Materials Week '97: Monday 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 Monday morning, September 15.

PHASE TRANSFORMATIONS: General Abstract Session

Session Chair: Mukul Kumar, John Hopkins University, Laurel, MD 20723

CONTINUOUS COOLING OF CAST NICKEL ALUMINUM BRONZE (NAB): T.A. Marsico¹, P.R. Howell², ¹The Applied Research Laboratory; ²The Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802

Cast NAB was continuously cooled from 1029°C to 500°C. The first phase to form during continuous cooling of NAB from 1029°C was proeutectoid a as grain boundary allotriomorphs at 989°C. By 823°C, planar interfaces has developed between the and the proeutectoid a phases. The proeutectoid a was found to grow via the ledge mechanism. The k_ii and k_iv phases were first observed at 832°C. The k_iii phase was first observed at 823°C which marked the start of the eutectoid reaction during continuous cooling. In contrast to what is observed during isothermal transformation, the k_iii phase is present on the is side of the a/ interphase interface. This was the first time the very early stages of the eutectoid reaction is continuously cooled NAB was clearly documented. At 823°C, both discrete and lamellar k_iii were observed. A model describing the genesis and evolution of the eutectoid reaction in continuously cooled NAB will be presented. EPMA x-ray maps and concentration profiles indicated that a certain level of solute, primarily in terms of Ni and Al contents, must be achieved before cooperation can occur. The NAB eutectoid reaction product was found to grow via ledge mechanism and found to multiply via branching. Also clearly evidenced at 823°C was the encapsulation of k_ii by k_iii in contrast to the previous belief that k_ii provided a nucleation site for k_iii. At 768°C, precipitate free zones (PFZ's) were found to form around the peripheries of the proeutectoid a "grains". Also observed at 768°C was the impingement of proeutectoid a "grains" and the sympathetic nucleation of a onto a phase. From 768°C to 500°C, the proeutectoid a phase and the eutectoid reaction product, a+k_iii, continued to grow at the expense of the phase. At 500°C, martensite was still observed in room temperature microstructures indicating the eutectoid reaction did not have time to reach completion.

8:50 am

ISOTHERMAL TRANSFORMATION OF CAST NICKEL ALUMINUM BRONZE (NAB): T.A. Marsico¹, P.R. Howell², ¹The Applied Research Laboratory; ²The Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802

Cast NAB was isothermally transformed at temperatures from 1000°C to 500°C. A semiquantitative TTT curve for NAB was generated. NAB was fond to solidify as 100% phase. However, the 100% phase field was found to be very narrow. The proeutectoid a reaction was impossible to suppress even during rapid quenches from the melt. The proeutectoid a phase was found to form on grain boundaries, as allotriomorphs, at 1000°C. The morphology of the a phase was found the follow the DubÇ classification. The k_ii and k_iv phases were first observed at 880°C. The k_ii phase was always found to nucleate in the phase while the k_iv phase was always found to nucleate in the a phase. At 880°C, the encapsulation of the k_ii phase by the growing a phase was observed. This was the first time this process was clearly documented. A model describing this process was presented. The k_iii phase was first observed at 815°C which marked the start of the eutectoid reaction in NAB. For the first time the early stages of the eutectoid reaction in NAB were documented. The k_iii phase was directly viewed forming on the side of the a/ interphase interface. A model describing the very early stages of the eutectoid reaction in NAB was presented. At 815°C, the eutectoid reaction was mostly discrete but became lamellar as the transformation temperature was lowered. Also observed at 815°C was the formation of the "k_iii films". This was the first time "k_iii film" formation was observed and documented. A model describing the formation and growth of these films was presented. The sympathetic nucleation of a onto a was observed and discussed. This was the first time sympathetic nucleation of proeutectoid a in NAB was clearly observed. At 500°C, no martensite was observed in the room temperature NAB microstructures indicating the eutectoid reaction had reached completion.

9:10 am

CHARACTERIZATION OF CONCENTRATION PROFILES WITH AVERAGE EFFECTIVE INTERDIFFUSION COEFFICIENTS AND CHARACTERISTIC DEPTHS: Y.H. Sohn, M.A. Dayananda, Purdue University, School of Materials Engineering, 1289 MSEE Building, West Lafayette, IN 47907-1289

Concentration profiles of isothermal multicomponent diffusion couples can be asymptotic or exhibit various features including relative maxima and minima in concentrations, zero- flux planes and flux reversals for individual components. Selected isothermal diffusion couples in the Cu-Ni-Zn and Ni-Cr-A1 systems that develop these features are examined on the basis of average effective interdiffusion coefficients D_i^eff and characteristic depth parameters d_i calculated directly from the concentration profile of component i on either side of the Matano plane. These coefficients and parameters are related to the composition at the Matano plane. The ratio of D_i^eff to d_i determined on one side of the Matano plane is compared with the corresponding ratio determined on the other side in order to characterize the various types of concentration profiles. For asymptotic concentration profiles the ratio of D_i^eff to d_i evaluated on the higher concentration side of the Matano plane is greater than that for the lower- concentration side. For a component developing a zeroflux plane (ZFP), the D_i^eff to d_i ratio determined on the ZFP side of the couple is lower than that for the other side. The D_i^eff to d_i ratios are observed to be negative for both sides of the Matano plane for concentration profiles exhibiting a minimum on the higher concentration side and a maximum on the lower concentration side of the diffusion couple.

9:30 am

QUALITATIVE BOND VALENCE STUDY OF TRANSITION METAL HYDRIDES WITH THE FLUORITE STRUCTURE: Bryan Molloseau, M. De Graef, Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213-3890

Transition metal intermetallics have potential applications in hydrogen-based propulsion systems for aircraft and future hypersonic vehicles. Hydrogen-induced degradation of structural materials is a major concern in the design of these systems. To obtain an in-depth understanding of the effect of hydrogen on the stability and properties of transition metals and their hydrides we employed at Monte Carlo implementation of the Bond Valence method (MCBVM), based on Pauling's crystal chemistry rules. The value of the MCBVM is to quantitatively predict bond lengths and unit cell dimensions, to test the validity of proposed crystal structures, locate sites for substitutionally disordered materials, and to locate weakly scattering atoms such as H and Li. Because the fluorite structure is the most prevalent dihydride crystal structure for transition metals, it was selected for this study. We will show that the MCBVM can be used to qualitatively predict charge transfer in metal hydrides. First-principals work on MgH₂, Pd-H and Ni-H compounds has shown a charge transfer from the metal to the hydrogen atoms; the MCBVM is in good agreement with these results, indicating that it may be used as a less-complicated first approach to the study of metal hydrides.

9:50 am

IN-SITU REACTION CONTROL IN THE Ti-Al-Si SYSTEM: J.S. Park, J.H. Perepezko, Department of Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Ave., Madison, WI 53706

In the ternary Ti-Al-Si system, TiSi₂ and TiA1 phases were used as reactants in order to produce the Ti₅Si₃ phase as a reinforcement in a TiAl matnx. Following reaction at 1373K for 100 hours, several intermediate phase layers were produced such as TiSi, porous Ti₅Si₄, TiAl₃, Ti₂Al₅ and TiAl₂, but the desired Ti₅Si₃ phase was not produced. An analysis of the diffusion path indicated that Ti₅Si₃ formation required an enhanced Ti flux which was provided by a kinetic bias layer of Ti. In the Ti biased reaction, the Ti₅Si₃ phase was produced along with several intermediate phases. The reaction products of the biased reaction showed stability under long term annealing at 1373K. The diffusion pathway control is discussed in terms of chemical potential changes during in-situ synthesis. The support of ONR (N00014-92-J1554) is gratefully acknowledged.

10:10 am BREAK

10:20 am

HYDRIDING OF GRADE II AND GRADE III TITANIUM: Z. Wang, C.L. Briant, K.S. Kumar, K.J. Van Vliet, Division of Engineering, Brown University, Providence, RI 02912

In this paper we report a study of hydriding of grades II and III titanium exposed to sea water environments. These environments included aqueous solutions of NaCl of various concentrations as well as simulated sea water. The pH values ranged between 1 and 8 and the temperatures of the exposures varied between room temperature and 90°C. The results show that hydrides form in the material if the electrochemical potential is below approximately -700m V(SCE). The corrosion potential of titanium is well above this value. However, this potential can be achieved by impressing a cathodic current on the sample or by galvanically coupling the titanium to zinc or aluminum. Galvanic coupling to a low alloy steel will also produce this potential at elevated temperatures. Mechanical tests show that the elongation to failure of grade II titanium is not significantly affected when a thick hydride layer is formed on its surface. However, the less pure grade III titanium shows a decrease in elongation when tested in an environment that produces hydrides on the surface. This work was sponsored by ONR Contract N00014-96-1-0272.

10:40 am

TEM CHARACTERIZATION OF LAYERED MAGNETIC STRUCTURES: H. Geng, M.A. Crimp, Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824-1226

Thin films and multilayers of magnetic materials are being studied for sensor and information storage applications. However, a clear understanding of the magnetic properties, including giant magnetoresistance (GMR) requires complete characterization of the structure. In the present study, conventional and high resolution TEM are being used to examine the micro and atomic structure of GMR multilayers and spin valves. Specimens comprised of (Nb//Cu/FeMn/Py/Cu/Py//Nb) and (Nb//Ag/Py/Ag/Py/FeMn//Nb) multilayers grown on Si (001) substrates have been characterized. TEM images reveal excellent contrast between Nb, having the BCC structure with a {011}growth plane and the spin valves. However, contrast between the individual layers of the spin valves is poor. HREM analysis indicates non-equilibrium structures may exist in some of the spin valve layers. The specifics of these structures have been found to be a function of layer thickness and composition. This research has been supported by the NSF through the Materials Research Science and Engineering Center (Grant No. 61-2268).

11:00 am

THE EFFECT OF COMPOSITION ON THE STRUCTURAL MAGNETIC AND ELECTRICAL PROPERTIES OF THE SYSTEM La_1-xCa_xMnO₃: Th. Leventouri, J.J. Neumeir, D. H. Goodwin, Alloy Research Center, Physics Department, Florida Atlantic University, Boca Raton, FL 33431

An unusually large magnetoresistance occurs in La_1-xCa_xMnO₃ compounds for 0.2<x<0.5. The effects of processing and composition on the evolution of the structure in the system La_1-xCa_xMnO₃ (0<x<1) will be presented through Rietveld analysis of x-ray powder diffraction data. Magnetic as well as electrical properties will be discussed.