Directional solidification experiments were carried out on peritectic steels to investigate the microstructure and volume shrinkage. A δ-dendritic structure developed intermittently and was enclosed by γ-austenite at growth rates of 15, 50, and 80 μm s−1. The primary dendritic spacing narrowed and the average spacing of dendrites decreased as the growth velocity increased. The functional relationship between the secondary dendrite arm spacing and cooling rates was established by statistical analysis. The influence of cooling rate on crack formation was analysed as a function of volume shrinkage. Reducing the cooling rate of the solidification delayed a massive type of peritectic transformation away from the ZST–ZDT brittle zone, thus reducing the probability of crack formation.
The electronic packaging shell, the necessary material for hermetic packaging of large microelectronic device chips, is made by mechanical processing of a uniform block. However, the property variety requirements at different positions of the shell due to the performance have not been solved. An independently developed liquid–solid separation technology is applied to fabricate the diamond/Al composites with a graded distribution of diamond particles. The diamond content decreases along a gradient from the bottom of the shell, which houses the chips, to the top of the shell wall, which is welded with the cover plate. The bottom of the shell has a thermal conductivity (TC) of 169 W/mK, coefficient of thermal expansion (CTE) of 11.0 × 10−6/K, bending strength of 88 MPa, and diamond content of 48 vol.%. The top of the shell has a TC of 108 W/mK, CTE of 19.3 × 10−6/K, bending strength of 175 MPa, and diamond content of 15 vol.%, which solves the special requirements of different parts of the shell and helps to improve the thermal stability of packaging components. Moreover, the interfacial characteristics are also investigated. This work provides a promising approach for the preparation of packaging shells by near-net shape forming.
The phase transformation and microstructure evolution of pearlite heat-resistant steel during heating were observed with an ultra-high temperature confocal scanning laser microscope. The α-ferrite completely disappeared earlier than Fe3C during the formation of γ-austenite, which is inconsistent with the fact that the Fe3C should disappear completely earlier under equilibrium conditions. After the Fe3C + α→γ transformation, static recrystallisation of γ-austenite occurred, accompanied by the dissolution of cementite. During the γ→δ transformation, the δ-cell first precipitated at the triple point of the γ-austenite grain boundaries, and then the δ-cell platelet with one tip appeared in the γ-austenite grain. The law of δ/γ inter-phase boundaries was analysed based on inter-phase boundary types and element diffusion.
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