Advanced Materials Research Vols. 306-307

Abstract: The face-centered cubic model is used to investigate the dynamic crushing behavior of high density closed-cell foams. The influences of the constant loading rate and the specimen aspect ratio on the crushing stress were discussed. It is demonstrated that the crushing stress is more sensitive to the constant loading rate than the specimen aspect ratio. To describe the dynamic crushing behavior of the foam theoretically, the idealized rigid-perfectly plastic-locking (RPPL) stress-strain model is extended to a more general case, in which both the density and the cross-section area are discontinuous. The good agreement between the finite element results and theoretical results confirms that the dynamic crushing behavior of foam can be described by the modified RPPL stress-strain model.

Abstract: Mechanical fatigue tests were conducted on specimens of A356-T6 casting alloys under multi-axial cyclic loadings with 5 loading paths (proportional, circular, square, diamond, and ellipse path). Optical microscopy and TEM were used to examine both undeformed and fatigue failure specimens. It is shown that undeformed material has coarse microstructure with low density of dislocation. Under every loading path, cyclic hardening dominates the whole fatigue process. Additional hardening is found in this casting material, but the rate and extent of cyclic hardening and additional hardening are quite dependent on particular loading paths. Various dislocation substructures are observed in the specimens including specimens fatigued to 20 cycles and fatigue failure specimens. While these dislocation substructures are determined by the moving ability of dislocation and interactions between dislocation and particles. Low cycle fatigue life is sensitive to the difference of loading paths, and the fatigue life increases as the extent of cyclic hardening decreases.

Abstract: TiAl based intermetallic compound claddings were produced on TA15 alloy surface by using laser depositing technology to melt Ti-46Al-2Cr metal powders. An interface layer between TA15 substrate and the TiAl claddings was formed. The influence of laser power, scanning speed and number of cladding layers on the interface layer and the hardness of TiAl claddings were investigated. Higher laser power and lower scanning speed made the interface layer thicker. Increasing the laser power and especially the scanning speed could improve the hardness of the TiAl claddings. When the second TiAl layer was deposited, there was no interface layer formed between the two TiAl layers, but the hardness of the first layer decreased and the second TiAl layer was softer than the first layer due to the rough microstructure.

Abstract: The microstructures and kinetic characteristics have been studied during heat treatment of AM60B alloy prepared by thixoforming. The results indicate that primary particles and secondary grains coarsened rapidly in initial 1 h solution treated at 430°C, and then slowed down. Their growth modes were different in the initial 1 h and then turned to be same. Accordingly, the alloy hardness dropped quickly before 1 h. Ageing hardness depends on both content of precipitates and the morphology. Adding content of the precipitates hardened the alloy till 8 h aged at 215°C after 24 h solution treatment at 430°C, and then the precipitates coarsening weakened the ally.

Abstract: Mechanical energy aided aluminizing process was applied on 1Cr18Ni9 stainless steel. Aluminized layer of 30-180μm was obtained after heat preservation for 3h at low temperature of 650-700°C. The microstructure of aluminized layer and the mechanical energy aided aluminizing mechanism were analyzed. Aluminized layer consists of three major phase, pure Al layer, Me₂Al₇phase layer and Me₂Al₅ & MeAl two-phase layer, respectively. Me₂Al5 and MeAl of the two-phase layer have different Ni content, with the precipitated phase of chromium-silicon compounds.

Abstract: The effects of sintering temperature on the properties of W-9.8Ni-4.2Fe alloys fabricated by spark plasma sintering were studied. The peak, base, repetition frequency of pulse electric current and the initial average no-load current were chosen as 3000A, 360A, 50Hz and 1200A during the sintering process, respectively. Research results show that W-9.8Ni-4.2Fe alloy, consolidated by SPSing for 6min at 1250°C under a pressure of 30MPa, has a relative density of 97.72%, hardness of 41.8HRC, transverse rupture strength of 1443MPa, and compression strength of 2640MPa. Its combination properties are topmost among all specimens sintered at various temperatures, which is attributable to the microstructural characteristics of dense microstructure, uniform distribution of binder, low connectivity of W-W and good metallurgic bonding. The optimal fracture surface in relation to the microstructure is dominating tungsten intergranular fracture, accompanied by local transgranular fracture in tungsten phase and ductile avulsion of the binder phase.

Abstract: This study adopted home-made embedded piezoelectric transducers and external commercial ultrasonic transducers respectively to monitor early hydration processes of cement. This paper discussed the development of ultrasonic amplitude and frequency spectrum during the hydration process. It is proved that the method is effective for monitoring the hydration of cement paste. The embedded transducer could resolve the coupling problem of ultrasonic monitoring and it could be used for the whole life health monitoring of concrete.

Abstract: Based on uniaxial tensile test, the complete uniaxal tensile stress-strain curve of Reactive Powder Concrete (the steel fiber content by volume is V_f =1%, 2%) was obtained, and the fracture energy of RPC specimens with cross-section of 100mm by 100mm was calculated. The test was finished through Universal Testing Machine without any stiffness-strengthen devices. In order to solve the stress concentration problem, a self-designed uniaxial tensile test equipment was developed, and a dumbbell-shaped specimen was used in the test. The results indicate that the fracture energy of RPC increased as well as the increasing of V_f.

Abstract: The cleavage fracture behaviors are studied in notched specimens of a low alloy hot rolled steel 16MnR. The results show that two types of cleavage initiation sites are existed in notched specimens, one being related to the inclusions ahead of notch root (IC type) and the other related to inclusions located ahead of string cracks far from the notch root (SIC type). The types of initiation sites are influenced strongly on temperature, changing from IC type at -196°C to SIC type at -130°C. In both IC and SIC initiation mechanisms, the crack nucleation is induced by inclusions and the final fracture is controlled by propagation of a ferrite grain-sized crack into matrix grain. The cleavage fracture of IC initiation type in notched specimens satisfies a dual-criterion model, i.e. a critical plastic strain ep ³e_pc for initiating a crack nucleus and a critical tensile stress syy³s_f for its propagation. While for SIC initiation type, the dual-criterion model is evolved with the expression of e_p+eps³e_pc and s_yy+syys³s_f.

Abstract: CaO-Al₂O₃-SiO₂ system glasses with MgO were prepared by melt quenching. The glass structure was characterized by Infrared spectra (IR) and Differential Thermal Analysis (DTA). Dielectric properties, devitrification and chemical durability have been studied. IR spectra and DTA indicated that glass network structure change by doping MgO. The glass chemical improved and dielectric constant decreased after doping with MgO. The glass shows a severe tendency of phase separation.