Showing papers by "University of Science and Technology Beijing published in 1999"

Recent and Prospective Development of Functionally Graded Materials in Japan

Abstract: The concept of functionally graded materials (FGMs), originally developed for the purpose of reducing thermal stress, has now greatly expanded and is being applied to the development of electrical, optical, magnetic, and biochemical materials. Here in Japan, several FGM research projects have been promoted with the support of the government and other organizations. Many societies concerned with material science now include special sessions or symposia on FGMs at their annual conferences to promote FGM research, and FGM research in Japan has been increasing year by year Many novel fabrication processes have been developed. and various new materials with unique properties have been prepared by introducing graded structures For example, achievement of successful control of electrical properties of perovskite-type (La 1-x Ca x ) MnO 3 has led to a new approach to the control electronic transport behavior by a graded structure, and the advanced optical filter characteristics of graded TiO 2 /SiO 2 multilayer film provide a new avenue not only for application of FGMs but also for understanding their mechanisms In this paper, some recent interesting findings on the design, process, and characteristics of FGMs are described. Future possibilities and trends in FGM research are also discussed.

Fabrication of Monolithic Ti3SiC2 Ceramic Through Reactive Sintering of Ti/Si/2TiC

Abstract: Fabrication of monolithic Ti3SiC2 has been investigated through the route of reactive sintering of Ti/Si/2TiC mixtures. Significant phase differences existed between the surface and the interior of as-synthesized products due to the evaporation of Si during the reaction process. The use of a 3Ti/SiC/C mixture as a powder bed could control the evaporation of Si and develop monolithic Ti3SiC2. A reaction model for the formation of Ti3SiC2 in the Ti/Si/2TiC system is discussed.

Combustion synthesis of α–Si3N4 whiskers

Abstract: α–Si3N4 whiskers were fabricated first through combustion synthesis (CS) technology. The as-synthesized crystals with [001] as the elongated axis contain a large amount of linear defects including (a + b)-type, (a + b + c)-type, and c-type of dislocations. The growth of α–Si3N4 whiskers is mainly controlled by vapor-condensation (VC) mechanism. Special additive plays an important role in promoting the growth of α–Si3N4 whiskers and suppressing the phase transformation of α–Si3N4 to β–Si3N4.

Fluid inclusion characteristics of mesothermal gold deposits in the Xiaoqinling district, Shaanxi and Henan Provinces, People's Republic of China

Abstract: Fluid inclusions were studied in quartz samples from early (stage I) gold-poor quartz veins and later (stage II) gold- and sulphide-rich quartz veins from the Wenyu, Dongchuang, Qiangma, and Guijiayu mesothermal gold deposits in the Xiaoqinling district, China. Fluid inclusion petrography, microthermometry, and bulk gas analyses show remarkably consistent fluid composition in all studied deposits. Primary inclusions in quartz samples are dominated by mixed CO2-H2O inclusions, which have a wide range in CO2 content and coexist with lesser primary CO2-rich and aqueous inclusions. In addition, a few secondary aqueous inclusions are found along late-healed fractures. Microthermometry and bulk gas analyses suggest hydrothermal fluids with typically 15–30 mol% CO2 in stage I inclusions and 10–20 mol% CO2 in stage II inclusions. Estimates of fluid salinity decrease from 7.4–9.2 equivalent wt.% NaCl to 5.7–7.4 equivalent wt.% NaCl between stage I and II. Primary aqueous inclusions in both stages show consistent salinity with, but slightly lower Th total than, their coexistent CO2-H2O inclusions. The coexisting CO2-rich, CO2-H2O, and primary aqueous inclusions in both stage I and II quartz are interpreted to have been trapped during unmixing of a homogeneous CO2-H2O parent fluid. The homogenisation temperatures of the primary aqueous inclusions give an estimate of trapping temperature of the fluids. Trapping conditions are typically 300–370 °C and 2.2 kbar for stage I fluids and 250–320 °C and 1.6 kbar for stage II fluids. The CO2-H2O stage I and II fluids are probably from a magmatic source, most likely devolatilizing Cretaceous Yanshanian granitoids. The study demonstrates that gold is largely deposited as pressures and temperatures fall accompanying fluid immiscibility in stage II veins.

Deformation mechanisms in a high-Nb containing γ–TiAl alloy at 900°C

Abstract: Extensive twinning, the glide and climb of ordinary dislocations, and much activity of 〈011] and 1/2〈112] superdislocations were observed in the γ phase of Ti–45Al–10Nb alloy after deformation at 900°C at a strain rate of 5×10 −4 s −1 . The presence of moderate density of superdislocations is likely to arise from a high-Nb alloying in γ phase. Two important effects of high-Nb alloying in γ phase were identified: the increase of the critical resolved shear stress (CRSS) for ordinary slip, and the decrease of the stacking-fault energy (SFE), as a result of Nb solid solution and/or low Al concentration in high-Nb alloying γ phase. The CRSS and the SFE were estimated approximately to be on the magnitude of 180 MPa and ∼20 mJ/mm 2 , respectively. The high CRSS and the difficulty of dislocation climb due to low SFE are believed to be the major factors responsible for the enhanced high-temperature strength retention of Ti–45Al–10Nb alloy.

Improvement of room temperature tensile properties for Fe3Al-based alloys by thermomechanical and annealing processes

Abstract: The effects of thermomechanical and annealing processes on room temperature tensile properties of Fe 3 Al-based alloys have been investigated. It is found that the thermomechanical process affects the room temperature tensile properties of Fe 3 Al-based alloys greatly. It is very important to control the hot deformation process so that a fine grain microstructure can be obtained before the final warm-rolling process. A higher room temperature ductility combined with a higher yield stress can be obtained by refining grains of Fe 3 Al-based alloys and obtaining the B2 ordered structure with a suitable surface state, such as oil film, Al 2 O 3 film and an aluminium-poor region near the surface. The intrinsic factors, such as microstructure and crystal structure, and extrinsic factors, such as surface condition, must be considered when the sensitivity to environmental embrittlement is investigated. An advanced thermomechanical process, named controlled thermomechanical process, to further improve room temperature tensile properties has been developed. It consists of hot forging an ingot to 20 mm at 1000–1200°C, hot rolling it to 10 mm at 850–1000°C, warm rolling to 5 mm thickness at 500–680°C, then annealing at 800°C for 1 h, then warm rolling to 2 mm thickness at 500–680°C.

Effect of rare earths on the carburization of steel

Abstract: The effects of rare earths (RE) on the carburization of steel are examined with four 0.2%C steels doped and undoped with RE. Clearly, both RE in carburizer and RE in steel may accelerate the carburizing process. RE in carburizer is more effective at enhancing carburization than RE in steel. The mechanism for this enhancement of carburization is that the RE oxide enhances the medium-sample interface reaction during carburizing.

Investigation of the abnormal effects of phosphorus on mechanical properties of INCONEL718 superalloy

Abstract: The effects of phosphorus on mechanical properties of the world’s most widely used superalloy INCONEL718 was examined. The results indicate that P has little effect on the tensile properties of INCONEL718 at room temperature, 500 or 650°C, but it increases stress rupture life and ductility. Phosphorus also improves the creep property and crack propagation resistance of INCONEL718. Further microstructure analysis by means of optical, SEM and TEM does not reveal the obvious effect of P on grain size and precipitation of INCONEL718. Some theoretical discussions were held aiming at further understanding the role of P in INCONEL718. It was suggested that the interaction of P–GB–M (metal element) complex might be responsible for this.

Comparison of precipitate behaviors in ultra-low carbon, titanium-stabilized interstitial free steel sheets under different annealing processes

Abstract: Ultra-low carbon, titanium-stabilized interstitial free (ULC Ti-IF) steel sheets are widely used in the automobile industry because of excellent deep drawability. The annealing process is critical to their final property, and there are two different annealing processes used in industrial production of interstitial free (IF) steel sheets, namely batch annealing and continuous annealing. In this study, precipitation behaviors of titanium IF steels in the two annealing processes were investigated. Among the most common precipitates in titanium IF steels, that is, TiN, TiS, Ti4(CS)2, and TiC, the size and dispersion of TiN, TiS, and Ti4(CS)2 remained almost unchanged after either annealing process. Conversely, the average size of a TiC particle increased substantially after both annealing processes, while TiC after continuous annealing was larger than that after batch annealing due to the higher heating temperature of continuous annealing. Two new particles, FeTiP and (Ti, Mn)S, were also observed in the batch annealing process but not in continuous annealing. The structure of FeTiP and (Ti, Mn)S were studied, and furthermore the evolution of FeTiP precipitation was found to be closely related to recrystallization in batch annealing. Finally, the interrelation among processing parameters, precipitation behaviors, and final property was studied.

Analysis of the Dynamic Performance of an Underground Excavation in Jointed Rock under Repeated Seismic Loading

Abstract: Results from field observations of dynamic behaviour of an underground excavation have been compared with numerical studies of the rock deformation history. The field behaviour shows progressive accumulation of rock displacement and excavation deformation under successive episodes of dynamic loading. It is possible to reproduce the modes of rock response quite well using a Distinct Element model of the rock mass, but the way displacements develop is dependent on the joint model used in the analysis. It is suggested that, in rock masses subject to repeated dynamic loading, excavation design may need to take account of the prospect of repeated episodes of transient loading at the excavation site.

Positron annihilation study on the stress-field pinning mechanism in (Eu,Y)-123 superconductors

Abstract: The simultaneous measurements of Doppler-broadened radiation spectra and lifetime spectra have been performed on (Y 1− x Eu x )Ba 2 Cu 3 O 7− y ( x =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1) superconductors. The mean lifetime and S -parameter, respectively show a characteristic significantly symmetric to Eu-doping concentration. Since lifetime and Doppler broadening spectroscopies are rather sensitive to the variation of the lattice distortion and defects induced by the stress-field of lattice mismatch, such symmetric behaviors of the S -parameter and the mean lifetime suggest a striking characteristic of stress in Y 1− x Eu x -123 superconductor. When the doping concentration x reaches 30%, the density of defects corresponding to stress-field appears the maximum, and there is an optimum distribution of the stress-field so as to increase critical current J c in Eu-doped Y-123 superconductors. Our results of the critical current density obtained by means of measurements of susceptibility χ ′( T , H AC ) and χ ″( T , H AC ) also suggest that the enhancement of the critical current density to reach an optimum when Eu doping x is 30%. We also qualitatively discuss experimental results in the framework of the model of stress-field pinning. The model calculating results also show the stress-field induced by lattice mismatch is an effective pinning center, and further conform that the stress pinning mechanism is dominant in RE-doped Y-123 superconductors.

Microstructure and thermokinetics analysis of combustion synthesized AlN

Abstract: The mechanism for combustion synthesis (CS) of AlN was investigated in the present paper. A novel quenching technique was developed to retain the high-temperature structure formed in the sample during the combustion process. Based on the microstructural observation of the quenched specimen and thermodynamic analysis, a two-step reaction mechanism for CS AlN, i.e., vaporization of Al and subsequent reaction with N2, is proposed.

The 1100 °C isothermal section of the Ti-Ni-Si ternary system

Abstract: The isothermal section of the Ti-Ni-Si ternary system at 1100 °C was constituted. Twenty-one single-phase areas were determined. The binary Ti3Si phase proposed to occur at 1100 °C was not found in the investigation. A new phase, with the composition of NiTi4Si4 named H, was detected. Some of the ternary compounds show noticeable composition ranges, especially G′-Ni3Ti2Si.

Formation of α phase in the massive and feathery γ -TiAl alloys during aging in the single α field

Abstract: Precipitation of {alpha} phase in massive and feathery microstructures was studied during aging of a Ti-48 pct Al-2 pct W-0.5 pct Si alloy in the single {alpha} field. It was found that the {alpha} phase mainly precipitates along the {gamma}-plate interfaces as laths in the feathery structure, while it nucleates at various sites in the massive structure in the form of idiomorphs and especially of plates. The {gamma}{sub m} {yields} {alpha} reaction proceeds by the growth of pre-existing {alpha} precipitates and chiefly by the development of new {alpha} plates. The {alpha} plates are likely to originate from the splitting of unit dislocations into Shockley partials and to grow by the diffusional ledge mechanism, which shows both diffusional and shear character. During aging, the stacking faults (SFs) in the massive {gamma} domains evolve into SF-shaped {alpha} precipitates through a transition {gamma}{prime} phase.

Lead free solder materials In–Sn–Zn system

Abstract: We investigated the bulk as well as surface properties of the In–Sn–Zn system because of its potential as lead free solder materials. Differential thermal analysis X-ray and microprobe analysis were performed on forty ternary samples. Our X-ray investigation showed a single phase at 10 and 25 at.% Zn and two phases (γ + Zn) at 40 and 55 at.% Zn. Quenched samples showed an invariant effect in γ + [Zn] solid solution. Heat treatment did not produce any effect on the ternary eutectic temperature. A statistical mechanical theory based on a layered structure has been used to develop expressions for the surface tension (γ) and surface composition (Cs) in the frame-work of self-association and compound formation models. These expressions have been utilised to compute γ and Cs of In–Sn and Sn–Zn systems as a function of concentration and temperature. Our study suggests an increase in γ of both (In–Sn and Sn–Zn) systems with the addition of In to Sn and Zn to Sn, respectively. It is interesting to observe that Sn atoms segregate to the surface of both the systems at all bulk compositions – with the larger extent of segregation in Sn–Zn. Segregation of Sn atoms and formation of di-atomic like atoms clusters (Sn2) might be influencing the wetting and adhesion in solder joints.

Influence of the second-phase particle size on grain growth based on computer simulation

Abstract: A three-dimensional Monte Carlo simulation method for grain growth in two-phased materials is set up, basing on a micro-physical analysis of the interaction between the second-phase particle and the grain boundary Two-phased systems containing second-phase particles with the same quantity but different sizes are designed, and the complete processes of grain growth are simulated The influences of the particle size on grain growth are observed and studied quantitatively