Structural Characterization Of Boride With Large Aspect Ratio In Titanium-aluminum Alloy

Titanium-aluminum alloy is the main material used in low pressure turbine blades of aero-engines at present,and near-net-shape investment casting is one of the important production methods.In order to avoid the uneven structure and performance anisotropy caused by the coarse columnar grains in the as-cast structure,grain refinement is particularly important.Boron is one of the most effective grain refiners.The large aspect ratio borides formed during the fabrication process can easily cause cracks to propagate rapidly,leading to reduced plasticity and even failure of the alloy.Therefore,controlling the morphology of boride is one of the urgent problems in engineering.In this paper,Ti-45Al-2Nb-2Mn-1B alloy as the research object,using SEM,TEM and other methods to explore the microstructure of different forms of borides with large aspect ratio,and the crystal characterization of the internal fine structure,the distribution of different elements was analyzed,and the nucleation and propagation mechanism of microcracks were studied.The main contents are as follows:Combining the structural characteristics of the turbine blades,stepped cast plates with different thicknesses were prepared,and scanning electron microscope was used to explore the effect of cooling rate on the matrix and borides.The grain size of lamellar agglomerates decreases with the increase of cooling rate.The boride is mostly block or slab at slow cooling rate,and the aspect ratio is several to tens;with the increase of cooling rate,the boride gradually transforms into flake or strip shape,and the aspect ratio increases accordingly to several hundred.The transmission electron microscope was used to explore the microstructure of different forms of boride with large aspect ratio.Ribbon,short rod and flaky borides are generally composed of two parts:a blocky structure and a layered structure.The blocky structure is usually located at the end and crystallized well.It is single phase C32 or D7_b,and is easily corroded and perforated during the double spray process,accompanied by growth steps.The layered structure is a multi-phase,composed of a large number of parallel flakes,the width of which is from a few nanometers to tens of nanometers,and it contains up to 4 kinds of boride structures and the B2 phase.The?g parallel rule is used to accurately characterize and analyze the moiréfringes in the dark-field image.Boride has an inhibitory effect on the growth of?/?₂lamellae during the solid-state phase transformation.An energy spectrometer was used to analyze the distribution of elements inside the boride.The content of Mn in the boride is lower than in the matrix,and the content of Mn in the layered structure is higher than that in the blocky structure.Mn is enriched at the B2 phase/boride interface.Compared with the matrix,the content of Nb in the boride is higher and that in the B2phase is lower.Inside the boride,Ti and Nb present a decreasing gradient distribution from the boundary to the core,and the content of Nb in the layered structure is higher than that in the blocky structure.The transmission electron microscope was used to compare the microstructures before and after the appearance of micro-cracks in equiaxed?grains to explore the mechanism of crack nucleation and propagation.A large number of misfit dislocations on the?/?₂interface provide convenience for crack nucleation,and the formation of stacking faults will consume part of the energy of the system and hinder the propagation of cracks.Deformation twins can change the orientation of crystals in a local area and contribute to the occurrence of plastic deformation.The propagation path of the crack is in a"Z"shape,and it deflects at an angle when passing through a stacking fault or grain boundary,which can greatly delay the propagation speed of the crack.