The effect of copper addition on martensitic structure and reversion from martensite to austenite behaviours upon heating were investigated to clarify mechanism of grain refinement of austenite in Fe-8wt.%Ni-Cu alloys. Upon water-quenching, the alloys underwent a martensitic transformation that exhibited a typical lath-martensitic structure. It was found that prior-austenite grain and martensite-packet sizes were refined with increasing copper content. The grain refinement was not due to a decrease of grain growth rate of the austenite. However, it was found that nucleation rate of the austenite on reversion was increased by the copper addition. In Fe-8wt.%Ni alloy heated in (austenite+ferrite) region, reversed austenite grains were formed at high angle boundaries such as prior austenite grain boundary and packet boundary. On the other hand, TEM observation of the Fe-8wt.%Ni-3wt.%Cu alloy revealed that fine copper particles precipitated within the martensitic structure and the reversed austenite grains also formed within lath-structures and lath boundary. It means that the copper addition promoted formation of the reversed austenite within martensitic matrix and resulted in the grain refinement of the prior-austenite in Fe-8wt.%Ni-Cu alloy.
In this study, the effect of Fe addition on the phase stability and corrosion resistance of Ti-15%Cr alloys was investigated. The alloying phenomenon in the specimens was also investigated to determine the effectiveness of the application of pure metallic powders as raw materials for the powder metallurgy method. Ti-15%Cr-1%Fe alloys exhibited needle-like structures within equiaxed structures, while Ti-15%Cr-5%Fe and Ti-15%Cr-10%Fe alloys only showed equiaxed grains. XRD results showed that the β phase could be stabilized by the addition of 5% or more Fe to the alloy. Although the pure powders were used as raw materials, the designated chemical composition, i.e. Ti-15%Cr-(1~10)%Fe can be achieved during sintering. The alloying phenomenon occurred upon sintering due to the high diffusivity of Cr and Fe within the β Ti matrix. The corrosion resistance of the newly developed Ti-15%Cr alloys was significantly improved compared with a commercial Ti-6%Al-4%V alloy.
This study aimed to investigate the effects of element diffusion on the alloying behaviour and microstructure of a Ti-10%Mo10%Cr
alloy during sintering and furnace cooling. A theoretical calculation of the average diffusion distance for each
element was performed to predict the alloying behaviour during sintering and furnace cooling. The Ti-10%Mo-10%Cr
alloy was fabricated using a blended element powder metallurgy approach. Micrograph of the samples after sintering
showed bright-circle structures and significantly decreased equiaxed structures. The number of plate-like structures
increased with prolonged sintering time. Microstructural changes occurred because of element diffusion resulting from
the prolonged sintering time. Moreover, the diffusion distance of each element also increased with prolonged sintering
time. Although elements can sufficiently diffuse during both sintering and furnace cooling, the diffusion distance during
sintering was considerably higher than that during furnace cooling for all elements. The diffusion distances of Cr and
Mo were the highest and lowest, respectively, during sintering and furnace cooling. This study showed that alloying
behaviour mostly occurred during sintering and was controlled by the diffusion of Mo atoms.