Effect of Thermal History on Tensile Yield Behavior of Tungsten Fiber-Copper Composites

Abstract

Axial tensile yield behaviors of tungsten fiber-copper composites have been studied in terms of thermally induced residual stresses. The composites with 50 vol pct and 400μm diam tungsten wires were fabricated by a liquid infiltration technique followed by hot rolling. In the temperature range of 288K-673K, X-ray diffraction technique was employed to measure the matrix stress parallel to the fibers due to thermal expansion mismatch. Tensile tests were performed at room temperature by the use of the Instron Model 1122.
Two series of specimens were prepared, which were as follows. 1) Annealed at 873K and slowly cooled to room temperature (designated as A). 2) Immersed in the boilling liquid nitrogen after the A treatment and slowly heated to room temperature (designated as N).
The experimental results of the present study are summarized as follows:
(1) The composites heated up to 673K indicated a fine X-ray diffraction profile. So it is possible by the X-ray analysis to measure the thermal stress at such a high temperature.
(2) The copper matrix deformed elastically to accommodate thermal expansion mismatch when the temperature change was small. For the large temperature change, however, the matrix subsequently deformed plastically. Thus, the resulting thermal stress depends upon the thermal history of the composites.
(3) The tensile yield behavior was significantly influenced by the thermal history of the composites. The A treated specimens indicated poor transition from stage I (both phases elastic) to stage II (fiber-elastic, matrix-plastic), while the N treated specimens indicated clear transition. These phenomena observed are well explained by the thermally induced residual elastic strains in the copper matrix obtained by the X-ray analysis.