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The influence of current direction on the Cu-Ni cross-interaction in the Cu/Sn/Ni joint configuration was investigated in this study. During current stressing, an electric current towards or away from the Ni-side of Cu/Sn/Ni was imposed at 150°C. It was observed that the (Cu,Ni) 6 Sn 5 ternary compound was the dominant reaction product at both interfaces, and its growth at the Ni-side strongly depended upon the direction and magnitude of the electron flow. When the electron flow was towards the Ni-side, more Cu was found to be driven to the Ni-side, resulting in an increase in the thickness of (Cu,Ni) 6 Sn 5 . This is due to the chemical-potential-induced Cu flux (J Cu chem ) that was enhanced by the electromigration (J Cu em ). In the case of electron flow away from the Ni-side, the supply of Cu to the Ni-side was retarded due to the fact that J Cu em was in the opposite direction to J Cu chem : The results of this study revealed that the Ni-side (Cu,Ni) 6 Sn 5 thickness remained almost unchanged under current stressing of 10 4 A/cm 2 at 150°C, which suggests the inward Cu flux is approximately equal to the outward flux, i.e., J Cu chem % J Cu em :

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This study systematically investigated the influence of high nitrogen (N) addition (0.205 wt.%) on microstructure and mechanical properties of as-cast M42 high speed steel. The results demonstrate that the conventional and high-nitrogen M42 cast ingots are mainly composed of martensite, retained austenite and various precipitates (M 2 C, M 6 C as well as MC in M42 cast ingot or M(C, N) in M42N cast ingot). The addition of N could increase the retained austenite content, trigger the transformation of MC to M(C, N), favor the formation of M 2 C at the expense of M 6 C, and improve the distribution uniformity of M 6 C at the macroscopic scale. Moreover, the addition of N could lead to the reduction of the secondary dendrite arm spacing as well as the decrease of the thickness and area fraction of eutectic carbides, and improve the distribution uniformity of eutectic carbides at the microscopic scale. The M(C, N) particles form directly from the liquid phase prior to the formation of primary austenite, which could act as the heterogeneous nuclei of primary austenite and thus promote the refinement of the as-cast microstructure. The addition of N slightly decreases the macro-hardness and ultimate compression strength of the cast ingot but increases its ductility, which could be ascribed to the increase of retained austenite content and the reduction in the amount of eutectic carbides. Therefore, high N addition can significantly improve the as-cast microstructure of M42 high speed steel, which is promising for the further enhancement of the mechanical property and service life of the final product.KEY WORDS: M42 high speed steel; pressurized metallurgy; nitrogen; as-cast microstructure; precipitates; mechanical property. N i J J J J J D