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Microstructure And Mechanical Properties Of NiAl-based Eutectic Alloys And Alloying Behavior Of Rare Earths

Posted on:2003-12-11Degree:DoctorType:Dissertation
Country:ChinaCandidate:W L RenFull Text:PDF
GTID:1101360092480358Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
NiAl-based eutectic alloys consisting of NiAl and refractory metal phase have been recognized as a success system because they show a combination of improved toughness and creep strength. This paper systematically and thoroughly studied the creep and the brittle to ductile transition(BDT) behavior of NiAl-9Mo eutectic alloy and the effect of rare earth elements(REEs) on microstructure and properties of eutectic alloy NiAl-28Cr-5.5Mo- O.SHf. The microstructure and the intrinsic relationship between the microstructure and deformation mechanism were analyzed and characterized by means of X-ray diffraction, auger electron microscope, scanning electron microscope and transmission electron microscope.The influences of strain rate and temperature on the tensile behavior of as-cast and HEPed NiAJ-9Mo eutectic alloy were researched at the temperature range of 700-950 癈 and over a strain rate range from 2.08xlO-4s-1 to 2.08xlO-2s-1. The results indicate that HDP process causes an enhancement in ductility and a decrease in ultimate tensile strength, yield strength, average strain hardening rate as well as a drop in brittle to ductile transition temperature(BDTT) under the same condition. It is noticed that the BDTT of as-cast NiAl-9Mo is more dependent on strain rate than that of EDPed material. And the brittle to ductile transition process in the alloy is related to a sharp drop in strain hardening rate. Regardless of strain rate, the fracture morphology changes from cleavage in NiAl phase and debonding along NiAl/Mo interface below the BDTT to microvoid coalescence in nature above BDTT. The apparent activation energy calculated for the BDT of HEPed and as-cast material are 327 and 263KJ/mol, respectively, suggesting that the mechanism is associated with lattice diffusion in NiAl phase. In order to determine which temperature is mostly related to the brittle fracture and to gain a physical understanding of BDT, the fracture behavior over the BDT range was thoroughly investigated under a constant strain rate. The fracture behavior over the BDT range can be characterized by four temperature stages: brittle cleavage, ductile cleavage, mixed fracture(fibrous and cleavage), ductile fracture. The characteristic temperature TAB of overlapping point between brittle cleavage and ductile cleavage can be defines as brittle to ductile transition point. Because the temperature corresponds to the resistance to cleavage starts to sharply increase and the dislocation density starts to dramatically increase.The creep behaviors of HIPed NiAl-9Mo eutectic alloy were investigated in the temperature range from 850 to 950 癈 under the applied stress of 50-100MPa. All the creeprvcurves exhibit similar shapes: very short primary creep and long steady-state creep stages. The steady-state creep rate is found to depend on the applied stress and temperature. The power law stress exponent for steady state creep was determined to be 4.75+0.25 and apparent activation energy for creep to be 410.5+4.5KJ/mol. The nucleation and propagation of creep cracks caused the beginning of the ternary creep stage. The creep fracture data can be described by the Monkman-Grant relationship. The alloy fails by mainly intergranular creep fracture and small partially ductile fracture. The proportion of the former decreases as the increasing stress at the same temperature or the increasing temperature at the same stress. The steady-state creep behavior is controlled by the dislocation locally climbing over the relative hard Mo particle. The particle does not exert an attractive force on the dislocation, which make the creep process showing a natural power-law dependence of dislocation velocity on applied stress with n=3~4 and an unrealistically small threshold stress. The large anelastic strains are observed for the alloy during the stress change-strain transient creep tests. When the composite is deformed and unloaded, the elastic strain energy stored in the fibers provides a driving force for the recoverable strain. The anelast...
Keywords/Search Tags:NiAl intermetallic compound, Rare earth elements, Eutectic alloy, Mechanical properties, Grain-boundary segregation, Oxidation properties, Creep, Brittle to ductile transition
PDF Full Text Request
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