In-Situ Observations on the Fracture Mechanism of Diffusion-Alloyed Ni-Containing Powder Metal Steels and a Proposed Met
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THE mechanical properties of conventional pressedand-sintered parts are usually poor compared to those of their wrought counterparts. One of the main problems is that large numbers of pores are present in the microstructure. The shapes, sizes, and numbers of pores are critical in determining the final mechanical properties.[1–6] The other problem is that most alloying elements are not homogenized. This is partly due to the inadequate powder mixing, to powder segregation during shipping and handling, and to the slow dissolution rate of the powder into the iron matrix, particularly in the case of Ni, which has a slow diffusion rate compared to the other main alloying elements such as C, Cu, and Mo. The resulting inhomogeneous microstructures, particularly those around sintered necks and pores, thus impair the final mechanical properties.[7,8,9] An intuitive approach to solving the homogenization problem of Ni is to use prealloyed steel powders. However, the poor compressibility of the hard prealloyed powder has prohibited its wide application. Another M.W. WU, Graduate Student, and K.S. HWANG, Professor, are with the Department of Materials Science and Engineering, National Taiwan University, Taipei 106 Taiwan, Republic of China. Contact e-mail: [email protected]. edu.tw H.S. HUANG, Researcher, is with the Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 310 Taiwan, Republic of China. Manuscript submitted September 11, 2006. Article published online June 29, 2007. 1598—VOLUME 38A, JULY 2007
alternative is to employ diffusion-alloyed powders, in which iron powders and alloying elemental powders are diffusion-bonded together at low temperatures.[10] These powders alleviate the segregation problem while maintaining good compressibility. However, the as-sintered microstructure of this powder, while improved, is still not homogeneous, unless extremely long sintering times or high sintering temperatures are employed. For example, in Fe-4Ni-1.5Cu-0.5Mo-0.5C alloys, which are designated by the Metal Powder Industries Federation (MPIF) as FD-0405, many Ni-rich areas are still present when industrial sintering practices are used.[9,11] Moreover, most Ni-rich areas are soft and are located near sintered necks or in pore-rich regions, which are stress concentration sites. Thus, these Ni-rich areas become the most likely crack-initiation sites during mechanical testing and are responsible for the poor mechanical properties of the Ni-containing sintered steels.[11,12,13] Although several studies have correlated the mechanical properties to the pore characteristics and fracture surfaces,[12–15] little direct evidence of the identification of the weak phases has been reported, nor has evidence that links these phases with the mechanical properties. This is possibly because the weak Ni-rich areas are quite small, only a few microns wide, thus making it difficult to perform structure analysis. Moreover, the microstructure is very inhomogeneous and large amounts of pores are present; both of thes
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