高性能SiC—AlN复相陶瓷

采用热压烧结工艺,通过合理的组成设计和烧结温度控制,制备出了高性能SiC-AlN复相陶瓷,在较佳条件下,复合材料的室温强度、断裂韧性、显微硬度分别高达1130MPa、6．2MPa·m^1/2、28．6GPa．显微结构研究表明,随着AlN的加入,复合材料的晶粒尺寸明显细化,并呈多层次效应,即由固溶体的形成所引起的一次晶粒细化和晶内亚晶界所引起的二次晶粒细化．     

High Preformation SiC-AlN Composites

High performance SiC-AlN composites were fabricated by hot-pressing with Y2O3 as additive via liquid phase sintering. The SiC-AlN composites containing 5 vol. pct AlN exhibit superior mechanical properties with flexural strength, fracture toughness and Vickers hardness of 1131 MPa,6.1 MPa·m1/2 and 28.6 GPa respectively Microstructure observations indicate that the grain size of the composites is obviously inhibited due to the formation of solid solution. TEM-EDS analysis demonstrates the existence of the solid solution. In addition, subgrain boundaries induced by dislocations in the matrix SiC grains, seem to divide a large grain into numerous nano-sized small grains, which significantly increase the mechanical properties of the composites     

Joint strength and interfacial microstructure in silicon nitride/nickel-based Inconel 718 alloy bonding

Joining of Inconel 718 alloys to silicon nitrides using Ag–27Cu–3Ti alloys was performed to investigate the microstructural features of interfacial phases and their effect on joint strength. The Si3N4/Inconel 718 alloy joints had a low shear strength in the range 70.4–46.1 MPa on average, depending on joining temperature and time. When the joining time was held for 1.26 ks at 1063 K, shear, tension, and four-point bending strength were 70.4, 129.7, and 326.5 MPa on average. The microstructures of the joints typically consisted of six types of phases. They were TiN and Ti5Si4 between silicon nitride and filler metal, a copper- and silver-rich phase, island-shaped Ti–Cu phase, a Ti–Cu–Ni alloy layer between filler and base metal, and diffusion of titanium into the Inconel 718 alloys. With increasing joining temperature, the thickness increase of the Ti–Cu–Ni alloy layer was much greater than that of the reaction layer. Thus the diffusion rate of titanium into the base metal was much greater than the reaction rate with silicon nitride. This behaviour of titanium results in the formation of a Ti–Cu–Ni alloy layer in all the joints. The formation of these layers was the cause of the strength degradation of the Si3N4/Inconel 718 alloy joints. This fact was supported by the analyses of fracture path after four-point bending strength tests.     

Composition, structure, and dielectric properties of SiC-AlN ceramic materials

We have studied the structure, relative dielectric permittivity (?), and dielectric loss tangent (tan??) of SiC-AlN ceramic materials. The results demonstrate that both ? and tan?? are anomalously high in the composition range 30?C50 wt % AlN at low frequencies (0.1 kHz). We show that the increase in ? may be due to a barrier effect on silicon carbide and aluminum nitride grain boundaries and to migration polarization.     

Joining of silicon nitride to SUS304 stainless steel using a sputtering method

Silicon nitride with thin sputter-deposited titanium and nickel films was joined to SUS304 stainless steel (18% Cr-8% Ni) using metallic buffers in a series of silicon nitride/nickel/ molybdenum/nickel/SUS304, and the joining strength and microstructures were investigated. Four-point bending tests showed fracture strength of the joints up to 169 MPa. Cracks were formed at the interface between the silicon nitride and its adjacent nickel buffer, and frequently extended into the silicon nitride. Microstructural analyses revealed that the silicon nitride reacted with the sputter-deposited titanium producing titanium nitride and isolated silicon atoms, and that silicon and titanium diffused into the nickel buffer. Calculations using a finite-element method indicated a marked reduction in thermal stress induced in the joined silicon nitride with increasing thickness of the molybdenum buffer. The strong interfacial bond inducing the fracture of the joined silicon nitride was interpreted in terms of a good interfacial reaction, the interdiffusions and the reduction of thermal stress being due to the insertion of the molybdenum buffer.     

Optimization of the Process of Machining of Inhomogeneous Built-Up Parts of Tractors

Materials Science - We study the fracture of inhomogeneous built-up materials by cutting tools made of hard alloys and superhard materials based on boron nitride and determine the characteristics...     

Gefügemodifizierung durch Bornitrid

Grain refinement by boron nitride Grain refinement of magnesium alloys aims at better deformation behaviour, higher strength and improved corrosion resistance. Besides mechanical treatment like pressing, it is possible to refine the grainsize by using nucleation materials. Whereas calcium and rare earth elements are already widely used, the use of boron nitride offers a cheap alternative to refine grains of magnesium aluminum alloys. The effect is achieved by the reaction of boron nitride with aluminum which cracks the chemical compound to form aluminum nitride with the nitrogen while boron is forming different magnesium borides. These two compounds both exhibit very high melting points and are stable in this environment so that they can act as seed crystals. Because boron nitride shows a bad wettability to metal molds, it would float on top of the mold. Therefore, it is necessary to produce pellets out of boron nitride and aluminum powder to improve contact to the mold and enhance reaction velocity.     

Strength degradation of Si3N4 exposed to simulated gas turbine environments

Silicon nitride, sintered with the aid of alumina and yttria, was exposed at 1000°C to two different simulated gas turbine environments. The composition of the reaction gas was varied by delivering either a high or low sulphur fuel into a burner rig. The characteristics of the corrosion product varied markedly with the sulphur content of the fuel. The extent of silicon nitride degradation was examined by two techniques: weight change and 4-point flexural strength. Strength measurements were conducted both at room temperature and at 1000°C. Whereas the weight gain information revealed that corrosion was enhanced in the low sulphur combustion gas, the strength of the corroded silicon nitride did not vary significantly from that of the as-received material. Scanning electron microscopy of the fracture surfaces was utilised to identify the fracture origins in the as-received and corroded samples. Strength, even after corrosion, was controlled primarily by defects introduced during manufacture.     

Fracture of silicon nitride joined with an aluminium braze

Mechanical Properties of silicon nitride joined with aluminium braze have been investigated using fracture mechanics. The highest bonding temperature, 1133 K, produced the highest four-point bend strength of 417 MPa, the strength depending strongly on stress rate. The fracture parameter,N, for slow crack growth in the joint was 29.7 which was near to that of the silicon nitride. Stress corrosion cracking is believed to be one of the serious problems associated with ceramic joining.     

锰在7000系铝合金中的作用及机理

介绍了过渡族金属元素锰在铝中的固溶度变化特点及锰的存在形式;重点讨论了过渡族元素锰对7000系高强铝合金时效行为和力学性能(拉伸强度、延伸率、断裂韧性及抗应力腐蚀性能等)的有益作用及作用机理.铝合金中添加一定量的锰,不影响铝合金的时效动力学,但却明显提高铝合金的强度,对铝合金韧性(延伸率和断裂韧性)、抗应力腐蚀性能等有一定的作用.在现代高强铝合金的研发中,锰已成为一种重要的合金化元素.     

Effect of microstructure and texture on the mechanical properties of the as-extruded Mg–Zn–Y–Zr alloys

This work mainly studied the influence of the microstructure and crystallographic texture on the mechanical properties of the as-extruded Mg–Zn–Y–Zr alloys with different Y contents. The samples were machined from thick plates obtained by extrusion and the tensile tests were performed parallel to extrusion and transverse directions, respectively. Microstructure observation firmly indicated that the grain-refining effect of icosahedral quasicrystal phase (I-phase) was superior to that of the cubic W-phase. In addition, the tensile results indicated that I-phase could effectively improve the strength (yield strength and ultimate tensile strength) of alloys. However, strengthening effect of W-phase was lower. With the quantity of W-phase increasing, the strength of alloys was degraded. It also showed that the alloys were mechanically anisotropic, i.e. the longitudinal strength was higher than that of the transverse direction. However, the ductility of the transverse direction was superior. With the increase of Mg–Zn–Y phases, the anisotropy of the ultimate tensile strength (UTS) between the longitudinal and transverse directions increased remarkably. SEM fracture observations showed that the fractures of the TD samples were characterized by the typical “woody fracture”, with a large amount of cracked Mg–Zn–Y particles (I-phase and W-phase) distributed at the bottom of dimples. With Y content increasing, the average spacing of the zonal distributed Mg–Zn–Y particles on the fracture surface became narrow, which influenced the transverse mechanical properties greatly.     

Mechanical property improvements in Nicalon SiC fibre reinforced silicon nitride ceramics by oxide coating of Si3N4 starting powders

Ceramic matrix composites are attractive as candidate materials for high-temperature applications offering some advantages compared to monolithic ceramics and high-temperature metal alloys. SiC fibre reinforced silicon nitride is one such composite system. However, the processing route is critical to the production of a reliable composite. In this study, silicon nitride matrix densification was improved and sintering temperature was lowered by coating of Si₃N₄ particles with oxides deposited from hydrolysed metal alkoxides. The solution containing oxide coated Si₃N₄ powders was used as a slurry to infiltrate Nicalon SiC fibre tows. Following previous studies, the fibres were heat-treated in carbon monoxide to improve mechanical and surface properties. Infiltrated green bodies were hot-pressed at elevated temperatures to produce dense composites. The results showed that particle coating accelerated densification kinetics, eliminated pores and reduced the required hot-pressing temperature. There was also less fibre degradation as a result of the lower temperature of densification. Bending strength and fracture toughness of the composites were measured and fractography was conducted using scanning electron microscope. Composites manufactured using coated Si₃N₄ powders showed improved properties, specifically matrix stiffening and delayed crack initiation under load.     

Spherical-impact damage and strength degradation in Si3N4-SiC composites

Four types of SiC-whisker/silicon nitride and SiC-particle/silicon nitride composites were produced, and their mechanical properties and impact damage behaviour examined. All of the composites exhibited elastic response behaviour at spherical impact with Hertz cone crack initiation. Impact resistance behaviour, however, was different for each composite. This was due to the different mechanical properties produced by their microstructures. A SiC-platelet/silicon nitride composite displayed the highest resistance to crack initiation and propagation, which resulted in high impact resistance to strength degradation. On the other hand, SiC-particle/silicon nitride, SiC-whisker/silicon nitride, and large SiC-whisker/silicon nitride composites showed less impact resistance, even though they have higher mechanical properties such as bending strength and fracture toughness.     

Effect of rapid solidification on mechanical properties of Cu-Al-Ni shape memory alloys

Mechanical properties of -phase shape memory alloys with the addition of small amounts of chromium or zirconium were studied at various temperatures. In addition to the conventional casting process, Cu-Al-Ni-Zr alloys were produced by rapidly solidified processes, including the gas atomization and melt spinning methods. Buck materials were also made by hot-press sintering from the Cu-Al-Ni-Zr alloy powders and chopped ribbons. It was found that the brittleness of Cu-Al-Ni alloys was improved by the addition of either chromium or zirconium. It is suggested that this improvement is due to increase of grain-boundary strength, not just because of grain refining. The better mechanical properties were obtained in the rapidly solidified and sintered alloys which show fine grain size, fracture strength as high as 780 MPa and 7% fracture strain.     

机械零部件用高强镁合金加工工艺与组织性能研究

针对Mg-Gd-Y合金塑性较差的问题,研究了固溶态和不同温度锻造加工态高强Mg-Gd-Y合金的组织与性能。结果表明,固溶态Mg-Gd合金的晶粒尺寸不均匀,平均尺寸约225μm;当锻造加工温度为440℃和410℃时,合金中第二相的数量较多,大量弥散分布的第二相的存在可以抑制动态再结晶的形成;随着锻造加工温度的降低,Mg-Gd合金的抗拉强度和屈服强度呈现逐渐升高的趋势,在锻造加工温度为470℃时,Mg-Gd合金的断后伸长率达到最大值19.2%,降低锻造加工温度至440℃和410℃时,断后伸长率反而有所降低;固溶态Mg-Gd合金的拉伸断口呈现脆性断裂的特征;锻造加工温度为500℃的拉伸断口呈现混合断裂特征,而锻造加工温度为410℃、440℃和470℃时Mg-Gd合金的断口都呈现为韧性断裂特征。     

Compressive properties of high-pressure die cast IP 75 alloy with strengthening Laves phase

The NiAl-2Ta-7.5Cr-0.5Nb alloys (IP 75 alloy) were prepared by high-pressure die cast (HPDC), and tested for compressible strength and fracture behavior in the temperature range 300-1373 K. The fine structures with a homogeneous distribution of Laves phase at the boundary regions created by high-pressure die cast led to improvements in both the compressible yield strength and fracture strain. The high temperature (1373 K) 0.2% compressible yield strength of the HDC IP 75 alloy (160 MPa) is larger than that of the IP 75 alloys prepared by other processes. The room-temperature compressible fracture strain of the HDC IP 75 (14%) is also superior to the IP 75 alloy (5%) prepared by an ingot-casting process. The effects of size refinement and the more homogenous distribution of Laves phase and the formation of a ductile Cr-rich phase due to a rapid solidification contribute to the increments of the compressible yield strength and the fracture strain of the HPDC alloy.     

Failure behavior of Cu–Ti–Zr-based bulk metallic glass alloys

Microstructure fracture and mechanical properties of Cu-based bulk metallic glass alloys were investigated. Centrifugal casting into copper molds were used to manufacture basic Cu₄₇Ti₃₃Zr₁₁Ni₉, and modified Cu₄₇Ti₃₃Zr₁₁Ni₇Si₁Sn₁ alloys. Although the alloys show an amorphous structure, TEM images revealed the formation of nanoparticles. At room temperature compression tests reveal fracture strength of 2000 MPa, elastic modulus of 127 GPa, and 1.8% fracture strain for the unmodified basic alloy. Whereas the modified alloy exhibits a fracture strength of 2179 MPa, elastic modulus reaches 123 GPa, and 2.4% fracture strain. So, with the addition of 1 at.% Si and Sn, the fracture strength improves by 9% and the fracture strain improves by 25%, but the fracture behavior under compression conditions exhibits a conical shape similar to that produced by tensile testing of ductile alloys. A proposed fracture mechanism explaining the formation of the conical fracture surface was adopted. The formation of homogeneously distributed nano-size (2–5 nm) precipitates changes the mode of fracture of the metallic glass from single to multiple shear plane modes leading to the conical shape fracture surface morphology.     

The influence of manganese addition on mechanical properties of commercial Al‐4Mg casting alloys

The aim of this study was to investigate the mechanical properties of Al‐Mg‐Si alloys aged to peak hardness with different dispersoid volume fraction. It was found that the tensile strength increases with dispersoid content, for alloys having similar ductility. The effect of an increasingly triaxial stress state on a fracture strain above mentioned alloys were measured using a series of notched tensile specimens whose notch root radius of curvature was changed. The alloy ductility was found to increase with dispersoid content and root radius and to decrease with increased stress triaxiality. The fracture toughness of these alloys was determined as a function of dispersoid content and notch root radius of curvature. It was observed that the fracture toughness increased as the dispersoid content and the notch root radius increased. scanning electron microscope analysis of the fracture surfaces revealed that fracture mechanism was transgranular fracture with dimples formation. It is argued that optimum mechanical properties in these alloys can be achieved at about 0.5 % Mn content.     

钛氮合金的铸态组织与性能

采用熔铸工艺法制备了含氮量为0.045%～0.27%的原位自生氮化物增强钛基复合材料,分析并测试了合金的铸态组织和力学性能.研究结果表明:在Ti-N合金中,随着氮含量的增加,合金中氮化物的形态和相组成发生了明显的改变;当氮含量在0.045%～0.18%时,合金的基体为α-Ti,增强相为TiN0.3;氮含量增加到0.225%时,增强相转变为块状Ti2N;复合材料的硬度、抗压强度和弹性模量均高于纯钛基体且随着氮含量的增加而增加;当增强相由TiN0.3转变为Ti2N时,抗压强度显著增加;由压缩断口分析可知,基体为韧性断裂,随着氮含量增加合金由韧窝 解理断口向具有解理特征的脆性断裂转变.     

非连续增强铝合金复合材料的力学性能

用粉末冶金法制备了ＳｉＣｐ／铝合金复合材料，并对其进行了力学性能测试和断裂特性分析；综述了用不同工艺生产的非连续增强ＭＭＣ的性能及影响因素；试图说明增强体／基体界面结合力是铝合金复合材料性能的控制因素；指出寻求适当的界面结合力是复合材料设计中的一个重要内容。