The project will aim to produce a novel composite: High entropy alloys composite. For this report is aiming to demonstrate the summary of the background reading, research progress and the forward plane.
The literature review parts concentrate on three aspects: Metal Matrix Composites (MMC), Bulk Metallic Glass (BMGs) and the Bulk Metallic Glass Matrix composites (BMGMCs). This project is mainly considering the Ti-based materials.
Metal Matrix Composite (MMC)
Metal matrix composite (MMC) is a kind of new engineering material developed rapidly in the recent years. On the basis of the matrix materials, it can be classified as: Al-base, Ti-base, Mg-base and Cu-base etc. According to the reinforcement of the materials, it can be divided into continuous and discontinuous fiber reinforced metal matrix composite. (For the discontinuous fiber reinforced aspect, the particle reinforcement is an important methodology). Metal matrix composites material combines the advantages of the metal matrix and the reinforcing constituent. It reveals kinds of characteristics: high specific strength, high stiffness, good electrical and thermal conductivity, and fatigue properties. According to these properties, the materials are applied in the aerospace and mechanical industries etc. The microstructures of the material are depending on the reinforcing constituent. The microstructure will contribute to the properties change of the materials. For the high strength and high stiffness properties, the fiber and particles have been added into the matrix that will increase the strength of the composites. Because of that the metal matrix still accounts main structure of the composites (Usually over 60%), the electrical conductivity is very good. The state between the reinforcements and the interface of the metal matrix, distribution of the reinforcements will contribute to the fatigue ability .
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MMC is a kind of composite, which means it is a multiple phase material. Because of the contact of the different components of the material, the interface will occur. It encompasses the original contact area of the reinforcements and the matrix, the new reaction product of the matrix and reinforcements, the oxide of these two components, etc. So the chemical composition is very complicated in this region. That leads to the discontinuous or step by step change of the properties of the materials in this region. The high temperature condition or heat treatment will contribute to the interface change. The segregation, diffusion, solid solution will occur vary with the cooling rate and the solidification .
Matrix is a very important factor for forming the MMC. For the particle reinforced MMC, the matrix is the main component to endure the external load. The strength of the matrix is the key to make the composite. In order to get the high performance composites, the selection of the matrix is very important. There are kinds of metal can be the matrix: Al, Cu, Ni, Ti, Zn, etc. During the preparation of the metal matrix, the reaction between the matrix and reinforcements must be considered. Because of the high temperature will cause the different interface reactions, the stability of them will lead to the property change, so during the experiments how to make the reinforcements have a positive reaction with the matrix must be solved. For this project, Ti will be the metal matrix. Element Ti will form the stable compounds with the oxygen and hydrogen under the high temperature. Titanium alloys reveal high specific strength and the excellent corrosion resistance. For the pure titanium, when the temperature below the 882Â±2ï¸’C, it will be the hexagonal close packed structure, it is often called Î± titanium. Above this temperature range it is body centered cubic structure Î² titanium. Because of the elements of the alloys are different, the allotropy transformation temperature will be different. If we want to increase the Î± titanium phase, and increase the allotropy transformation temperature, we can use Al, O, C, to boost the Î² phase decomposition and transfer to the Î± phase. In order to enlarge the Î² phase Cu, Ni, Co can be used. During the heat treatment of the titanium alloys the transition phase will occur, the ductility and the hardness will be affected. So we must avoid the growth of the martensite  .
1.2 Bulk Metallic Glass (BMG)
In order to introduce the term Bulk Metallic Glass, there are some basic knowledge should be mentioned. We know that the metal and alloys are considered to be the crystalline structure. If the atomic arrangement in the solid materials is random, it is referred as amorphous. For the amorphous alloys, they usually form at very high cooling rates. The aim of that is to limit the nucleation of the crystalline phase. But if the materials can be quenched at a sufficiently high cooling rate, the formation of the equilibrium crystalline phases will be suppressed, so that will retain the amorphous structure. According to the analysis, the assumption of control the critical cooling rate between the range has been made. And this assumption is proved by Klement et al in 1960. They performed the rapid-quenching process on the Al-Si alloys  . And the structure of this material is amorphous, that is referred as the metallic glass. Seeking the alloy compositions for the glass formers becomes a very important project. And the alloy combination which with the lower critical cooling rate will be investigated since that time. According to the demonstration of Chen and Turnbull, the glass composition Pt-Ni-P can be formed at critical cooling rate less than 1000. That is considered to be the first bulk metallic glass . We can find that the most important feature of the BMG is that the glass transition is from super cooled liquids to the glassy state which occurs when cooling from the temperature high to low. That can distinguish it from the other amorphous materials. In 1993 Johnson and Peker reported that the composition Zr-Ti-Cu-Ni-Be which contains 22.5% beryllium. They observed that the beryllium atom will fill the empty space of the glass structure that will stabilize the glass and liquid phase  .
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Along with the development of the BMGs, we can find that there are lots of attractive properties of them: high hardness and strength, good corrosion resistance etc. And the large size of BMGs also can be made. But the low ductility and fatigue resistance cannot be solved very well. The plasticity and toughness of this kind of materials must be improved  . The plastic deformation mechanism of the BMG is the deformation according to the initiation and propagation of the shear bands. This is because of the nucleation of the shear bands, which will grow and convert to the shear transformation zones (STZs). When people investigate the mechanical properties of the BMGs, uniaxial compression tests are often performed. Usually the sample size is 1 or 2mmï¼Œ but the ductility will be low in this situation. Researches to overcome this problem are increasing in the recent years. X.J.Gu et .al. reported that they use BMG to determine the changes of compressive plasticity and fracture by using different sample size. The sample size has been change from 1mm diameter rods to 5*5 rectangular bars. They found that the Poisson's ration and compressive plasticity decreasing with increasing the sample size .
1.3 Bulk Metallic Glass Matrix composites (BMGMCs).
Along with the research of Bulk Metallic Glass which shows a lot of excellent properties. But the low fracture toughness and the failure under the tensile loading lead to many problems and limit the application of the materials. In order to overcome these disadvantages, using the glassy matrix combine with the different length scale second phase particles has been developed. That is the BMG composites . There are two classifications of this kind of composites: ex situ and in situ. The ex situ composites can be formed by: 1.General alloying process that will contain the glassy phase and reinforcing second phase particles. 2.Adding the reinforcements (crystalline second phase) directly into the glass forming process. The in situ one can be made by: 1.Choosing the melts at the applicable time during the solidification, 2. "A secondary treatment of amorphous precursors". In the recent years the Bulk Metallic Glass Matrix composites are being investigated. This kind of materials is two phase alloys that consist the dendrites grow in the glass-forming matrix (Dendrites and the BMG matrix). This material not only has the advantages of the BMGs but also reveals good performances in tensile ductility and fracture toughness. Differ from other materials BMGMCs' grain structures are very unique. Multiple phases, dendrites and grain boundary, the combination of them contribute to the changes of the dislocation and microstructure features  .
Shape memory alloys are being investigated in the recent years. So develop the microstructure contains the BMG matrix and crystalline phases with the concept of transformation induced plasticity becomes popular. The Ti-based and Zr-based composites have been reported [11-12]. According to the demonstration of Y.Wu et.al, for the significant work-hardening capability, the BMG composites will have large tensile ductility. This research group also found that the CuZr crystalline phase (B2 phase) of the system is body-centred. And the martensitic transformation from B2 phase to the B19' phase  . The deformation mechanism of the dendrite composites has been clearly stated by J.Eckert group. The ductile phase starts to yield first, and some of the load will apply to the glassy matrix. (The interface between the beta phase and matrix will affect this). The shear band will initiated and propagate inside the glassy phase after the yielding. And then the slip occurs. Along with the increasing of the shear strain, the beta-phase will obstruct the propagation. So the shear band will move around the dendrites .
A recent research reported by S.Pauly et al. shows the Ti-Cu-Ni shape memory bulk metallic glass composites were machined by controlling the cooling rate very carefully. They found that the metastable microstructure of this material consists mainly of the spherical ductile maternsitic precipitates in an amorphous matrix. The multiple shear bands are formed because of the stress concentration applied around the precipitates area. And the composition of the B2/B19 phase can be changed by the Cu-rich phases around the grain boundaries .
Douglas C. Hofmann has summarizes the recent development of the shape memory bulk metallic glass composites. This new materials will crystallize the softer phase compare with the general metallic glass. In order to achieve the ductility of the material, the shear modulus of the crystal phase must be lower than the matrix. But now only add the beta-stabilizer to the material can be considered as a solution . But the discovery of the shape memory reinforced metallic glass composites still boost the structural application of the amorphous alloys.
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The plane for the following experimental process will be:
Continue practicing the arc melting machine (melting and casting).
Produce high standard materials based on the paper Ti-Cu-Ni shape memory bulk metallic glass composites.
Use XRD, DSC, EDX and SEM for the sample characterisation (Observe the microstructure and phase formation).
Mechanical test will be performed.