A Study Of The Superplasticity Of Representative Intermetallic Compounds

Superplasticity is the general potential property of materials of latticestructure. It's a process that materials establish a stable deformation course afterthey are deformed and become unstable. It's mostly a boundary behavior, i.e.boundary gliding, transferring and shifting occurred as a microcosmic physicalbehavior.Intermetallic compounds are formed by tantalum and metalloid, its atomsare arranged in the form of big length and high order, and its metal bonds andcovalent bonds are coexisting. Therefore, it has not only the better plasticity ofmetal but also the intension of ceramic under high temperature. Intermetalliccompounds are important constructional materials for industry such as aviation,spaceflight, traffic, chem., mechanism et al, also they are functional meterialsfor semiconductor, magnetism, hydrogen storing, superconductor et al, becauseof their low-density, high-melting point, and the property between metals andceramic.Intermetallic compound has generally a problem that it is brittle whichleads to a poor capability to be machined. Shaping by superplasticity offers it afeasible way for machining. My thesis aims at offering a prophase work for thefollowing study on superplasticity of intermetallic compound, which stretchesand extends fields of superplasticity on hard and brittle materials.The application of intermetallic compounds mostly is aluminide, includingSystem Ni-Al,System Ti-Al et al. System Ni-Al has two compounds ofLl2-type Ni3Al and B2-type NiAl. Ni3Al-based alloy has high-melting point andgood ability of oxidation resisting. Ni3Al based alloy can be applied for enginefirebox of turbine and supercharger of diesel engine. NiAl based alloy hashigh-melting point, low-density, good ability of oxidation resisting,high-conductivity, big E value, and has an analogy with metal abovetoughness-brittleness conversing temperature. NiAl has been developed bladeof single crystal NiAl for jet engine in newly phase. System Ti-Al consistsTi3Al and TiAl and is the system studied better. They have low density, highintension under high temperature and good capability for creep resisting, whichenable them suitable for application in aviation and spaceflight. Ti3Al may beused as uphold of startup under high pressure and bottom ring in firebox, and soon. TiAl-based alloy is used as engine blade of turbine, turbine supercharging,auto valve, depressor parts, etc.The superplasticity of L12 structure (Ni3Al), L10 structure (TiAl) and B2structure (NiAl) in single-phase intermetallic compounds was reviewed. Thesuperplasticity of these intermetallic compounds possesses all characteristics ofconventional superplastic materials. The superplasticity of L12 structure andL10 structure stems from the grain boundary sliding, while the dynamicrecovery and recrystallization are the superplastic deforming mechanism for B2structure. The superplastic deformation of NiAl,Ni3Al,TiAl,Ti3Al indual-phase(multi-phase)intermetallic compounds occur because microstructureand grain growth can be restrained and grain boundary gliding can continue.Transmission electron microscopy (TEM) and orientation image microscopy(OIM) observations show that in the superplasticity of large-grainedintermetallic compounds, there are great numbers of subgrain boundaries whichform a network and among which the proportion of low and high angleboundaries increases with the increase of the strain. The observed superplasticphenomenon was explained by continuously dynamic recovery andrecrystallization (CDRR). A dislocation gliding and climbing processaccommodated by subgrain boundary sliding, migration and rotation, enablesthe superplastic flow to proceed.There are two methods to improve the plasticity of NiAl based alloys: toimprove microstructure and to refine grain. The former includes increasing thequantities of independent slip systems ,introducing the plastic second phaseand alloying. The method to improve the plasticity of Ni3Al based alloys isprincipally alloying, while B is the alloy element that is studied better. Themethods to improve the plasticity of TiAl based alloys are alloying,improvingthe fabrication methods of alloys and thermal-mechanical treatment et al. Themethods to improve the plasticity of Ti3Al based alloys are appending the βsubstitution and stabilizing element and introducing the second phase.The brittleness of polycrystalline NiAl based alloys is due to theinconsistentness of grain boundary deformation which is caused by shortage ofslip systems. The toughening mechanism of microalloying is to strong the grainboundary;the toughening mechanism of alloy elements such as Fe,Ga,Mo,Cr are to have an effect upon electron structure and bonding characters, tochange dislocation core structure and to increase dislocation mobility;Theplastic second phase which distributes on grain boundary makes thedeformation of neighboring grains accommodate or passivates cracks andblocks cracks expanding. The room temperature brittleness of Ni3Al basedalloys mostly because the inherence of its grain boundary is weak. There arethree basic toughening mechanisms of B: segregation of B on grain boundarystrengthens the intensity of grain boundary, B makes dislocation slipping ofgrain boundary accommodate and the competitive mechanism of B and H;Feand Mn improve the combinableness intensity of grain boundary;Zr and Beimprove its essential toughness;Cr decrease its mid-temperature brittleness;multi-phase Ni3Al based alloys colligate the effects of some elements. Theessential reason of brittleness of TiAl is that dimensional distribution ofelectron structure in TiAl based alloys is asymmetrical. The tougheningmechanism of alloy elements such as Ti,Mn,Cr,V is that they occupy in thesubstructure of Al, absorb some electrons between Ti-Ti atoms, improvedistribution symmetry of electric charge around titanium atoms accordingly;Erand La capture oxygen atoms from matrix, decrease oxygen content of matrix,and increase purity and ductility of TiAl matrix;The toughening mechanism ofdual-phase(multi-phase)TiAl based alloys is that it improves strength andplasticity by controlling lamellar volume fraction and lamellar thickness of α2and α2-phase absorb interstitial atoms of γ-phase, so decrease bad effects ofinterstitial atoms. The essential reason of brittleness of Ti3Al is that only planeslipping systems can operate, which are typical, while cylinder slipping systemsand taper slipping systems can't at room temperature, so that it's hard to operatefor multi-slip systems and twin deformation. The toughening mechanism ofβ-phase substitution and stabilizing element is that a small quantity of Nb atomsreplace Ti and accelerate operation of more slipping systems, while larger Nbatoms accelerate precipitation of β-phase and O-phase which have moreslipping systems thanα2-phase;The toughening mechanism of dual-phase alloysis similar to it.Superplasticity of NiAl,Ni3Al,TiAl and Ti3Al based intermetalliccompounds is principally fine grain superplasticity, and some NiAl-based andTiAl-based alloys also have coarse grain superplasticity. Deformationcharacteristic of fine grain superplasticity in intermetallic compounds isanalogy with common fine grain materials, but dislocation slipping is morenotable in former. Coarse grain superplasticity is common in intermetalliccompounds. The foundamental reason for these phenomena is the specialstucture of dislocation in intermetallic compounds. Dynamic recrystallizationdoesn't act the same during deformation course: traditional dynamicrecrystallization (DRX) is the coordinate mechanism in the superplasticdeformation of fine grain alloy of intermetallic compounds, while continualdynamic recrystallization acts as the primary mechanism for coarse grain.My thesis entirely summarizes the characteristics,research actuality anddeveloping tends of NiAl , Ni3Al , TiAl and Ti3Al based intermetalliccompounds domestic and overseas, describes their brittleness and tougheningmethods, analyzes their essential reasons of brittleness and correspondingtoughening mechanism from the point of microcosmic. The emphasis of mythesis is their summarization of superplasticity, which sums up experimentalbases, experimental rules and experimental results, also contrasts and analyzesthe procedures and of superplasticity. It offers abundant theory preparations forfollowing research works and experiments and it also can be regarded as abackground and reference for the following study.