Liang Shuhua

Effect of Al Content on the Properties and Microstructure of Al2O3–Cu Composite Prepared by Internal Oxidation

Combining high-energy milling with internal oxidation, Al2O3–Cu composite was prepared. The effect of Al content on hardness and electrical conductivity of the composite material was studied. The exothermic effect of Al resulting from internal oxidation during fabricating Al2O3–Cu composite was also calculated and discussed. The results show that Al content has a great effect on the properties and microstructure of the Al2O3–Cu composite. When Al content is less than 0.8wt% and the internal oxidation temperature is at 1223 K, the properties of the composite are improved with the increase in Al content. In contrary, the properties of the composite will decrease while the Al content is more than 0.8 wt%. The main reason for this case is the melting of a part of Cu resulting from the greater exothermic effect with increasing Al content. The influence of exothermic effect on the oxidation and sintering temperature is predominant for the specimen preparing procedure. In this context, the choices of heating rate and the internal oxidation temperature should be varied with Al content.

Effect of TiB2 Particle Size on Erosion Behavior of Ag-4wt% TiB2 Composite

Abstract In order to clarify the effect of TiB 2 particle size on the arc erosion behavior of Ag-TiB 2 composite, Ag-4wt%TiB 2 composite reinforced by different grain-sized TiB 2 particles was prepared by mechanical milling and powder metallurgy. The microstructures of Ag-4wt%TiB 2 composites were analyzed, and the global distribution of TiB 2 particles was quantitatively evaluated by the enumeration method. The vacuum arc erosion of Ag-4wt%TiB 2 composite was tested in a modified TDR240A single crystal furnace. The surface morphology of the Ag-4wt%TiB 2 composite after arc erosion was characterized. The mass losses before and after arc erosion were measured. The arc duration was determined by the discharged waveform recorded by a Tektronix TDS-2014 dual channel digital memory oscilloscope, and the arc erosion mechanism was discussed as well. The results show that with the decrease of TiB 2 particle size, the TiB 2 particles are more uniformly dispersed in the Ag matrix, and Ag-4wt%TiB 2 composite presents less mass loss, shorter arc duration, larger arc erosion area and shallower erosion pits. It suggests that fine TiB 2 particle can effectively improve the arc erosion resistance of the Ag-4wt%TiB 2 composite.

Interfacial microstructure of CuCr/1Cr18Ni9Ti bi-metal materials and its effect on bonding strength

The CuCr/1Cr18Ni9Ti bi-metal materials were prepared by the solid-liquid bonding method. The microstructures, mechanical roperties and formation mechanism of the bonding interface were studied. The results show that there exists a serrated transition layer with a certain width at the interface of CuCr/1Cr18Ni9Ti bi-metal materials, and the transition layer consists of Fe-based and Cu-based solid solutions. The elastic modulus and hardness reach the maximum values at the interface closing to the 1Cr18Ni9Ti zone. The bonding temperature has a significant effect on the width and morphology of the transition layer. The interfacial bonding strength is at least 30% higher than that of the CuCr alloy, and the tensile fracture occurs at the side of the CuCr alloy rather than at the bonding interface.

Discontinuous Precipitation Reaction Front of Cellular Recrystallization for a Single-Crystal Superalloy Studied by Electron Microscopy*

Combining analytical transmission electron microscopy systematic tilting, scanning transmission electron microscopy mapping and nano-beam electron diffraction operations, we obtain direct experimental proofs on the boundary type, elemental distribution and structure of the cellular recrystallization reaction front for a single-crystal superalloy. It is demonstrated that the cellular recrystallization reaction front usually corresponds to coincidence site lattice boundaries, and a thin layer of γ-forming elements such as Re, Cr, Mo and Co invariably exists in the direct reaction front. Furthermore, the thin layer with γ-forming elements is proved to be γ phase, with the same orientation as the neighboring original matrix.

Experimental and Simulation Analysis of Thermal Shock with Rapid Heating Followed by Water Quenching for CuW70 Alloys

Abstract By means of the rapid heating followed by water quenching, thermal shock experiments were conducted for CuW70 alloys to study the variations of mechanical behaviour. On the other hand, the simulation with finite element method was implemented to analyze the variations and the distributions of the temperature and the thermal stresses of CuW70 specimens during thermal shock process. The results show that the strength of CuW70 alloys degenerates and the ductility is improved after repeated thermal shock, and the maximum thermal stress appears at the beginning of heating and quenching period. The alternated thermal stresses which vary between tensile and compression stress will be applied on the internal and the external of CuW70 alloys during repeated thermal shock.

Vacuum Arc Characteristics of CuW70 Alloy

Abstract The fine-grained and coarse-grained CuW70 alloys were prepared by sintering and infiltrating. The motion of cathode spots was observed by a digital high speed video camera and a scanning electron microscope (SEM). The results show that the fine-grained CuW70 alloy has larger breakdown strength, longer average arc life and lower chopping current. For coarse-grained CuW70 alloy, the cathode spots ignite and extinguish alternately beneath the anode, the breakdown is mainly located on copper phase, and the erosion pits are much deeper. However, the cathode spots of fine-grained CuW70 alloys have the characteristics of division and deviation, with the uniform distribution of the erosion pits on the entire cathode surface.

Mechanical Behavior and Microstructure of WCu Alloy under Hot Compression

Abstract Considering the application of WCu alloy at elevated temperature, hot compression was applied on WCu alloy and W skeleton to analyze the microstructure and mechanical behaviors at different temperatures and strain rates. The results show that the stress-strain behavior of WCu alloy is affected mainly by the temperature. The sintering necks disappear mostly after hot compression. The plastic flow of WCu alloy appears at 300 °C while W particles are mostly homogeneous approximately at 900 °C. In addition, the strength of W skeleton declines slightly at high strain rate, while round grains and cracking appear at the substructure of W particle under different strain rates.

Simulation of Meso-scale Accumulated Damage and Induced Crack Behaviors of CuW Alloy

Abstract Considering the relationship between Meso-scale accumulated damage and microstructure, external cyclic strain load was applied on 2D representative volume element (RVE) of CuW alloys, and the accumulated damage and induced crack behaviors of CuW alloy were stimulated using Darveaux model. The results show that the plastic slip bands appear on the copper phase at the beginning. Damage and induced cracks of CuW alloy initiate at the corners and then propagate along the edges of tungsten grains with the increase of cyclic number. The propagation path of micro-cracks is affected by the distribution of tungsten grains mainly, while the sintering necks slow down the propagation speed of micro-cracks to some extent.

Mechanical Properties and Thermal Shock Resistance of Zr, Cr Doped WCu Composite

Abstract WCu composites were fabricated by infiltrating Cu alloys doped with Zr and Cr into W skeleton in order to strengthen the W/Cu interfaces. The effects of Zr and Cr on the mechanical properties including flexural strength and impact toughness, and thermal shock resistance of WCu composites were investigated. The results show that the addition of a small amount of Zr and Cr can increase the flexural strength and the impact toughness of WCu composites. With the increase of thermal shock temperature, the flexural strength of WCu composites decreases. However, the addition of Zr and Cr in WCu composite can reduce the effect of thermal shock on mechanical properties obviously, improving the thermal shock resistance of WCu composites.

The 1.85 GPa AlSc Bulk Alloy with Abundant Nanoscale Growth Twins

An in situ ultrahigh-strength ductile Al50Sc50 bulk alloy is produced by the copper mold casting method with a composite microstructure of micron-/submicron-sized grains and nanoscale twins. According to the microstructural investigations, hierarchical nanotwinned lamellar AlSc bundles with embedded micron-/submicron-sized Al2Sc and AlSc2 are observed. The as-cast alloy displays a unique act of ultrahigh strength of ~1.85 GPa together with pronounced work hardening and a large plasticity of ~14%. Further microstructural investigations on deformed specimens indicate that abundant hierarchical nanotwinned lamellar AlSc bundles are effective to dissipate localization of shear stress or block dislocations from spreading throughout the alloy and hinder the propagation of microcracks formed by local stress transition.