimental results. Comparisons of the stress/strain response with the calculated yield surface and texture measurements will be shown to correlate with the relationships between the inplane and through thickness deformation.

8:50 am

THE MICROTEXTURE OF FRACTURE IN LITHIUM-CONTAINING ALUMINUM ALLOYS: R. Crooks, Analytical Services and Materials, Inc., NASA Langley Research Center, M/S 188A, Hampton, VA 23681-0001; P.N. Kalu, Department of Mechanical Engineering, FAM/FSU, Tallahassee, FL; A.P. Reynolds, Department of Mechanical Engineering, University of South Carolina, Columbia, SC

An Electron Back-Scattered Pattern (EBSP) system was used on a scanning electron microscope (SEM) to examine the crystallographic features of intergranular fracture in several lithium-containing aluminum alloys. The alloys examined have thin, flat grains with a "pancake" morphology, which tend to be highly elongated in the rolling direction. Metallographically prepared and electropolished samples of longitudinal sections were examined in the SEM where absorbed current images and diffraction data were collected through-thickness, with patterns indexed to the images. Delamination fracture was then induced in the samples by torsional loading, which often produced several cracks, and samples were returned to the SEM chamber to determine the crack locations relative to the previously acquired data. The results indicate that delamination occurs between grains near the S and R texture component orientations, which are known to form during deformation processing and as a result of grain boundary initiated recrystallization, respectively.

9:10 am

ELEVATED TEMPERATURE COMPRESSIVE SLOW STRAIN RATE PROPERTIES OF SEVERAL DIRECTIONALLY SOLIDIFIED NIAL-(NB, MO) ALLOYS: J. Daniel Whittenberger, Ronald D. Noebe, NASA-Lewis Research Center, Cleveland, OH 44135; Steven M. Joslin, B.F. Oliver, The University of Tennessee, Knoxville, TN 37996-2200

Three NiAl-based alloys containing 3Nb-lOMo, 5Nb-lOMo or 13.6Nb-18Mo (at. %) were directionally solidified (ds'ed) to develop interpenetrating networks of B2 crystal structure NiAl Laves NiAlNb and bcc metallic Mo. It was hoped that the Mo would improve the fracture toughness, while the Laves phase would promote good creep resistance. Examination of the alloys indicated that the as-grown structure consisted of essentially unalloyed NAL NiAlNb alloyed with ~8.5 Mo, and a Mo solid solution containing 27Nb-7Ni-7A1. Compressive properties were measured between 1200 to 1400 K in air with strain rates ranging from ~10-4 to ~10-8s-1. The flow strengths of the two alloys with 10Mo were nearly identical and much weaker than those for NAI-13.6Nb-18Mo. Comparison the properties of this latter alloy with other ds'ed NAl-based eutectics revealed that it was the strongest material under lower temperature/fast deformation conditions, but this advantage was lost at higher temperatures and/or slower strain rates.

9:30 am

DEFORMATION AND FAILURE MECHANISMS IN MATERIALS THAT DEFORM BY TWINNING AND MARTENSITIC TRANSFORMATION: Surya R. Kalidindi, R.D. Doherty, E. El-Danaf, S. Asgari, I Shaji, Department of Materials Engineering, Drexel University, Philadelphia, PA 19104

MP35N (35% Co, 35% Ni, 10% Mo, 20% Cr) exhibits an remarkable combination of ultrahigh strength, high ductility, high fracture toughness, high corrosion resistance, and it retains its room temperature strength upto about 600°C. This alloy hardens from 390 MPa (in the annealed state) to 1385 MPa after being 53% cold drawn and then to 1935 MPa on subsequent aging of the cold-drawn material at 600°C for four hours. Furthermore, the alloy was found to show a 50% reduction in area before failure in tension tests of the cold drawn and aged samples. The mechanisms of the primary and secondary hardening obtained through cold working and subsequent aging have been investigated in this study by optical and transmission electron microscopy investigations on deformed and deformed and aged samples. Some of the interesting observations made in our study include: (i) Plots of strain hardening rate versus stress (or strain) in simple compression of annealed MP35N indicate four distinct regimes of strain hardening which when suitably normalized were found to be in excellent agreement with the corresponding measurements in several other low stacking fault energy fcc alloys. (ii) The amount of secondary hardening in MP35N depended critically on the amount of plastic deformation imposed prior to aging. It was observed that a strain of about 0.2 was needed before secondary hardening could be detected and that the increase in strength by secondary hardening saturated at a strain of about 0.6. (iii) Simple compression deformation on samples that exhibited significant secondary hardening produced extensive micro-scale shear banding. The extent of shear banding increased with increased levels of secondary hardening and macro-scale shear bands were noted in compression of heavily deformed and aged samples. (iv) Measurements of plane strain fracture toughness revealed that the 53% cold-worked samples retained high toughnesses (about 120 MPa m112) even after the secondary hardening. The formation of shear bands noted above did not seem to effect either the fracture toughness or the ductility of the deformed and aged samples compared to the deformed only samples upto about 53% reduction levels in cold-drawing. Beyond about 60% reduction levels in cold-drawing, the ductility of the deformed and aged samples was observed to drop precipitously. A detailed investigation using the optical microscope and TEM was undertaken to explain the above observations. We are presently attempting to correlate the microscopy observations to the results from the mechanical testing. These results and our current understanding of the physical origin of the properties described above will be presented.

9:50 am

MECHANICAL BEHAVIOR OF PM U720 FORGED ALLOY AT ELEVATED TEMPERATURES: Chih-An Yin, and K. A. Green, Allison Engine Company, P. O. Box 420, S-52, Indianapolis, IN 46204-0420

Material characterization tests for tensile, creep-rupture, low cycle fatigue and fatigue crack growth responses were conducted on P/M U720 forged disc alloy subjected to a specified heat treatment. The purpose of this investigation is to examine the effects of the thermomechanical processes on the high temperature mechanical behavior of this material with a special attention to the creep-rupture, low cycle fatigue and fatigue crack growth rate. The results of analysis are mainly based on the phenomenological approach and predicted methods so that the quantitative comparison can be presented. in addition, the fracture morphologies of the material at 538°C and 650°C were also investigated by SEM and optical metallography to determine the relationships of failure modes, relative fatigue and creep-rupture life to the grain size, local microstructure and defect inclusions resulted from different consolidation processes.

10:10 am BREAK

10:20 am

STUDY AND COMPARISON OF (111) DEFORMATION MICROBANDS ASSOCIATED WITH IMPACT CRATERS AND PENETRATION CHANNELS IN THICK COPPER PLATES: L.E. Murr, J.M. Rivas, E.P. Garcia, C-S. Niou, and E. Ferreyra T., Department of Metallurgical and Materials Engineering, The University of Texas at El Paso, El Paso, TX 79968

While copper, having critical grain sizes greater than about 20 µm, will twin above a critical, plane-wave shock pressure of about 20 GPa, {111} deformation twins are essentially absent below impact craters or along dense rod penetration channels in copper plates (1.3 to 3.5 cm thick) where instantaneous (plane-wave impact) shock pressures exceed 100 GPa; for spheres and rods impacting at velocities between 1.5 and 6.7 km/s, with densities ranging from 2.2 to 17.6 g/cm3. However, deformation microbands consisting of elongated, dense dislocation wall structures (0.1 to 0.4, am wide), coincident with primary {111}planes; occur in bands or zones removed several millimeters from the crater or penetration channel walls, and predominantly in grains which exhibit little or no (plastic) distortion. These microbands possess misorientations ranging from 1 to 3° and appear to be created as a consequence of intense shear wave phenomena associated with these unique, high strain, high-strainrate deformation regimes. They also occur amongst dense, equiaxed, dislocation cells whose misorientations are <1°, and appear to be related to {111}deformation twins or twin faults which also emerge from or occur amongst, dislocation cells in plane-wave shock loaded copper, for example. Supported by NASA-JSC Grant NAG-9-481 and U.S. Army Contract DAAE30-94-C-0059.

THIS PAPER IS WITHDRAWN

10:40 am

EFFECTS OF HEAT TREATMENT AND MO CONTENT ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF FE-NI-CR-MO HIGH TEMPERATURE GEAR STEELS: Cha-Yong Lim, Sung-Joon Kim- Korea Institute of Machinery and Materials, #66 Sangnam-Dong, Changwon, Kyungnam, Korea 641-010

The effects of heat treatment and Mo content on microstructure and mechanical properties of high temperature gear steels have been investigated. High temperature gear steels require temper resistance, hot hardness, fracture toughness and impact strength for operation at elevated temperatures. The compositions (wt.%) of gear steels studied in this work were Fe-2.0Ni-l.0cr-(1~3.15)Mo-2.0Cu-0.4Mn-0.llC steels. The ingots manufactured by the vacuum induction melting were hot-rolled at 1473 K to 15mm thickness. The hot-rolled plates were quenched and tempered at various conditions. The degree of secondary hardening is increased with the Mo content due to the precipitation of fine alloy carbide (Mo2C) during tempering treatments. Continuous cooling transformation diagrams were obtained. High temperature mechanical tests (Hardness, Toughness, Impact energy) were conducted. Cylindrical compressive specimens of 8mm diameter and 12mm height were machined from the hot-rolled plates. High temperature compressive tests were carried out in an compression testing machine at various strain rates. The microstructure aRer deformation was investigated. The deformed structure was dynamically recrystallized at higher temperatures and lower strain rates. Activation energy and flow equation at high temperatures were discussed.

11:00 am

MICROSTRUCTURAL CHANGE DURING SUPERPLASTIC DEFORMATION IN LOW ALLOYED Zn-Al ALLOY: W.B. Lee, T.K. Ha, C.G. Park, Center for Advanced Aerospace Material, Univ. of Sci. & Tech., POSTECH, Pohang, 790-784, Korea

Microstructural change and deformation behavior have been observed in superplastic Zn-Al alloys. Row-alloyed Zn-Al alloys, which contains a small fraction of precipitates at grain boundary, showed superplastic deformation behaviors even at room temperature, while pure Zn is very brittle. In the present study, the effects of aluminum composition (up to 1 wt.%) on the change in the microstructure (the role of precipitates) as well as dislocation structure have been investigated during the superplastic deformation. TEM in situ deformation was also performed to investigate dislocation behavior at grain boundary during superplastic deformation of low alloyed Zn-Al alloys.

11:20 am

EFFECTS OF YIELD STRESS AND TEXTURE ON DELAYED HYDRIDE CRACKING IN Zr-2.5Nb PRESSURE TUBE: In Sup Kim, Je Yong Oh, Korea Advanced Institute of Science and Technology, Taejon 305-701, S. Korea

The delayed hydride cracking (DHC) in pressure tubes is caused by diffusion of hydrogen atoms into crack tip region, precipitation of hydride, and consequent fracture of the hydrided region under stress. To explain DHC quantitatively, many models have been proposed and the Puls model has been well recognized among them. In this study, the effects of yield stress and texture on DHC have been investigated in Zr-2.5Nb pressure tube material and experimental results were introduced and compared to Puls DHC model. The delayed hydride cracking velocity (DHCV) increased with yield stress, while the threshold stress intensity factor increased as yield stress decreased. The activation energy of DHCV was dependent on the texture. When the direction of crack propagation was longitudinal, the activation energy was 34kJ/mol. However, in the case of circumferential propagation, the energy was measured as 17kJ/mol.

11:40 am

DEVELOPMENT OF TEXTURE AND MICROSTRUCTURE DURING COLD ROLLING OF Ni3Al(B,Zr): B. Bhattacharya, R.K. Ray, Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur-208016, India

The present report deals with textural as well as microstructural developments during cold rolling of a two-phase Ni3Al(B,Zr) alloy. Basically a pure metal type texture was obtained at lower levels of deformation, while a mixed type (both pure metal and alloy type) was observed at high strain. The transition was detected after about 35% reduction which was correlated with a possible structural transformation in the /phase. The change of deformation mode from slip to twinning in was confirmed by electron microscopy. However, the second phase (Ni-base, disordered phase) in the alloy is not known to suffer any change in the deformation mode with straining. Therefore, it is suggested, that the textural development in the two phases will be of different nature; the combined effect will be reflected in the form of the textural change for the whole sample, as mentioned above.