Research On The Mechanical Behaviors And Yield Criteria Of Directionally Solidified Ni₃Al Based Alloys

Ni3Al which has been extensivley studied is one of the most promising industrial intermetallic compounds. It has the properties of high melting point, high specific strength and high strength and creep resistance as well as oxidation resistance at high temperatures. Those excllent properites make it very attractive in both aerospace and civil fields. Alloy IC10, a Ni3 Al based multiphase LI2 alloy, is produced by directionally solidification in [001] orientation. As a new generation of Ni3 Al based directionally solidified superalloy developed in China, IC10 is used in turbine engines. It has different mechanical properties compared to triditonal disorderd alloys. However, the previous research has concentrated only on the solidification direction [001]. None studies focus on the anisotropy and tension/compression asymmetry as well as the yielding behavior of alloy IC10. Obviously, studies of those aspects are essential for the application of the alloy. In this paper the mechanical properties of IC10 are studied through macro and micro tests.Plastic deformation of the Ll2-long-range ordered intermetalic alloy IC10 was investigated focusing on the anomalous yield behavior, yield anisotropy as well as tension/compression asymmetry and the yielding behavior in plane stress state.And new yield criteria used to describe the yielding behaviors of IC10 are developed based.The main work done in this thesis includes:Tension and compression tests along different orientations of IC10 have been carried out, including:(1) tensile tests along [001] at the temperature range from 298~1273K(2) tension and compresstion tests along different orieations. Based on the experimental results, the temperature and orientation denpendence of the yield strength as well as the tension/compression asymmetry of IC10 have been invesgated. The following conclusions were obtained: the yield strength of the Ni3Al-based superalloy IC10 increases with increasing temperature up to 1073 K followed by a sharp decrease which has been observed in many other Ll2 ordered materials. Based on the results of TEM observation, it is concluded that the abnormal temperature dependence of yield strength is closely related to the Kear-Wilsdorf mechanism which involves cross slip of screw dislocations from {111} planes to {010} planes. The decrease in the flow stress above peak temperature was attributed to macroscopic slip on {100} planes. The anomalous yield behavior of IC10 is less pronounced compared to other LI2 ordered single crystals. This may be explained by the microstructure of the directionally solidified polycrystalline. Except for g’ phase, there is a considerable volume fraction of g phase.The CRSS of alloy IC10 is anisotropic and tension/compression asymmetric.The grain boundaries of alloy IC10 affect the CRSS value in tension significantly rather than in compression. Biaxial tensile and compressive tests of IC10 have been carried out using cruciform specimens. The experimental yield loci of IC10 in plane stress space have been successfully determined and compared with the yield loci calculated from several existing yield criteria. It is found that the yield loci of superalloy IC10 is asymmetric about the balanced biaxial tension line, which indicates strong plastic anisotropy of the material.Moreover, the yield locus for alloy IC10 has an asymmetrical shape between tension and compression.The isotropic Mises criterion, which contains only one material parameter, can’t describe the anisotropic yield behavior of alloy IC10. Some existiong yield criteria fit the data points well in the biaxial tension region, but significant discrepancies are found for some stress ratios in biaxial compression region between the theory and experiment results.Traditional yield functions are based on the hypothesis of equal yield strength in tension and compression, which failed to describe the strength differential(SD) effect(different tensile and compressive yield stresses) of IC10.Based on the yielding behavior of Ll2-ordered materials, a criterion deduced from the one introduced by Barlat and Lian has been proposed in the full plane stress space. In comparison to the original one, the new yield criterion can describe both anisotropy and tension/compression asymmetry of solids. It was shown that the proposed yield function appears to be appropriate for the description of the anisotropy and asymmetry observed in the Ni3Al-based intermetallic alloy IC10.A new yield criterion that can model the asymmetry and anisotropy in yielding of pressure insensitive metals, in terms of accuracy and simplicity of formulation, is developed in this thesis. First, a new isotropic yield criterion, which can model the asymmetry in yielding of pressure insensitive metals, is proposed. Further, using Cazacu’s generalizations to anisotropic conditions of the invariants of the deviatoric stress, the proposed isotropic yield criterion is extended to orthotropy. The proposed anisotropic criterion has a quite simple form, and the number of material constants involved is only half of that of Cazacu’s(2004) yield criterion. Compared to Hill’s(1948) yield criterion, the proposed anisotropic yield criterion has three additional constants, which are used to model the tension-compression asymmetry of materials. All material constants involved in the criterion can be determined by simple tests. The proposed criterion reduces to Hill’s(1948) yield criterion if the tensile and compressive yield stresses are equal. In other words, the proposed anisotropic yield criterion can be considered as an extension of Hill’s(1948) criterion to tension-compression asymmetry materials. The anisotropic criterion was then applied to describe the asymmetry and anisotropy observed in the Cu-Al-Be shape memory alloy and the Ni3 Al based super-alloy IC10. Very good agreement between theoretical and experimental initial yield loci was obtained. Compared to the existed criteria with the ability to capture SD effects, the proposed criterion has a much simpler formulation and involved fewer coefficients. We believe that the simplicity of the proposed criterion will make it very attractive for applications.