Study On Magnesium Matrix Interpenetrating Composites Reinforced By TiAl₃ Network Structure

Owing to the lower density, high strength density ratio, good castability and excellent machinability, magnesium alloys exhibit prominent application in automotive and navigation. However, the poor high temperature properties (work temperature can not exceed 120°C) have limited their use to certain specific application. In order to improve the high temperature properties and reduce the cost in practice, brittle reinforced phase are interpenetrated into the ductile magnesium alloy matrix. Interconnected, inter-twisted and interpenetrated composites are thus formed resulting in some novel and strong composites. The objective of present paper focus on the following aspects: discussing L1₂ -TiAl₃ structure characteristioc, discussing brittle improvement mechanism, enriching reinforcement theory; discussing the topology structure characteristic of TiAl₃, opening out moulding technology of reticulated preform; researching the vacuum pressure infiltration behavior of IPMMCs, discussing the influence factor such as processing and interface characteristic, discussing the crack expanding behavior of interpenetrating composites. The mostly research content are as follows,At first, TiAl₃-Intermetallics superfine powders are fabricated by mechanical alloying and annealing process. The microstructure modification from tetragonal DO22 to cubic L1₂ superlattice has been achieved by addition of Cr to Ti-Al system. The fracture toughness improvement of Al₃Ti, obtaining sufficient number of slip systems, is so far dominantly realized by high energy mechanical alloying and subsequent annealing. At present work, rare earth lanthanum has been selected as ternary element addition to transform crystal morphology and refine crystal dimension. A powder of intermetallic compound L1₂ TiAl₃ has been tested by XRD (D8 ADVANCE), laser granularity appearance(LS13 320, BECKMAN COULTER, USA), DSC(TG209-C), SEM (S—2500) and TEM (JEOL JEM2010). Microhardness of hot isostatic pressing sample were tested by HXD-1000 hardness appearance. mechanical alloying and annealing treatment and an alloy bulk based on Ti-Al-La system has been fabricated by hot isostatic pressing (HIP) technology. The fracture toughness of sample are test by DCM. Next, TiAl₃ intermetallics network structure(NS), processing high porosity(70～95%), larger surface area ratio, smaller expanding coefficient, high chemistry and dimension stability and good oxidation resistance and excellent filtration absorbing property, was fabricated by precursor ablation technology. The cell units of network structure is fourteen polyhedron, extending and forming 3-D network porous structure which follow the Euler law, Aboav-Weaire law and Levis rule. Shape anisotropy is that R₁₂=1.2, R₁₃=1.0. The compressive strength and fracture toughness of NS, influenced by many factors such as porosity, sinter temperature, topology structure, crack in struts and particle size, are improved by reducing cracks, decreasing porosity and increasing sinter temperature. The crack bridge mechanism and blunt crack growth mechanism were discussed. The relative density (ρ*/ρ_s -relative density,ρ*-the density of porous structure andρ_s-the density of the fully dense material) as a important property of NS was measured using the Archimedes' method. The pore size and the width of strut (t) and the length of strut (l)were measured by bubble methods, in which, the cross-section and surface micro-structural characterization of the samples were performed on a scanning electron microscopy SEM (Model No. S-2500) and EDS (OXFOED INCA).Third, IPMMCs was fabricated by vacuum pressure infiltration technology. Chemical compatibility between reinforced phase and matrix phase were discussed according to thermodynamics theory and at the same time, potential chemical reactions of interface field were proposed. Microstructure and topology characteristic of interface field, interface composition, element distribution of two side of interface are researched. Mg is an active element in the matrix alloy and inclined to congregate onto the surface of molten alloy. From the line scanning image of bonding micrographic, we can see that the Mg element had a higher content in the interface than that in the inner of the matrix alloy. The more content of Mg on the surface of molten alloy would thin the oxide surface and decreased the surface tension of molten alloy which assisted the bonding and reaction of interface. The total infiltration processing may be divided three stages: hatch, middle and final stage. Wetting angle of IPMMCs was taken as the critical parameters. The IPMMCs exhibits an excellent wettability under moderate conditions. Aluminum had a difficulty to infiltrate preform without the volume of Mg driven in infiltration processing. The increasing of Mg content (2 wt%-10 wt %) resulted in an increased amount of infiltration. However, the increasing of Mg content beyond 10wt% has an adverse effect. The infiltration process is controlled by an interfacial mass transfer mechanism. Light chemical reaction occurs in the interface of IPMMCs and the reaction productions reduce the surface tension of melt and impulse the infiltration. Infiltration temperature and infiltration time were the key parameters which turn into the infiltration impetus. Three dimensional network structure reinforced aluminum magnesium matrix composites had been fabricated via pressureless infiltration technology. Infiltration temperature and infiltration time are the important variables in fabrication. The presence of Mg content resulted in an increased amount of infiltration. When the volume fraction of reinforcement is 6%, the 0.2%YS is 212MPa and the UTS is 278 MPa. Then with the increases of volume fraction of reinforcement, the strength (0.2%YS and UTS) decrease. The cracks in skeleton result in the reduction in ductility. The presence of second brittle phase results in the increases in hardness of composites. 3-DRMMC with volume fraction of reinforcement as 6% exhibit K_IC of 25.5 MPam^1/2. The failure features as cracking and void in reinforcement, interface cracking and interface debonding as well as matrix damage result in the decreases of fracture toughness. With the increases of volume fraction of reinforcement, 3-DRMMC exhibits excellent wear-resistance property.In addition, a model for abrasive wear of IPMMCs is proposed based on the lower and upper extreme cyclic wear behavior. Owing to the special topology structure characteristic, aluminum alloy reinforced with Al₃Ti network structure can improve dry sliding wear resistance. Reinforcements inhibit plastic flow and restrict propagation of wear cracks. However, due to the cracking in struts or at matrix/reinforcement interface and hollow struts, the fracture toughness and elastic modulus of IPMMCs deceased, therefore, the wear rate increase appreciably compared with the theoretical model. Increase in the volume fraction of reinforcement leads to improvement in the wear resistance. Reinforcements are crushed into small pieces regardless of the morphology of original reinforcement present in the composite, hence wear resistance of composite is marginally affected by the reinforcement volume fraction. The wear mechanisms in IPMMCs could be classified into three modes: adhesive wear, abrasion wear and fatigue wear failure.Finally, the cutting tool materials and cutting parameter had an important influence on the lathe turning properties of IPMMCs. Tool wear increased with increasing of the reinforced phase weight percentage, the VB_max of 0.45wt%C steel is large, the VB_max of K10 took second place and the VB_max of PCD is smallest, so the cutting materials of PCD is one of the optimal materials to machine MMCs. The surface roughness of IPMMCs decreased with increasing of cutting velocity at first, then increased with increasing of cutting velocity owing to reinforced phase participating in cutting and surface roughness linearity increased along with the increase of feed rate not only AZ91 alloy but also IPMMCs, whereas the cutting depth had very little effect to surface roughness. The metal matrix composites reinforced by three-dimensional continuous network structure reinforcement have a particular characteristic of interpenetrating phase with brittle reinforcement inter-twisting ductile matrix. These outstanding features portray IPMMCs to be special machinability characteristics. The fluctuation ranges of cutting force increase rapidly with the increases of volume fraction of reinforcement, and the wear volumes of cutting tool increase constantly. Cutting parameters as cutting speed, feed rate and depth of cut affect distinctly the cutting force in machining on IPMMCs. With the increases of cutting speed, the cutting force decreases gradually in machining IPMMCs. Consequently, increasing the effective cutting speed acts to reduce cutting force. The cutting forces increase with increase in feed rate and depth of cut. Owing to the large fluctuation of cutting force, there were a cratered surface caused by reinforcement pulling-out and fiaking-off and some brittle phase protruding from the machined surface.In conclusion, TiAl₃ intermetallic field become the hot point of research owing to its low density, high strength ang good heat-resistance. While the interpenetrating, intertwisting and interconnecting composites exhibit comprehensive application foreground in automobile, electron, machine manufacture and spaceflight and aviation, owing to their high fracture strength, high strength density ration and control electricity and thermal properties. This project I is forces on not only developing magnesium matrix composites but also extending its application scope, exhibiting impartment theory direct significance. At the present, a patent about this field has been declared. This research was granted by Shandong Science Foundation (Y2006F03)...