Preparation And Physical Metallurgy Of Bulk Fe-based Nanocrystalline Soft Magnetic Material

In this paper, with combination of Natural Science Foundation Project of Hunan Province entitled with "Preparation of Nanocrystalline Bulk Soft Magnetic Alloys Under Ultra Conditions" (01JJY2056), Shanghai Scientific Development Foundation Project entitled with "Basis research and applications of high Bs Fe-based nanocrystalline soft magnetic alloy prepared by super high pressure thermal consolidation method" (0252nm054), Shanghai Scientific Development Foundation Project entitled with "Basis research on high temperature and low loss FeCo based bulk nanocrystalline alloy" (0452nm086), and Natural Science Foundation Project of China entitled with "Research on high frequency nanocrystalline soft magnetic materials" (50501008), preparation methods of Fe-based nanocrystalline bulk soft magnetic alloy was explored as well as some related basis problems of preparation.Firstly, the feasibility of preparing nanocrystalline soft magnetic materials in FeMB and FeCoMB(Cu) systems by mechanical alloying (MA) method. The results showed that, during MA process, with the ball milling time increased, the alloying degree of Fe, Co, M (Nb,Zr) and B mixing powder was improved and grain size decreased. lattice distortion increased gradually and after reached a certain critical value it decreased, at lastα-Fe super saturated nanocrystalline solid solution with bcc structure and grain size 10-15nm formed in microstructure. In FeNbB alloy system, increased Nb content prolonged the alloying process, while the addition of Zr and Nb at the same time accelerated the alloying process apparently. The replacement of Fe by Co also accelerated alloying and nanocrystallization process in (FeCo)-M-B-Cu alloy. With increasing annealing temperature, the grains of FeMB and FeCoMB(Cu) nanocrystalline powder grew up gradually and the internal stress relaxed. However, below 600℃annealing nanocrystalline grain grew slowly and it's grain size was in 10-15nm range. When annealing temperature was over 650℃, the grain size of nanocrystalline grew up rapidly, the annealing temperature up to 700℃, second impure phase appeared.During MA process, dislocation pump mechanism and lamellar structure provided rapid channel for elements' atomic diffusion. MA produced lots of micron and nanometer lamellar structure inside the particle, atomics can form non-equilibrium solid solution through the diffusion of interface between lamellar. The accumulation of large amount of dislocations resulted in the formation of dislocation cell, which then developed to nanocrystalline.During MA process, the grain size D ofα-Fe phase was functioned as mill time t describing by a power function:D=atb, the power exponent b is related to the species and content of alloying elements and can also show the nanocrystalline forming ability during MA process, b is in the range of-0.4640～1.0122.On the basis mentioned above, FeMSiB bulk materials were prepared by MA+Hot Pressing Sintering method, MA+Spark Plasma Sintering (SPS) method, MA+Super High-pressure Moulding method, as well as by spun melting amorphous powder+SPS sintering and spun melting amorphous powder+Super High-pressure Moulding method. The results showed that, (1) under the hot-pressure sintering conditions of 850℃～900℃/30MPa/30min, bulk alloys with 94.7%～95.8%relative density and fine (lOOOnm) or superfine (lOOnm) grains were prepared by 8-15nm MA Fe84Nb7B9,Fe80Ti8B12 and Fe32Ni36Nb (V) 7Si8B17 nanocrystalline or amorphous powder. Except that bcc single phase microstructure formed in Fe80Ti8B12 alloy, the other alloys own multiphase microstructure; (2) under the spark plasma sintering conditions of 30MPa/5min/850～1050℃, bulk alloys with 99%relative density and mainly composed of 50～100 nm nanocrystalline a-Fe phase. The sintering temperature of amorphous powder was lower for 50℃than that of MA nanocrystalline powder under same density, and the densification velocity was quicker for amorphous powder. (3) under the super high-pressure moulding conditions of 5.5GPa/820W-1150W/3-15min, no matter what kinds of powder were used,8-10nm MA nanocrystalline powder of Fe73.5Cu1Nb3Si13.5B9, Fe84Nb7B9 and Fe84Nb4W3B9, amorphous powder of Fe78Si9Bi3 and Feg6Zr5.5Nb5.5B3, nanocrystaline (10-20nm)α-Fe single phase bulk alloy samples could be prepared, the relative density reached 98.4%, the size of samples wasφ20×(7～10) mm. When sintering time was invariable, the grain size and relative density of bulk alloy increased apparently with improving sintering power; when sintering power Pw was invariable, the grain size and relative density of bulk alloy increased slowly with increasing sintering time. (4) bulk alloys prepared by MA nanocrystalline powder and melt spun amorphous powder have high saturation intensity and high coercive force Hc. The hindrance influence of lattice distortion and inner micro strain on magnetic domain wall resulted in high Hc, appropriate annealing reduced Hc effectively.Hydraulic high press machine is a key equipment to synthesis diamond. By the use of near high static pressure field produced in hydraulic press machine to prepare large size bulk nanocrystalline materials was firstly proposed in this paper, and provided technical basis for researching and obtaining large size nanocrystalline bulk soft magnetic alloy. The results showed that, increasing pressure of super high-pressure moulding not only restrained the formation of impure phases Fe-Nb, Fe-B and etc, but also prevented nanocrystalline grain from growing. This is significant to guide the development of super high-pressure moulding technique to prepare nanocrystalline soft magnetic bulk materials. By the use of this technique bulk nanocrystalline materials were obtained in several alloy systems, especially for Fe78Si9B13 amorphous powder.Crystallization kinetics behaviors under super high-pressure of Fe78Si9B13, Feg6Zr5.5Nb5.5B3 (Nanoperm) and Fe55Co18Nb4Al2Si4B15Sm2 amorphous alloys were discussed, their activation energy was calculated by Kissinger and Ozawa methods, the results showed that, the calculated activation energy can denote crystallization activation energy during crystallization process of amorphous alloys.The continuation Friedman method was used to calculate the local crystallization activation energy during initial nanocrystallization process of Fe86Zr55Nb55B3 amorphous alloy. At the beginning stage of crystallization process, local crystallization activation energy was 315-325kJ/mol (00.9), local crystallization activation energy increased evidently, whenα=0.95, activation energy reached at 493kJ/mol. During the initial nanocrystallization process of this amorphous alloy, the average activation energy was 371.4kJ/mol.