Toughness variation with test temperature and
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Testing Surface finish Strain rate Temperature Stress concentration
INTRODUCTION
THEheavy alloys are tungsten based two phase composites used in applications requiting high density.1 The alloys are liquid phase sintered from blended elemental powders. After sintering, the microstructure consists of a rounded tungsten phase (typically 50/~m in diameter) surrounded by a matrix phase containing dissolved tungsten. The typical chemical composition ranges from 80 to 98 pct W with either Ni-Cu, Ni-Fe, or Ni-Fe-Co additions. Understandably, the mechanical properties are variable with microstructure, chemistry, and processing. Yield strengths in excess of 500 MPa are fairly common; however, ductility and toughness tend to be unpredictable. Generally, the Ni-Fe alloys exhibit superior mechanical properties and a 7 : 3 ratio of nickel to iron is observed to be optimal. 2'3'4 In spite of numerous studies on the heavy alloys dating back to the 1930's, there is still uncertainty as to the sources of toughness variation. Considering the large number of parameters associated with this material, the observed variability in toughness is not surprising. Generally, the factors influencing toughness can be divided into three categories. First are those factors which produce differing results between studies such as composition, sintering temperature, test geometry, sintering atmosphere, and heat treatment. Second are those factors which, give differing properties between similarly processed heats such as density, pore size, impurities, and particle size. Third are the factors which contribute to property variations within a single heat of heavy alloy such as thermal and gravitational gradients. These variables are summarized in Figure 1, and it is emphasized that all these factors are interrelated. Hence, studies aimed at optimizing specific properties like toughness must be performed carefully to avoid confusing results from the other factors. Many previous studies have optimized mechanical properties of the heavy alloys through either rapid quenching or slow cooling from temperatures above 1000 ~ The conflicting results from such investigations are summarized in Table I and clearly illustrate the complexity of the problem. R.M. GERMAN is Professor, School of Engineering, Materials Division, Rensselaer Polytechnic Institute, Troy, NY 12181. J. E. HANAFEE, Project Leader, and S. L. DiGIALLONARDO, Metallurgist, are with the Chemistry and Materials Science Department, Lawrence Livermore National Laboratory, Livermore, CA 94583. Manuscript submitted January 24, 1983.
METALLURGICAL TRANSACTIONS A
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Purity Packing density Blending Pressing
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Toughness
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Time Temperature Atmosphere Density Matrix gradient
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Microstructure Grain size Intermetallic phases Grain coalescence Interfacialarea Pore size Segregation
~ 1 [ PostCycle Processing Heat treatment Deformation Cooling rate Degassing
\Chemistr_y Wcontent Ni:Fe ratio Substitutional
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