Engineering topics

German, R. M.

Publications and source records attributed to German, R. M..

5 records

NASA NTRS1992

Generic grain size distribution for liquid phase sintering

In order to develop an empirical model to represent a generic grain size distribution for liquid phase sintering, experimental data and theoretical models are analyzed using the plot of standard normal vs normalized size. From the experimental 3D grain size plot, the most representative line is deduced. The line best fits the Lifshitz-Slyozov encounter modified model except for the small size ranges of the lower 15 percent of the distribution. It is shown that grain coalescence might be a cause for this departure from theory.

German, R. M.↗

NASA NTRS1992

Sintering time and atmosphere influences on the microstructure and mechanical properties of tungsten heavy alloys

The present paper traces the microstructure and mechanical property variations vs sintering time under conditions where the atmosphere was adjusted to suppress pore generation. Attention is given to tracing the mechanical property links to the grain size under optimized sintering conditions. The optimal cycle is used to sinter two heavy alloy compositions (88 and 95 wt pct W) for times up to 600 minutes at 1480 C. The 88-pct-W samples slumped, but the 95-pct-W samples were fully densified and suitable for tensile testing. At long sintering times, the tungsten grains flattened and the tungsten contiguity decreased, indicating a transition to low-energy configurations for the solid-liquid interfaces. The cube of the mean grain size varied linearly with the isothermal sintering time. This allowed the determination of grain size effects on mechanical properties, showing a decreasing yield strength with increasing time in agreement with the Hall-Petch behavior. The tensile strength and elongation were highest for sintering times from 30 to 90 min, reflecting a minimum in the residual porosity.

German, R. M.↗

NASA NTRS1990

Processing effects on the mechanical properties of tungsten heavy alloys

Tungsten heavy alloys exhibit significant mechanical property sensitivities to the fabrication variables. These sensitivities are illustrated in this examination of vacuum sintering and the effects of composition, sintering temperature, and sintering time on the mechanical properties of tungsten heavy alloys. Measurements were conducted to assess the density, strength, hardness, and elongation dependencies. A detrimental aspect of vacuum sintering is matrix phase evaporation, although vacuum sintering does eliminate the need for postsintering heat treatments.

Kishi, Toshihito↗

NASA NTRS1988

Gravity and configurational energy induced microstructural changes in liquid phase sintering

Experiments were performed with W-Ni-Fe heavy alloys in order to measure the microstructural changes vs. position that occur in liquid-phase sintering under normal gravity. The experimentally observed segregation is less than the calculated equilibrium segregation due to the retarding effect from the rigid solid skeleton formed during sintering. These results improve understanding of microstructure, mechanical properties, component shape, and dimensional stability benefits that may be realized from low-gravity sintering.

Kipphut, C. M.↗

NASA NTRS1987

Gravitational contributions to microstructural coarsening in liquid phase sintering

Preliminary experiments for determining the role of gravity in liquid phase sintering have been carried out. Tungsten heavy alloys were selected for this investigation because of the large density difference between solid and liquid, extensive interest in the alloys and considerable data on these alloys. By identifying and isolating the role that gravity plays in shape distortion and microstructural coarsening, further insight into the mechanisms of coarsening kinetics may be realized. Improvements in mechanical properties, shape complexity, and dimensional stability may be realized in the future from low gravity sintering.

Kipphut, C. M.↗