B. Yan’s scientific contributions

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 high-pressure sintering conditions on density and grain size of α-Fe phase was investigated to obtain Fe86Zr5.5Nb5.5B3 nanocrystalline bulks. Results show that single phase α-Fe nanocrystalline (16.3 nm) bulk of 98.3% relative density could be acquired with precursor amorphous powders prepared by melt-spinning and then ball milling, and under the sintering conditions of P=5.5 GPa/t=3 min. With the increment of Pw and t, its relative density and grain size slightly increased. The specific saturate magnetization σs and coercive force Hc of the nanocrystalline bulk alloy was 119.6 emu/g and 63.8 Oe, respectively.

Microstructures and magnetic properties of Fe84Nb7B9, Fe80Ti8B12 and Fe32Ni36 (Nb/V)7Si8 B7 powders and their bulk alloys prepared by mechanical alloying (MA) method and hot-press sintering were studied. The results show that: 1) After MA for 20 h, nanocrystalline bcc single phase supersaturated solid solution forms in Fe84 Nb7B9 and Fe80Ti8B12 alloys, amorphous structure forms in Fe32Ni36Nb7Si8 B17 alloy, duplex microstructure composed of nanocrystalline γ-(FeNi) supersaturated solid solution and trace content of Fe2B phase forms in Fe32Ni36V7Si8 B17 alloy. 2) The decomposition process of supersaturated solid solution phases in Fe84Nb7B9 and Fe80Ti8Bi2 alloys happens at 710-780°C, crystallization reaction in Fe32Ni36Nb7Si8 B17 alloy happens at 530°C (the temperature of peak value) and residual amorphous crystallized further happens at 760°C (the temperature of peak value), phase decomposition process of supersaturated solid solution at 780°C (the temperature of peak value) and crystallization reaction at 431°C (the temperature of peak value) happens in Fe32Ni36V7Si8 B7 alloy. 3) under 900°C, 30 MPa, 0.5 h hot-press sintering conditions, bulk alloys with high relative density (94.7%-95.8%) can be obtained. Except that the grain size of FeNb7B9 bulk alloy is large, superfine grains (grain size 50-200 nm) are obtained in other alloys. Except that single phase microstructure is obtained in Fe80Ti8B12 bulk alloy, multi-phase microstructures are obtained in other alloys. 4) The magnetic properties of Fe80Ti8B12 bulk alloy (Bs=l.74 T,Hc=4.35 kA/ m) are significantly superior to those of other bulk alloys, which is related to the different phases of nanocrystalline or amorphous powder formed during hot-press sintering process and grain size.

The tests of low cycle fatigue of 316L stainless steel under uni-axial and multi-axial non-proportional loading were carried out, and the microstructures in this material were observed also. The micro-mechanism of the dependence of non-proportional cyclic additional hardening on the strain path was studied. The distributive rule of distance within which dislocations move freely is analyzed. The results show that the non-proportional cyclic additional hardening of 316L stainless steel is directly related to the shape and size of the multi-slip dislocation structures. Distance within which dislocations on slip planes move freely is normal distribution. There is the logarithmic linear relationship between the macro effective saturated stress amplitude value and the statistical mean distance within which dislocations move freely. Based on the above results, the new non-proportionality of loading path is defined with the statistical mean distance within which dislocations move freely. The result show that the new non-proportionality defined in this paper is considered as entering uni-axial cyclic constitutive equation, which can describe the effect of the strain path on the non-proportional cyclic deformation behavior of the material.

D.C. magnetic properties and A.C. magnetic properties of nanocrystalline Fe72.8Cu1Cu1Nb1.7 V1.5Mn0.5Si13.5B9 alloy with high-permeability, as well as its dispersion behavior and core loss, were reported. The equation to describe the high-frequency dispersion and core loss was given. The relationship between the core loss and the amplitude flux density and the relation-ship between the core loss and the frequency were described.

Based on the microstructural investigation of low cycle fatigue under multi-axial non-proportional loading of 316 L stainless steel, the distance within which dislocation moves freely was measured, while its distribution rule was analyzed. The quantitative relation between the macroscopic effective saturated stress and the statistical mean distance within which dislocations moves freely was given. The new definition of non-proportionality of loading path was presented on the basis of the statistical mean distance within which dislocation moves freely, which is considered as entering uniaxial cyclic constitutive equation to describe the stress and strain behavior under multiaxial loading.