Effect of Temperature and Phase Constitution on Kinetics of La Diffusion

The diffusion of La in ε-Fe 2～3N(ε)and γ′-Fe 4N (γ′)phases produced by plasma nitriding was studied. It is found that with increasing nitriding time, the ε phase continuously decomposes into γ′ one under employed experimental conditions and compared with γ′ phase or ε+γ′ ones, the mono one ε has an impedient effect on depth of La diffusion. The growth of La layer in dual phases and mono one follows an approximate exponential law and a parabolic one, respectively, and this kinetics law does not change with increasing temperature. The effect of increasing temperature in the range of 520 to 560 ℃ on La diffusion depth is less than that of phase constitution change such as the decomposition of ε phase into γ′ one.

EFFECT OF LASER SCANNING SPEED ON PHASE CONSTITUTION AND WEAR RESISTANCE OF LASER CLADDING HARD FACING ALLOY STELLITE 12

Effect of laser scanning speed on phase constitution,microstructure,microhardness and wear resistance of laser-cladding hard facing alloy Stellite 12 has been investigated by means of a cross-flow-type CO_2 laser of 1.5 kW.The laser-cladding layer on the alloy consists of ma- trix phase of suppersaturated solid solution γ(Co,Cr)and complex hexagonal carbide M_7C_3.Under certain condition,at higher laser scanning speed,the suppersaturated solubility of elements increases in γ(Co,Cr),and the precipitation of the complex carbtde M_7C_3 de- creases,so the microhardness increases and the wear resistance decreases.But at slower laser scanning speed,more complex carbide M_7C_3 may precipitate,and the wear resistance may be improved.Therefore,it is believed that the principal factor in improving wear resistance is the precipitation of carbide M_7C_3,while increment of microhardness is the suppersaturated solu- bility of elements in matrix phase γ(Co,Cr).

Phase Constitution in Mixed Gd_2O_3 and B_4C by Sintering at High Temperature

The phase constitution in mixed Gd2O3 and B4C by sintering in graphite tube furnace at the temperature of 100～1489 ℃ in argon atmosphere was studied by means of XRD and TG-DTA. The results show that the impurity C reacts with O in the B4C at the temperature of 367～458 ℃. When the temperature is 800 ℃, Gd2O3 reacts with B4C, and the reaction products include GdB6, GdB4, GdBO3, GdBC and B. GdBC changes into GdB4 at 1200 ℃. When the temperature is 1470 ℃, GdB6 is obtained by the reaction of GdB4 with elemental B. The content of GdB6 is increasing with the prolonging time.

PHASE CONSTITUTION AND MORPHOLOGIES OF Mg-Zn-Zr ALLOY(MB15)

The equal-width lath-shaped Zn_2Zr_3 phase and the irregular lump MgZn_2 phase were ob- served in the Zr-and Zn-enriched nucleus region and along the grain boundaries of the as-cast alloy.The very fine ZnZr_2 phase occurs in the grains and the superfine MgZn phase disperses in the α-Mg solid solution.In extruded shapes,the coarse grains of Zn_2Zr_3 phase and a great many dispersively distributed ZrH_2 phase and MgZn_2 phase particles appear in the Zr-and Zn-enriched lath region.They play an important role in impeding the proceeding of recrystallization.The precipitation of Mg_2Zn_3 phase may be led to the non-lath region by ageing at 170℃ for 10h.

Reactive Hot-press Sintering of Al-B_4C Composites at Relative Low Temperature

A new process of reactive hot-press sintering with boron carbide(B4C) and aluminum powders was proposed to overcome difficulties in the sintering of dense B4C ceramic materials.The B4C powder with different content of pure metallic aluminum particle were milled,hot-pressed and sintered at 1600 ℃ for 1 hour.The mechanism of sintering at relative low temperature was analyzed.The phase constitution of the composites was determined.Effects of Al content on the hardness and fracture toughness of the composites were discussed.The results show that thermite reaction procedure in B2O3+Al was the mechanism of sintering at relative low temperature,B4C,Al2O3 and metallic aluminum are the major constituents of the composites.The microhardness of the composites decreases with the increasing of Al content,but the fracture toughness increase obviously.The composite with 5wt% Al content has the best microhardness and fracture toughness in all the composites.

Phase precipitation and magnetic properties of melt-spun ternary Gd_(2)Fe_(14)B alloy and advantages of gadolinium substitution in Y_(2)Fe_(14)B alloy

To take the advantage of gadolinium(Gd) in developing and manufacturing RE-permanent magnets,the magnetic properties and phase precipitation behavior of Gd_(2)Fe_(14)B alloys prepared by melt spinning were investigated in this work.The results show that optimally direct quenched nanocrystalline Gd_(2)Fe_(14)B alloy exhibits the magnetic properties with remanence J_(r) of 0.51 T,coercivity H_(c) of 187 kA/m,and maximum energy product(BH)_(max) of 33.1 kJ/m^(3).It also shows excellent thermal stability with a small negative temperature coefficient of J_(r)(α=-0.066%/K) and a positive temperature coefficient of H_(c)(β=0.171 %/K).The phase precipitations of as spun Gd_(2)Fe_(14)B alloy during melt spinning and over-quenched amorphous Gd_(2)Fe_(14)B alloy during heat treatment were clarified.The magnetic properties of the crystallized alloys annealed at 800℃ are close to those of the optimized directly quenched as-spun alloys,indicating good performance stability under various processes.In addition,the role of Gd substitution in(Y_(1-x)Gd_(x))_(2)Fe_(14)B alloys was systematically studied.Gd substitution can significantly enhance the thermal stability and decrease the irreve rsible remanence loss of Y_(2)Fe_(14)B alloy.With increasing Gd conte nt,Gd-doped Y_(2)Fe_(14)B alloys show continuous enhancement of H_(c) at elevated temperatures.The highest β value of 0.22%/K was obtained in(Y_(0.2)Gd_(0.8))_(2)Fe_(14)B alloy at 300—400 K.Gd substitution is not only effective in improving the exchange coupling effect and corrosion resistance of Y_(2)Fe_(14)B alloy but also superior to Nd in terms of improving the oxidation resistance.The present work has certain instructions for designing and developing new low cost RE permanent magnets with improved thermal and environment stability.

High energy product of isotropic bulk Sm-Co/α-Fe(Co)nanocomposite magnet with multiple hard phases and nanoscale grains

Nanocomposite magnets consisting of hard and soft magnetic phases have potential applications to be the next generation of permanent magnets with very high energy product and less expensive rare-earth elements.But it is still a big challenge to obtain bulk magnets with ideal microstructure and high performance.In this work,two-step warm processing at relative low temperatures had been adopted to obtain nearly theoretical density bulk nanocomposite magnets from amorphous/nanocrystalline powder precursors.Novel nanostructures consisting of multiple Sm-Co hard phases(SmCo_(5)as main phase,SmCO_(3),SmCo_(7),Sm_(2)Co_(17)as minor phases)and 25 wt%α-Fe(Co)soft phase,nanoscale grain size below 20 nm for both the hard phase and soft phase,and the diffusion of Fe and Co compositions had been obtained in bulk isotropic magnets.Besides the ideal nanostructures,a high coercivity of 5.9 kOe,M_(r)/M_(s)value of 0.78 and a high square degree of demagnetization curve S=0.47 were obtained.All of these factors together brought a new record-high energy product(BH)_(max)of 23.6 MGOe.These results make an important step toward fabricating novel nanostructure with high performance.

Oxide Scale Growth on High Carbon Steel at High Temperatures

The structure and formation process of oxidation on high carbon steel were investigated with the aid of X-ray diffraction （XRD）, scanning electron microscope （SEM） and Laser Raman spectroscopy （LRS）. The oxide scale formed comprised a three-layer structure, similar to that formed on pure iron and low-carbon steel. For the high carbon steel, however, the scale was essentially a two-layered because of the low proportion of hematite （Fe2O3） formed. The scale thickness increased with the temperature and time of oxidation. The rate of scale thickening rapidly increased above 900 ℃, at which the rate was particularly fast in the first 20 s of oxidation. The proportion of wüstite （Fe1-yO） increased with time and temperature of oxidation, while the magnetite （Fe3O4） remained constant at about 2 μm.

Influence of Re on the Properties of a NiCoCrAlY Coating Alloy

MCrAIY can serve as stand-alone overlay coatings or bond coats in thermal barrier coating systems, and its properties play a vital role in determining the performance of these coating systems. In order to further understand the behavior of MCrAIY coatings, several NiCoCrAIY model alloys with different levels of Re （0.3 wt%, O wt%, and 9 wt%） were investigated. Microstructural observation showed the addition of Re promoted the precipitation of Cr-rich phases, such as α-Cr and σ. The presence of α-Cr lowered the coefficient of thermal expansion （CTE） of the coating alloys, which could reduce the CTE mismatch at the scale-metal interface. The solid solution strengthening effect of Re is responsible for an increase in Rockwell hardness of the coating alloys. But the isothermal oxidation resistance at 1100℃ was deteriorated due to the precipitation of brittle α-Cr phase, a phase of inferior oxidation resistance compared with β-NiAI and γ-Ni.

Effect of Carbon Addition on Microstructure and Properties of WC-Co Cemented Carbides

Based on a unique method to synthesize WC-Co composite powder by in-situ reactions of metal oxides and carbon, the effects of the carbon addition in the initial powders on the phase constitution, microstructure and mechanical properties of the cemented carbides were investigated. It is found that with a suitable carbon addition the pure phase constitution can be obtained in the sintered bulk from the composite powder. The mechanical properties of the cemented carbides depend on the phase constitution and the WC grain structure. To obtain the excellent properties of the WC-Co bulk, it is important to obtain the pure phase constitution from the appropriate carbon addition in the initial powders and a suitable grain size.

Microstructure and performance of WC-Co-Cr coating with ultrafine/nanocrystalline structures

The WC-lOCo-4Cr composite powder was synthesized firstly. Then the composite powder was agglomerated to prepare thermal spraying feedstock. The ultrafine/nanostructured WC-lOCo-4Cr coating was prepared by high velocity oxygen fuel thermal spraying. The phase constitution, elemental distribution and microstructure of the coating were characterized by X-ray diffraction and transmission electron microscopy, respectively. The wear resistance and corrosion resistance of the prepared composite coating were tested. The results show that the main phases of the coating include WC, binding phase with partial amorphous structure, with a little WC and Co(Cr)coexisting. The distributions of Co and Cr elements from the phase boundary to the eutectic area then to Co zone were analyzed quantitatively. The mechanisms for the formation of the microstructure and effects of Cr on the performance of the composite coating are proposed.

SYNTHESIS OF STRONTIUM-AND MAGNESIUM-DOPED LANTHANUM GALLATE BY GLYCINE-NITRATE COMBUSTION METHOD

Sr- and Mg-doped lanthanum gallate powders with the composition of La0.55Sr0.15Ga0.85Mg0.15O2.85were synthesized by a glycine-nitrate combustion method. Powders prepared under different fuel combustion conditions were investigated by XRD and TEM. The results show that, under slightly rich fuel condition, the product powders contain less impurity phases, and powders prepared by the glycine-nitrate combustion contain far less impurity phases and have smaller particle sizes than those prepared by solid-state reaction method or acrylamide polymerization technique.