R. Ma’s research while affiliated with Tongji University and other places

The thermal stability of milling Fe86Zr11-xNbxB3 (x = 5.5, 6) melt-spun strip powders and the influence of high-pressure sintering conditions on phase component and grain size of bulk alloys were investigated by X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The results show that milling melt-spun powder remains in the amorphous state, and the crystallization temperature of which is 480-530°C, the apparent activation energy Ep of crystallization process is 294.1-219.5 kJ/mol. The increasing Nb content can increase crystallization temperature and decrease Ep. Under the sintering conditions of 5.5 GPa/3 min, when Pw is 1150 W, single phase α-Fe nanocrystalline (20.6-26.7 nm) bulk alloy with relative density higher than 99.0% can be obtained. Under the sintering conditions of 5.5 GPa/1150 W/3 min, the magnetic properties of these nanocrystalline bulk alloys are Fe86Zr5.5Nb5.5B3 alloy, Bs = 1.15 T, Hc = 5.08 kA·m-1; Fe86Zr5Nb6B3 alloy, Bs = 1.26 T, Hc = 4.27 kA·m-1.

The influence of spark plasma sintering and high-pressure sintering technique on microstructure and magnetic properties of bulk alloys of MA Fe73.5Cu1Nb3Si13.5 B9 nanocrystalline powder were investigated. The results show that after milling for 70 h, the single phase α-Fe nanocrystalline supersaturate solid solution powder with average grain size 9.5 nm and bcc structure can be obtained. In DSC heating-up curve of MA powder, four exothermal peaks with different strength appear, which are corresponding to the structure relaxing process of distorted nanocrystalline supersaturate solid solution, crystallization process of trace amorphous phases and phase precipitation of supersaturate solid solution, respectively. The precipitation process of supersaturate solid solution is decided into two stages. Under the SPS sintering conditions at p = 30 MPa, t = 5 min, when sintering temperature is 1050°C, nanocrystalline (100 nm) bulk alloy with relative density of 99% and main phase of α-Fe can be obtained, the magnetic properties of bulk alloy are: Bs = l.34 T, Hc = 7.34 kA/m. Under the high-pressure sintering conditions of p = 5.5 GPa, t = 5 min, when Pw = 1150 W, single phase α-Fe nanocrystalline (21.4 nm) bulk alloy with relative density of 99.1% can be obtained, the magnetic properties of which are: Bs = 1.14 T, Hc = 8.22 kA/m.