Renjie Wu

Microstructural characterization of TiB in in situ synthesized titanium matrix composites prepared by common casting technique

Abstract TiC reinforced titanium matrix composites were produced by non-consumable arc-melting technology utilizing the self-propagation high-temperature synthesis (SHS) reaction between titanium and graphite. X-Ray diffraction (XRD) was used to identify the phases in the composites. Microstructures of the composites were observed by optical microscope (OM) and transmission electron microscope (TEM). The results show that there are two phases in the composites: TiC and titanium matrix alloy. TiC has two different shapes: dendritic shape, equiaxed or near-equiaxed shape. The in situ synthesized TiC grows by dissolution–precipitation. Analysis of the binary phase diagram determined that the solidification path undertook the following three stages: primary TiC, binary eutectic β-Ti+TiC and solid transformation. Primary TiC grows in dendritic shape due to the formation of composition undercooling. Binary eutectic TiC grows in equiaxed or near-equiaxed shape. A small quantity of TiC may form twin structure during nucleation and growth. The twin plane is the (111) plane.

The interfacial characterization of oxidized SiC(p)/2014 Al composites

Abstract The passive oxidation behavior of SiC particles at elevated temperatures has been studied. The evolution of interfacial reaction products in 2014 aluminum alloy composite reinforced with oxidized-SiC particles after extended thermal exposure at elevated temperatures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction. Although it could prevent the interfacial reaction between SiC particles and aluminum alloy, the preoxidation of SiC particles resulted in the formation of other interfacial reaction products. The results showed that at elevated temperatures nano-MgO formed initially on the surface of the oxidized SiC particles and then turned into nano-MgAl 2 O 4 crystal due to the reaction between the SiO 2 and aluminum alloy containing Mg. TEM observations indicated that the oxidized layer on SiC particles was uniform and had a good bonding with SiC and aluminum alloy.

The interfacial reaction characteristics in SiC/Al composite above liquidus during remelting

The development of interfacial reaction between matrix and reinforcement during the remelt recycling is critical to the commercialization and sustainable-development of metal matrix composite, but it is very difficult to characterize the interfacial reaction between the reinforcement and matrix melt above the liquids using a conventional experimental method. In this paper, the interfacial reaction characteristics in SiCp/Al composite above liquids were investigated using liquid metal X-ray diffraction and differential scanning calorimeter (DSC). A solute-rich region with much more Si content above liquids was observed in the composite melt, and the experimental results also showed that the interfacial reaction feature is associated with the solute-rich region with much more Si content. Mechanism for the observed phenomena was discussed in detail.

Microstructure and tensile properties of in situ synthesized (TiBw + TiCp)/Ti6242 composites

In the present work, (TiBw+ TiCp)/Ti6242 composites were fabricated via common casting and hot-forging technology utilizing the SHS reaction between titanium and B4C. The XRD technique was used to identify the phases of composites. The microstructures were characterized by means of OM and TEM. Results from DSC and analysis of phase diagram determine solidification paths of in situsynthesized Ti6242 composites as following stages: β-Ti primary phase, monovariant binary eutectic β-Ti + TiB, invariant ternary eutectic β-Ti + TiB + TiC and phase transformation from β-Ti to α-Ti. In situsynthesized reinforcements are distributed uniformly in titanium matrix alloy. Reinforcement TiB grows in whisker shape whereas TiC grows in globular or near-globular shape. TiB whiskers were made to align the hot-forging direction after hot-forging. The interfaces between reinforcements and Ti matrix alloy are very clean. There is no any interfacial reaction. Moreover, the mechanical properties improved with the addition of TiB whiskers and TiC particles although some reduction in ductility was observed. Fractographic analysis indicated that the composites failed in tension due to reinforcements cracking. The improvements in the composite properties were rationalized using simple micromechanics principles. The strengthening mechanisms are attributed to the following factors: undertaking load of TiB whiskers and TiC particles, high-density dislocations and refinement of titanium matrix alloys grain size.

The effect of Si upon the interfacial reaction characteristics in SiCp/Al-Si system composites during multiple-remelting

The composite interfaces play an important role in determining the resultant composite properties, especially the development of interfacial reaction during remelting is critical to the commercialization and sustainable-development of metal matrix composites. In this paper, the interfacial reaction characteristics of SiCp/Al-Si system composites during multiple-remelting were investigated by Differential Scanning Calorimeter (DSC). It was found that the interfacial reactions were not sensitive to remelting number, remelting temperature and reinforcement volume fraction after a degree of reaction, and the results also suggested that the preventation effects of Si upon the interfacial reaction SiCp/Al were mainly attributed to the Si released from the interfacial reaction, while the original Si content in the master alloy also has the same effect only after a given Si content.

The oxidation of SiC particles and its interfacial characteristics in Al-matrix composite

The passive oxidation behavior of SiC particles has been studied in an electric furnace at atmospheric pressure and in dry air, the weight change due to the transformation from SiC into SiO2 is descibed as a function of exposed temperature and holding time. According to the oxidation data of SiC particles, the oxidation parameters and the degree of oxidation for SiC particles can be controlled. Controllable preoxidation of SiC particles is one of the keys for designing interface and interphase to achieve high performance aluminum composite. Consequently, the evolution of interfacial reaction products in 2014 aluminum alloy composite reinforced with oxidized-SiC particles after extended thermal exposure at elevated temperatures were further characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and X-ray diffraction. While it could act to prevent the interfacial reaction between SiC particles and aluminum alloy, the preoxidation of SiC particles led to the formation of other interfacial reaction products. The observation of the microstructure revealed that at elevated temperatures nano-MgO formed initially on the surface of the oxidized SiC particles and then turned into nano-MgAl2O4 crystal due to the reaction between the SiO2 and aluminum alloy containing Mg. TEM observations indicated that the oxidized layer on SiC particles was uniform and had a good bonding with SiC and aluminum alloy.

The passive-oxidized behavior of SiC particles and their jointing characteristics

Abstract The passive-oxidized behavior of as-received α-SiC particles has been studied on the basis of the weight changes of the transformation from SiC into SiO 2 , which are dependent on the elevated temperatures and time, the volume fraction of SiO 2 and its thickness by assuming a spherical dependence on the elevated temperatures and time that also shows the same parabolic trends as the curves of the weight gain. According to the oxidation data of SiC particles, the oxidation parameters for SiC particles and the oxidation degree of the preform made from as-received SiC particles can be selected. An oxidized-jointing network skeleton among the particles of the preform is formed, as observed by SEM. These oxidation characteristics of SiC particles are beneficial for producing complicate and strong preforms for composites application in electronic packaging and in the aerospace industry.

Polytypism of SiC and interfacial structure in SiCp/Al composites

In the present work, the polytypism of SiC and the interfacial structure between SiC and Al were investigated using X-ray diffraction (XRD) and high resolution transmission electron microscopy (HREM). It was approved that 15R could be juxtaposed with 6H stacking sequences in the same SiC reinforcement and a structural transformation zone was also observed. The Al4C3 compound can nucleate on SiC at the SiC/Al interface with the growth orientation parallel to the C axis of SiC. Mechanisms for the observed phenomena were also discussed in detail.

Interfacial reaction between the oxidized SiC particles and Al-Mg alloys

The interfacial reactions of oxidized SiC particles reinforced Al-Mg matrix composites were investigated by the field emission-scanning electron microscopy (FE-SEM), TEM and X-ray diffraction. It was found that the nanoscale MgO forms initially due to the interfacial reaction, then whether it reacts with molten Al continuously or not depends on the content of Mg in the matrix and its covering densification at the surface of particles. When there is not enough Mg in the matrix for the formation of dense MgO layer, MgO will transform into MgAl2O4 crystal owing to the continuous reaction with SiO2 and molten Al. When dense MgO layer forms at the surface of the particles due to the affluence of Mg for the initial reaction, it will protect the inner SiC from the attack of molten Al. However, the reaction products of both MgO and MgAl2O4 are thermo-stable phases at the surface of the particles under high temperature. The results clarify the interfacial reaction route and they are of great value to the control of the interfacial reactions and their interfacial design of the composites.

The melt structures above the liquidus in SiCp/Al composite

The properties and the structures of liquid melt above the liquidus play an important role in determining the resultant properties of the alloy and the composite. In this paper, the melt structures of SiCp/Al composites with two different reinforcement volume fractions were investigated using liquid metal X-ray diffraction and differential scanning calorimetry. The experimental results showed that the melt structures of SiCp/Al composite are different from that of liquid matrix alloy, a Si–Si hump in the pair radial distribution function in SiCp/Al composite melt and the DSC trace indicated that Si is not well distributed above the liquidus, the size change of short-range-order (SRO) in SiCp/Al composite melt indicated the diffusion of Si from chemical reaction between SiC and molten Al is possible only after a given temperature or after some extent concentration fluctuation of Si. The mechanisms for the observed phenomenon are also discussed in this paper.