Research On The Mechanical Properties And Constitutive Equations Of Alloy IC10

Alloy IC10, one of the newly developed multiphase Li₂ alloys, is considered to be near-future candidate for advanced high temperature structure materials in aeroengine hot sections due to its excellent mechanical properties over a wide range of temperatures. IC10 is subjected to adverse and complex operating conditions and exhibits unusual thermo-mechanical flow behaviors. However, the studies on the mechanical properties and constitutive equations for IC10 are still very rare. In this thesis, the mechanical properties of IC10 were studied through both macro and micro tests. And the constitutive equations used to describe the flow behaviors of IC10 were developed through both phenomenological and physically scales. The main work done in this thesis includes:The tests on mechanical properties of IC10 were conducted. And the variation rules of mechanical properties were studied. In order to investigate the features of stress-strain curves and the mechanical properties (such as elastic moduli, elongation, yield stress and strain hardening rate), the tensile tests were conducted over a wide range of temperatures (25～1100℃) and strain rates (10-4～10-2/s). The fracture morphology and the micro-mechanisms of IC10 were studied based on the results of SEM and TEM tests. Experiments show: There are only work hardening behaviors in stress-strain curves over the range of 25～700℃. And the dynamic recovery behaviors are obvious in the stress-strain curves at 800℃. There are only dynamic recrystallization behaviors during the whole deformation above 800℃. IC10 exhibits the anomalous features of yield stress and strain hardening rate. During the special range of temperatures, yield stress and strain hardening rate are insensitive to strain rate and they increase slightly with temperature. Above the peak temperature of them, they drop off significantly and exhibit strong strain rate sensitivity. When the temperature is below 800℃, the fracture is transgranular cracking mixed with a few dimpled and intergranular fractures. The fracture is ductile cracking above 900℃. The TEM observations reveal that the macro mechanical properties are the reflections of the dominating long screws which are connected by superkinks and locked by KW locks.The mechanistic model and its numerical method were developed in this thesis. The mixed unit of the phase ofγ′andγis considered to be the micro-features unit of IC10. And two assumptions were proposed: the yield and hardening of IC10 are the net effect of point obstacles which resist the motion of mobile dislocations. The mobility of dislocations is connected with its superkink heights. The evolution of the mobile dislocation density was developed based on the evolution of whole dislocation density and the distribution of their superkink heights. And the evolution of obstacles density was derived from the evolution of the mobile dislocation density. The UMAT program of the model was written. And the flow behaviors over the range of 25～700℃were simulated by the model. The results show that the model is valid.A new constitutive equation, which can be used to describe the dynamic recovery/ recrystallization behaviors, was developed in this thesis. In the new model, the whole stress-strain curve is divided into the strain hardening part before the peak stress and the strain softening part after the peak point. The relationships between the model parameters (such as hardening factor K_h, softening factor K s, hardening exponent n_h and softening exponent n_s ) and the mechanical properties (such as yield stressσ_0.2,critical stressσ_c , critical plastic strainε_c ,broken stressσ_b and elongationε_b) were developed. The new model was used to predict the flow behaviors of IC10 above 800℃. Compared with the prediction of Cho's model and the experiments, the new model is proved to be valid.The L-H model, J-C model and ZA model, which were widely used to describe the hardening flow behaviors, were modified, that is the modified L-H model, the modified J-C model, the modified ZA model 1 and 2. All these four modified models were used to predict the flow behaviors of IC10 during the range of 25～700℃. And the results show that: the modified L-H model and the modified J-C model are easy to use due to their simple formulation, few parameters and high accuracy.The strain rate sensitivity was defined in this thesis. The strain rate sensitivities and the thermal activation volume were analyzed quantificationally based on the strain rate jump tests and the stress-relax tests. The strain rate jump tests were conducted over the range of 600℃～1100℃. And the relax tests were conducted over the range of 600℃～1000℃. Experiments show that: the strain rate sensitivity is very low below 800℃. And it increases lightly at 800℃. It is apparent above 800℃. The thermal activation volume is very small. And the volume decreases with the increase of the deformation. At the range of 600～900℃, the volume decreases with the increase of the temperature. The volume increases with temperature at the range of 900～1100℃.