Materials Week '97: Tuesday PM 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 Tuesday afternoon, September 16.

ALLOY MODELING AND DESIGN: Session IV: Physical, Chemical, and Mechanical Properties of Alloys II

Session Chair: Dr. M. Asta, Sandia National Laboratories, Livermore, CA 94551

SELECTIVE VARIANT GROWTH OF COHERENT PRECIPITATES UNDER EXTERNAL CONSTRAINTS: D.Y. Li, L.Q. Chen, Department of Materials Science and Engineering, The Pennsylvania State University, State College, PA 16802

An anisotropic distribution of coherent precipitate variants may result in anisotropic behavior of multiphase materials. The anisotropic distribution or the selective variant growth of the coherent precipitate could be obtained by constrained aging. The present study demonstrates that the selective variant growth results from the anisotropic coupling between the applied stress/strain and the local strain caused by the lattice mismatch. Under a certain constraint, the coupling energy is different for differently oriented precipitate variants. As a result, the variant growth becomes selective and this, in turn, modifies the material's performance. Selective variant growth of Ti₁₁Ni₁₄ precipitates in Ti-51.5 at %Ni alloy was investigated as a particular example. It was demonstrated that the selective variant growth of Ti₁₁Ni₁₄ precipitate can be predicted based on the symmetry analysis and the elastic energy calculation. The research was conducted in combination with TEM analysis and computer simulation. A positive corroboration between the theoretical analysis and the experiment was found.

2:30 pm

MECHANICAL BEHAVIOR ANALYSIS OF PST TiAl INTERMETALLIC COMPOUNDS WITH FINITE ELEMENT METHODS: Leilei Zhang, David G. Atteridge, Oregon Graduate Institute of Science and Technology, Department of Materials Science and Engineering, P.O.Box 91000, Portland, OR 97291-1000

Finite element methods was used to simulate the mechanical behavior of PST TiAl intermetallic Compound during tensile test under room temperature. Different lamellar orientation to the axis of the tensile stress was studied. The relationship between the lamellar orientation and mechanical properties agree with the experimental results. With the experimental results of PST TiAl under high temperature, grain rotation mechanism was proposed for the deformation characteristic of TiAl intermetallic compounds. It will help people to understand the deformation characteristic of TiAl and improve its room temperature ductility. That will improve the wide use of TiAl intermetallic compounds.

3:00 pm

MICROSTRUCTURAL DESIGN OF HIGH STRENGTH ALUMINUM ALLOYS: J.F. Nie, B.C. Mudle, Department of Materials Engineering, Monash University, Clayton, Victoria 3168, Australia

A common feature of high strength (_y > 450 Mpa) and ultra-high strength (_y > 700 Mpa) aluminum alloys is that maximum strength and hardness are achieved through precipitation hardening involving predominantly plate-shaped precipitates formed on rational habit planes {100} and {111} in an -Al matrix. However, there is currently little detailed quantitative understanding of the strengthening mechanisms operative and the relationship between the form and distribution of the strengthening precipitate phases and the observed tensile strength. The development of structural alloys remains largely empirical, and there is a need for an improved theoretical basis for alloy design. This presentation will include a review of those microstructures associated with maximum strength in this class of alloys and attempt to identify those microstructural parameters important in optimizing precipitation hardening or dispersion strengthening. It will also review recent attempts to model the effects of precipitate shape and orientation on yield strength, using appropriate versions of the Orowan equation and models of precipitation strengthening developed for rationally oriented plate or rod-shaped precipitates.

3:30 pm BREAK

3:40 pm INVITED

FERROMAGNETIC SHAPE-MEMORY COMPOUNDS WITH HEUSLER-TYPE STRUCTURE: K. Inoue, Department of Materials Science and Engineering, University of Washington, Box 352120, Seattle, WA 98195-2120; M. Taya, Mechanical Engineering, University of Washington, Box 352600, Seattle, WA 98195-2600; M. Kitamura, Electrical and Electronic Engineering, Utsonomiya University, Utsonomiya, Japan

There is a strong interest in the development of high performance materials for various applications including actuators for aircraft, and biomedical devices, in which large controllable strains and rapid response are most desired to have as important properties besides mechanical properties such as appreciable specific strength and high durability. Shape memory (SM) materials and piezoelectric ceramics are considered to be candidates for actuator materials but they only exhibit either large controllable strains or rapid response. We have shown effects of applied magnetic field on displacive phase transformation for off stoechiometric Ni₂MnGa-based compounds, which is indicative of potentiality of magnetically induced SM effects. We will report on displacive phase transformation and magnetic properties of ferromagnetic Ni₂MnGa-based compounds with and without quaternary additions (FE, Co, Ge, Si, Al). Effects of elemental additions on transformation temperature and magnetic properties will be presented, and relationships between phase stability and magnetic properties will be discussed.

4:20 pm

MAGNETISM AND POINT DEFECTS IN INTERMETALLIC FeRh: Luke S.J. Peng, Gary S. Collins, Department of Physics, Washington State University, Pullman, WA 99164-2814

Stoichiometric FeRh is a highly-ordered B2 intermetallic that is antiferromagnetic below about 400 K and ferromagnetic above. There is considerable interest in large changes in unit-cell volume and electrical conductivity and large magnetostrictive effects that take place at the AF-F transition. We are beginning a study of magnetism and defects in FeRh using 57Fe Mossbauer spectroscopy. In the AF phase, annealed, stoichiometric FeRh has a large hyperfine field (27.28(2) T at 20 K) and narrow lines indicating very little lattice disorder. In the present talk will be described measurements just beginning on annealed samples with 48 and 52 at .% Fe, for which there will be either 4 at .% of Rh or Fe antisite atoms on the Fe or Rh sublattices, respectively. Observed hyperfine-field-shifts due to antisite defects in the AF phase are expected to distinguish between two alternative magnetic structures that have been proposed. Supported in part by the NSF under grant 96-12306 (Metals Program).

4:50 pm

ATOMIC SCALE STUDIES OF POINT DEFECTS IN INTERMETALLICS: Gary S. Collins, Department of Physics, Washington State University, Pullman, WA 99164-2814

Over the past five years we have been using the method of perturbed angular correlation of gamma rays (PAC) to study properties of point defects in ordered intermetallics such as NiAl, CoAl and PdIn. Defects such as lattice vacancies and antisite atoms produce localized charge-density disturbances that modify the nuclear quadrupole interaction in nearby probe atoms. Using the 111 In/Cd probe, good signal-resolution is obtained for defects out to several atomic shells. Measurements on quenched NiAl, CoAl and PdIn have helped determine that the high-temperature equilibrium defect in each system is the Schottky vacancy-pair. The temperature dependence of site fractions of probe atoms with neighboring vacancies has yielded formation enthalpies and defect concentrations. We are now beginning measurements at high-temperature that will serve to test the methodology used for analysis of measurements on quenched samples and to provide information about diffusion. An overview of this work will be provided.