Synthesis and characterization of amorphous-nanocrystalline Fe${}_{70}$Cr${}_{10}$Nb${}_{10}$B${}_{10}$ powders by mechanical alloying

In this study, structural, morphological, thermal and magnetic properties of amorphous-nanocrystalline Fe${}_{70}$Cr${}_{10}$Nb${}_{10}$B${}_{10}$ (at.%) alloy are discussed. The formation and evaluation of amorphous-nanocrystalline structures of the alloy were followed by XRD, SEM-EDX, TEM, DTA, and VSM techniques. After 50 h of milling Cr, Nb, and B were completely dissolved into the Fe lattice forming 82% of the amorphous phase of the alloy. A gradual dissolution of the alloying elements also increased the lattice parameters concurrently. The lattice parameters reached a maximum value of 2.908 Å after 20 h of milling and then leveled off to a value of 2.891 Å at the end of 50 h of milling. Based on the XRD data, crystallite size and lattice strain of the alloy were calculated as 3.2 nm and 3.34% respectively. TEM analyses revealed that the alloy particulates comprised needle-shaped nanoparticles of an average size of 21 nm. The room temperature magnetic hysteresis loops showed that the increased duration of milling decreased the saturation magnetization from 91 to 24 emu/g. This was mainly due to the upsurge on the amorphous phase content in the alloy as the milling progressed. The increase in amorphous phase content and the subsequent reduction of the saturation magnetization were due to the inter-diffusion of the non-ferromagnetic Cr and B atoms into the Fe lattice. Thermal studies revealed that around 350 °C the amorphous phase of the alloy began crystallizing. The magnetic saturation of the heat-treated alloy also increased with the growth in the crystalline phases. The 50 h milled sample annealed at 700 °C was found to have the highest magnetic anisotropy as observed from the temperature-dependent zero-field cooled and field cooled magnetization measurements. The high-pressure X-ray diffraction measurements revealed that the amorphous state of the alloy remained stable up to 11.3 MPa. It also revealed the structural similarities of the Fe${}_{70}$Cr${}_{10}$Nb${}_{10}$B${}_{10}$ alloy with those of the Fe${}_{70}$M${}_{10}$B${}_{20}$ (M = Nb and Cr) types. For all practical purposes, the microstructural stability under high compressive pressure represents the consolidation properties of the nanostructured magnetic materials since both pressure and temperature-induced phase transformations are the primary controlling factors for the specific magnetization properties of the alloy.