Research On The Morphology And Mechanical Property Of Fe-Ni-P-B Alloy Solidified Under Deep Undercooling

Research on the morphology and property of alloys solidified at deep undercooling is of great significance in understanding the solidification behaviour and microstructure evolution of alloys at the condition far away from equilibrium state, and in pursuing to enhance the glass forming ability of metallic alloys and to develop high performance materials. Extensive investigation on these problems will benefit to establish the experimental and scientific basis of the microstructure controlling and the properties optimizing of metal materials. In this study, fluxing technique has been applied to purify the alloy, which results in obtaining the deep undercooling of Fe-Ni-P-B alloy and the solidification morphology at different undercooling. Then the microstructure evolution, the mechanical properties and the fracture behavior of the as-prepared alloys have been studied.The results show that the crystalline morphologies of the Fe40Ni40P14B6 samples solidified at different undercooling are quite difference. As the undercooling increases, the solidified morphology of the alloy varied from hypoeutectic structure to eutectic structure, together with the refinement of grain size. When the undercooling exceeds a critical value, the alloy melts will experience spinodal decomposition, resulting in the formation of network-like morphology and significant refinement of the microstructures. The bulk nanocrystalline alloys with grain size less than 50nm have been obtained under large undercooling with the promotion of liquid spinodal decompositon. Despite of that the morphology of the solidified alloy is quite sensitive to the undercooling, the crystalline phase constituents of the alloys obtained at different undercooling are all the same of (Fe,Ni)₃(P,B) andγ-(Fe,Ni) phases. By use of water quenching method, bulk metallic glassy bars with the diameter of 1.6mm and nanocrystalline/glass dual-phase bars have been prepared.It has been found that the compressive plastic strain of the as-prepared Fe40Ni40P14B6 bulk metallic glass (BMG) is as high as about 5.21% at room temperature, which is much larger than those of other Fe-based bulk metallic glasses (ε≤0.5%) and most other glassy alloys (ε<2%). Its'high plasticity is attributed to the nanoscale inhomogeneity in composition resulted from liquid separation. The Fe₄₀Ni₄₀P₁₄B₆ BMG also exhibits distinct strain-hardening characteristics which have not been found in other Fe-based amorphous alloys.It has been found the plasticity and fractography of Fe₄₀Ni₄₀P₁₄B₆ nanocrystalline/glass dual-phase alloy are closely related to the volume fraction of crystalline phases. When the volume fraction of the crystalline phase is about 1.53%, the plastic strain of Fe-Ni-P-B nanocrystalline/glass alloy is only about 0.33%. The nanosized corrugations have been found on the fracture surface of this dual-phase alloy within its amorphous matrix. While Fe₄₀Ni₄₀P₁₄B₆ full glassy alloy exhibits typical shear fractography of amorphous alloys and no nanosized corrugations on the fracture surface has been found.