Densification behavior of tungsten heavy alloy based on master sintering curve concept
- PDF / 403,793 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 44 Downloads / 241 Views
heavy alloys (WHAs) are used for a number of applications, including radiation shields, counterweights, electrical contacts, gyroscopic components, vibration dampeners, and kinetic energy penetrators.[1] The most common compositions consist of W along with some combination of Ni, Fe, or Cu, although small amounts of other transition metals are sometimes added. The WHAs can be readily processed to near theoretical density by sintering at temperatures at which a liquid phase is present. However, significant densification occurs by solid-state sintering during heating prior to liquid-phase formation.[2–5] The Ni and Fe additions, in particular, enhance the solid-state sintering of W.[6] Changes in the dominant sintering mechanism at different stages of the sintering cycle complicate analysis of densification behavior. In this article, the master sintering curve (MSC) concept is modified to analyze densification during heating of W-Ni-Fe heavy alloys with W contents ranging from 83 to 93 wt pct. II.
MSC THEORY
During solid-state and liquid-phase sintering of most materials, diffusion plays the primary role in densification,[7] although grain rearrangement can also be very important in liquid-phase sintering.[8] In solid-state sintering, either grain boundary or volume diffusion is the dominant densification mechanism. Surface diffusion is active with small powders, but does not contribute to densification. The following multiple mechanism model provides a means to predict densification behavior:[9]
S.J. PARK, Associate Research Professor, and RANDALL M. GERMAN, Director and CAVS Chair Professor of Mechanical Engineering, are with the Center for Advanced Vehicular Systems, Mississippi State University, Mississippi State, MS 39762. Contact e-mail: sjpark@cavs. msstate.edu J.M. MARTIN, Staff Researcher, is with the Centro de Estudios e Investigaciones Te´cnicas de Guipu´zcoa (CEIT) and TECNUN, 20018 San Sebastia´n, Spain. J.F. GUO, Graduate Student, is with the Center for Innovative Sintered Products, CISP Lab, The Pennsylvania State University, University Park, PA 16802. J.L. JOHNSON, Staff Engineer, is with Breakthrough Technology, Kennametal Inc., Latrobe, PA 15650. Manuscript submitted August 29, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
1 dr g V Gv D v Gb d b D b 1 5 sv 3r dt kT G3 G4
[1]
where r is the relative density, t is the time, gsv is the solid-vapor surface energy, V is the atomic volume, k is Boltzmann’s constant, T is the absolute temperature, G is a density-dependent function, D is a diffusivity, G is the grain size, db is the width of the grain boundary, the subscript v stands for volume diffusion, and the subscript b stands for grain boundary diffusion. Taking into account the exponential dependence of the diffusivities with temperature, the previous equation can be rewritten as 1 dr gsv V Gv Dv0 Qv Gb db Db0 Qb 1 exp exp ¼ RT RT 3r dt kT G3 G4 [2]
where R is the universal gas constant, Dv0 and Db0 are the pre-exponential factors, and Qv and Qb are the activation energies for volu
Data Loading...
