Microstructure Design And Deformation Behavior Of Ti-22Al-24Nb-0.5Mo Alloy

In response to the serious deficiencies,such as lower plasticity and fracture toughness of Ti₃Al-based alloys,which long-term service at 650-700℃,Ti₂AlNb-based alloys characterized with good ductility,fracture toughness,high temperature strength and oxidation resistance emerge as the time require.The high temperature service performances of Ti₂AlNb-based alloys are comparable to that of nickel-based superalloy,but the density of them decreased by 40%approximately compared with the latter,thus have the ability to realize the urgent demand for lightweight in the field of aerospace engines.Currently,Ti₂AlNb-based alloys are promising candidate materials to replace the Ni-based high-temperature alloys,which have broad application prospects in the aerospace field.However,for Ti₂AINb alloys with complex phase constitutions,achieving the optimal comprehensive tensile properties and breakthrough of fracture toughness at both room and elevated temperatures through microstructure design are still interesting and urgent problems to be solved.Thermomechanical processing and subsequent heat treatment are important approaches to regulate the microstructure and mechanical properties of Ti₂AlNb-based alloys.The design of the above two processes should be based on revealing the deformation mechanisms of each phase in alloy,thus clarifying the roles of each phase on mechanical properties.In this work,the hot deformation behavior of Ti_-22Al-24Nb-0.5Mo（at.%）alloy in B2 single-phase region was studied systematically,with the aim to provide important theoretical basis for the thermomechanical processing of this alloy.Moreover,the slip deformation mechanisms of each phase in this alloy and their effects on mechanical properties were clarified by adjusting the solution temperature,cooling rate,etc.Based on above research,Ti_-22Al-24Nb-0.5Mo alloy which dominated by coarse lamellar O phase was developed,achieving the breakthrough of fracture toughness and provide a novel microstructure design strategy for the toughening of Ti₂AlNb-based alloys.The main conclusions are as follows:In the B2 single phase region,grain size of B2 was increased with the enhancement of heating temperature,and the higher of heating temperature,the faster of grain growth rate.Under different heating temperatures,the growth index of B2 grain was increased with the increase of temperature,which are close to 0.5.Furthermore,with the extension of holding time,the growth rate of B2 grain presents a first fast and then slow down trend.The serrated grain boundaries have formed when the alloy subjected to hot deformation in the B2 single phase region.Moreover,such serrated grain boundary was analyzed quantitatively by fast Fourier Transform method,and the corresponding relationship between the wavelength of serrated grain boundary and thermal processing parameters was established.There are three formation mechanisms for such serrated grain boundary:strain-induced grain boundary migration,stress-induced B2 phase slip and reorganization of substructure which in the vicinity of grain boundaries.The solution temperature not only directly regulate the morphology and number fraction of primary O phases,but also indirectly affect that of secondary O phases.When the solution temperature was increased from 940℃ to 980℃,the volume fraction and size of the primary O phase gradually decreased and transformed into the rim-O+α2 phase which characterized with core-shell structure.Meanwhile,the volume fraction and size of the secondary O phase precipitated during aging treatment also improved.The yield and tensile strength of this alloy at room temperature both showed a linear positive correlation with the solution temperature.The yield strength is controlled by the B2 matrix phase,while the tensile strength is dominated by the secondary O phase.The fracture toughness of this alloy mainly depends on the relative content between B2 phase and secondary O phase.The secondary O phase,serving as the initiation sites of micro cracks,will increase the probability of micro-crack formation in the front of the main crack.The merging of micro-crack with the main crack promotes the rapid extension of the main crack and decrease the fracture toughness.On the contrary,B2 phase plays a significant role for blunting the cracks and toughening the alloy.The alloy microstructure which dominated by coarse O phase was designed by optimizing the forging process.The room temperature fracture toughness of this alloy is 37±2 MPa·m^1/2,which is higher than that of the reported Ti₂AlNb-based alloy so far.In addition,the alloy has a high temperature fracture toughness of 62±2 MPa·m^1/2 at 650℃,which is 1.7 times as high as the room temperature fracture toughness.It was found that the fracture toughness of this alloy at room temperature is dominated by the intrinsic toughness,and cracks can pass through the O phase by activating the basal,prismatic and pyramidal slip systems.For high temperature fracture toughness,the contribution of extrinsic fracture toughness increased,and the probability of crack propagation along the O/B2 interface at high temperature was also increased significantly.It should be noted that the adjacent O phase with a larger misorientation has a significant impediment effect on the slip transfer,leading to obvious crack deflection.According to the theoretical calculations,the heights of the plastic zone at the crack tip at room and high temperature are 0.11 mm and 2.45 mm,respectively.This is also the reason why the fracture toughness of this alloy at high temperature is significantly higher than that at room temperature.With the increase of tensile plastic strain,the slip mechanism of B2 phase underwent a transition from single-system slip to double-system slip,and then to cross slip.The α2 phase within the B2 grain has no obvious impediment effect to the slip deformation of B2 phase,while those at the grain boundaries can hinder the slip of B2 phase effectively.Moreover,the rim-0 and lamellar O phases are crucial to the improvement of plasticity and work hardening ability for Ti₂AlNb-based alloys.When the propagation of slip line along the O phase,the activated slip system of O phase is {110}<110>,while the activated slip system of O phase is {041}<114>as the slip line pass through the O phase.The secondary acicular O phase precipitated during aging treatment can effectively hinder the rapid and continuous propagation of the slip lines.Therefore,the secondary acicular O phase play an important role in improving the tensile strength and work hardening ability of Ti₂AlNb-based alloys.