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Session Chairperson: S.K. Varma, Dept. of Metallurgical and Materials Engineering, University of Texas at El Paso, El Paso, TX 79968
IDENTIFICATION OF CONSTITUENT PARTICLES IN 2024-T3 AND 7075-T6 ALUMINUM ALLOYS: Ming Gao, Robert P. Wei, Dept. of Mechanical Engineering and Mechanics, Lehigh University, 7 Asa Dr., Bethlehem, PA 18015; Jerry Feng, Metallurgy Division, Naval Research Laboratory, Washington, DC 20375
Constituent particles in 2024-T3 and 7075-T6 aluminum alloys were identified by analytical electron microscopy (AEM) to aid the understanding of particle-induced pitting corrosion. Convergent beam electron diffraction (CBED) was used, in conjunction with energy dispersive x-ray spectroscopy (EDS), for determining their structure and composition. Typical phases in these alloys (CuA12, CuMgAl2 and Fe4CuA123) were identified. In addition, a complex rhombohedral phase, with composition close to (Fe, Cu, Mn, Si)Al, was also identified. Detailed aspects of these particles are described, and their role in pitting corrosion are discussed. *Research supported by the Air Force Office of Scientific Research, Grant F49620-93-1-0426, and the Federal Aviation Administration, Grant 92-G-0006.
TEM STUDIES OF PARTICLE-INDUCED CORROSION IN 2024-T3 AND 7075-T6 ALUMINUM ALLOYS: Robert P. Wei, Ming Gao, , Dept. of Mechanical Engineering and Mechanics, Chi-Min Liao, Dept. of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015
To better understand particle-induced pitting corrosion in aluminum alloys, thin- foil specimens of 2024-T3 and 7075-T6 aluminum alloys, with identified constituent particles, were immersed in aerated 0.5 M NaCl solution and then examined by transmission electron microscopy (TEM). The results clearly showed matrix dissolution around the iron and manganese containing particles (such as, Fe4CuAl23). Matrix dissolution was also observed around CuAl2 particles, while CuMgAl2 particles tended to dissolve relative to the matrix. These results are consistent with previous SEM observations of pitting corrosion, and are discussed in terms of the electrochemical characteristics of the particles and the matrix. Research supported by the Air Force Office of Scientific Research, Grant F49620-93-1-0426, and the Federal Aviation Administration, Grant 92-G-0006.
RELATIONSHIP BETWEEN THE SOLUTIONIZED MICROSTRUCTURES AND THE TRANSIENT CURRENT RESPONSE FROM SCRATCH TEST EXPERIMENTS IN 6061 ALUMINUM ALLOY AND A COMPOSITE CONTAINING ALUMINA PARTICLES: Shane Andrews, S.K. Varma, Department of Metallurgical and Materials Engineering, The University of Texas at El Paso, El Paso, TX 79968-0520
The sensitivity of the scratch technique towards understanding the corrosion behavior of 6061 aluminum alloy reinforced with 0.1 volume fraction of alumina particles and in the monolithic form has been explored. Identical solutionizing treatment in the two materials shows different transient current responses. The depassivation and repassivation kinetics appear to differ considerably during the impact and continuous scratches since a balance between the two results in stabilized value of peak current during continuous scratching experiments only. The total charge density increases linearly with solutionizing time for both materials while grain growth law (square of the grain diameter is directly proportional to the solutionizing time) is valid for up to 25 hours of solutionizing time. Near surface microstructures have been examined with the help of TEM as well as SEM. This research has been supported by the National Science Foundation through the grant number HRD9353547.
SCC BEHAVIOR OF SENSITIZED ALLOY 600 LASER-SURFACE ALLOYED WITH Cr-NJ POWDER: Joung Soo Kim, Jin Kuk Shin, Sun Ki Woo, Jeong Hun Suh, Yun Soo Lim, I1 Hyun Kuk, Korea Atomic Energy Research Institute P. O. Box 105, Yusung, Taejun, Korea
In order to prevent and/or mitigate intergranular stress corrosion cracking(IGSCC) occurring in sensitized Alloy 600, the surface of the material was alloyed with Cr or Cr-Ni powder using a CO2 laser beam. The thickness of the alloyed region was measured to be around 200 250 µm. The Microstructure and the compositional variations of the alloyed layers were analysed with a transmission electron microscope (TEM) and analyzed using EDX attached to the TEM. The alloyed molten metal was observed to solidify epitaxially from the boundary between the melted and unmeted(matrix) regions, like the microstructure of sensitized Alloy 600 surface-melted by a CO2 beam. The compositional change in the alloyed layer was very small, i.e. very homogeneus. The surface-alloyed specimens were tested in a sodium tetrathionate (Na2S406) solution at room temperature using a CERT technique. IGSCC was completely prevented by the surface-alloying using a CO2 laser beam, i.e. the specimen was ruptured and the fracture surface was observed to consist of dimples, which is a typical morphology of specimen fractured by ductile failure(rupture). The relationship between the microstructure and the susceptibility to IGSCC will be discussed.
CORROSION-FATIGUE AND STRESS-CORROSION CRACK GROWTH IN PRECIPITATION-HARDENABLE STAINLESS STEELS: P.S. Pao, C.R. Feng, R.A. Bayles, Naval Research Laboratory, Washington, D.C. 20375; G.R. Yoder, Office of Naval Research, Arlington, VA 22217
The effect of load ratio on the corrosion-fatigue crack growth kinetics and the stress-corrosion cracking of the precipitation-hardenable stainless steels PH13-8Mo, 15-SPH, and 17-4PH in a 3.5% NaC1 solution were investigated. All three PH-class stainless steels exhibit good stress-corrosion cracking resistance with stress-corrosion cracking thresholds in excess of 70 MPa. These steels also demonstrate good corrosion-fatigue cracking resistance in a 3.5% NaC1 solution as the crack growth rates in a 3.5% NaC1 solution are less than twice the rates in ambient air. The corrosion-fatigue cracking thresholds progressively decrease as the load ratio (R) increases from 0.10 to 0.90. PH138Mo exhibits anomalous corrosion-fatigue crack growth kinetics and a concomitant change in fracture mode at R=0.90. The observed crack growth behavior, the deformation microstructure, and the significance of high load ratio corrosion fatigue will be discussed.
10:10 am BREAK
THE MICROSTRUCTURE AND PROPERTIES OF NITROGEN GAS ATOMIZED ALLOY 690: G.E. Fuchs, Lockheed Martin Company, P.O. Box 1072, Schenectady, NY 12301-1072
Alloy 690 exhibits excellent resistance to stress corrosion cracking (SCC) and is frequently used in steam generator applications. In order to achieve the desired microstructure for SCC resistance in A690, high temperature solution annealing heat treatments are required. These heat treatments, though, can result in extensive grain growth and relatively low strength. Powder metallurgy processing and the addition of nitrogen during atomization results in an alloy with significantly improved microstructural control and properties. This study examines the microstructure and properties of a nitrogen gas atomized A690 heat consolidated by HIP and extrusion. The effects of consolidation temperature and subsequent hot working is also discussed.
THE BRITTLE-TO-DUCTILE TRANSITION IN NiA1 SINGLE CRYSTALS: S. Shrivastava, F. Ebrahimi, Materials Science & Engineering Department, University of Florida, Gainesville, FL 32611
Intermetallic compound NiAl has the potential to be used as a turbine blade material for aerospace applications due to its low density, high thermal conductivity, and good oxidation resistance. However, it suffers from insufficient toughness at low temperatures. The purpose of the present study was to investigate the effects of displacement rate on the brittle-to-ductile transition (BDT) in NiAl single crystals. Double-notched-tensile specimens with  tensile axis were used for fracture toughness testing. The results of this study show a strong strain rate dependence of BDT temperature, however, the low temperature toughness level was insensitive to the applied displacement rate. Fracture paths of the specimens fractured at different temperatures and strain rates were analyzed and the crack initiation and propagation mechanisms were investigated. In this paper, the process of BDT in NiAl single crystal will be discussed in terms of thermally activated deformation processes and their effects on crack initiation and propagation.
PRECIPITATION BEHAVIOR OF GAMMA PRIME AND SIGMA PHASE OF HIGH STRENGTH STAINLESS STEEL FOR SEA WATER APPLICATIONS: Minoru Suwa, Hideto Kimura, Materials and Processing Research Center, NKK Corporation, 1 Kokancho, Fukuyama, 721, Japan
The effect of y'-Ni,(Ti, Al) and a phase precipitation on the mechanical properties and the corrosion resistance to sea water was investigated in Fe-(2025)%Cr-35%Ni-(4.5-6)%Mo-2%Ti-0.3%Alalloys. Though the hardness of the alloy increases after ' and a phase precipitation, pitting-corrosion resistance is deteriorated by a phase, while deterioration by ' phase precipitation is little. The precipitation of ' is found to occur almost simultaneously with a phase in 25%Cr-4. 5%Mo alloy. However, in 20%Cr-6%Mo alloy, the aging condition for ' phase to precede phase is found, which achieves both Vickers hardness of 320, and critical pitting temperature(CPT) of 323K.
MICROSTRUCTURE CHANGE IN SILICON NITRIDE DURING SUPERPLASTIC DEFORMATION: Naoki Kondo1, Eiichi Sato2, Fumihiro Wakai1, 1National Industrial Research Institute of Nagoya, Hirate-cho, Kita-ku, Nagoya, Aich 462, Japan; 2The Institute of Space and Astronautical Science, Yoshino-dai, Sagamihara, Kanagawa 229, Japan
Three dimensional (3-D) microstructures of =1.34 (280 % deformed) silicon nitride nitride specimen were calculated using the stereological analysis. The grain radius slightly increased during deformation. The aspect ratio remained almost constant up to = 0.88, and then, largely increased. The orientation angle decreased monotonously. A dominant deformation modes are considered to be grain sliding (= 0.88-1.34). Grain rotation and alignment also act as sub-dominant deformation mode all through the deformation.
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