Study On The Formation And Growth Behavior Of Lamellae In TiAl Alloy

The low ductility at ambient temperature and the insufficient high-temperature performance resulted from poor homogeneity of the microstructure are the main factors restricting the further expansion of applications of ?-TiAl alloys.The adjustment and control of the lamellar microstructure of these alloys is the effective way to improve the ductility and enhance the high-temperature performance.Therefore,a greater requirement is put forward for the understanding of the formation of lamellar microstructure and the precipitation of ? lamellae.The fact,the precipitation of ? lamellae and the formation of lamellar microstructure are dynamic processes at high temperature,makes the existing research works are mainly based on experimental results using the indirect approach.This prevents the full understanding of these processes.It is evident that the in-situ observation of these processes and the collection of the crystallographic information related in the microstructure evolution are of great realistic significance to get a deeper understanding of the principle of the microstructure evolution and the mechanism of the phase transformation.In view of this,the formation of the lamellar microstructure and the growth behavior of the ? lamellae under different cooling rates in Ti-48Al-2Cr-2Nb alloy were studied by using high-temperature in-situ observation combined with high-temperature quenching experiment in this paper.The microstructure evolution during the formation of the lamellar colony was investigated.The characteristics of precipitation of ? lamella nuclei,the lengthening and thickening of the ? lamellae at high temperature were gathered.The growth regularity of ? lamellae with different thickness and the formation mechanism of surface reliefs of different types were discussed by the examination of the detailed structures of surface reliefs produced by precipitation of ? lamellae.Based on the analysis of the video records of the in-situ observation under different cooling rates frame by frame,the data of the growth kinetics of ? lamellae were obtained and the kinetics under different cooling rates were contemplated.The mechanisms of the precipitation of ? lamella nuclei,the lengthening and thickening of the ? lamellae were investigated through the analysis of the atomic arrangement and element contents in the microstructure around the ? lamellar nuclei,growth tips and huge step under different cooling rates.The main conclusions are listed as follows:The high-temperature in-situ observation and the as-quenched microstructures show that the ? lamellae first nucleate at ? grain boundaries and initially develop in the ? grain with non-BOR then grow into the adjacent ? grain with BOR at their other growth tip to form primary ? lamellae.Intragranular nucleation of ? lamellae within ? grains also takes place and these ? lamellae grow at their both growth tips.During the successive growth process,? lamellae prefer to grow by nucleation next to the precedent lamella or branching of the precedent lamella,thus forming a ‘growth pair'.These lead to the formation of a close arrangement of new ? lamellae near the precedent ? lamellae,which manifests as growth groups of ? lamellae.The two ? lamellae in one growth pair have opposite stacking sequence,and exhibit a twin-related relationship.The branching at lamellar growth tips is due to the respective development of two variants with same stacking sequence in one ? lamella.One branched lamella would stop grow and form a cone shape,and a new ? lamella with opposite stacking sequence would precipitate beside this cone-shaped branch by sympathetic nucleation.Interface migration not only occurs in the intervals between growth pairs and untransformed ? matrix but also develops between two ? lamellae of growth pair.The former leads to the widening of ? lamellae.When the two ? lamellae in a growth pair grow one behind the other,a thin ? lamella and a much thicker ? lamella would form.The surface reliefs are produced by formation of ? lamellae and can be classified into three different types,single-tilt,regular terrace and irregular terrace surface reliefs,corresponding to narrow width,moderate width and large width lamellae,respectively.All of them are related to diffusional terrace-ledge-kink growth mechanism and only single-tilt surface relief combines shear character.The ? lamellae increase thickness by nucleation of growth islands one upon the other,forming a pyramid terraces configuration,and increase length by kink migration.The kink density and size control the lateral growth and further determine the lamellar width.One single-tilt surface relief consists of three regions:(i)? lamella part constituting the major portion of surface relief,(ii)accommodated matrix part occupying a portion in the bottom of the inclined surface,(iii)abnormal structure layer near the specimen surface.In the areas from the surface to the interior of the specimen,? lamella and the adjacent matrix,even the accommodated matrix,maintain the Blackburn orientation relationship.The initial stage of the precipitation of ? lamellae is demonstrated to be a shear process,which results in the formation of the inclined surface for the surface relief.Lateral growth and bunching of the growth steps of the ? lamellae through a diffusional process result in the formation of terraces with a needle-tooth shaped interface and step risers in atomic scale.With the increase of cooling rate,the start temperature of ? lamella precipitation decreases,and the domination of grain-boundary nucleation changes to the domination of intragranular nulceation and the stress-induced nucleation.With higher cooling rate,thickness of ? lamellae formed at high temperature changes into thinner,and the spacing,height and the size of rounded corners of terraces also declined.However,the increase of cooling rate almost has no effect on the thickness of the ? lamellae homogeneous nucleated at low temperature.The growth of the ?-allotriomorph is inhibited by the increase of cooling rate,which makes the morphology of grain boundary change from interlocked-shape to straight grain-boundary.The profile of the nucleation ratio curves at different cooling rates presents “S” type.At the initial stage and the final stage of phase transformationl,the nulceation ratio of ? lamellae is very low and it increases slowly with decrease of temperature.Only in the middle stage,the nucleation ratio increase significantly.The growth rate of ? lamellae becomes faster with the increase of the cooling rate,espcially when the cooling rate is highter than 6 °C/min.The thickening curves show a step shap in the stage II,and the curves obtained under all cooling rates show a parabolic shape.Both the ? lamella nuclei precipitated at high temperature and homogeneously precipitated at low temperature have fcc structure characteristics,and there is a narrow transition zone between the nuclei and the matrix.Structural transformation is achieved by the pruduction of tacking faults along the(0001)planes.At high temperature,the composition transformation falls behind the structure transformation,and the change of element content is concentrated only in the matrix at one side of the fcc structure.For the ? lamella nuclei precipitated at low temperature,the structure transformation and the composition transformation are finished almost at the same time.The lengthening of ? lamellae invovles only the transformation of two atomic layers.The analysis of the type of Shockley partial dislocations involved in the ? to ? phase transformation indicates that the phase transformation follws a displacivediffusive mechanism.The phase transformation produces a concentrated stress field around ? phase,and a directional diffusion of alloy elements may take place due to this concentrated stress field.The fact that the [11?00]?2 direction is not exactly parallel to the [112]? direction after the phase transformation induces a reduction in stability of the interface on one side of the ? lamella,and the Shockley partial dislocations is easier to be activated.The reduction in ?/? interface stability on one side of the ? lamella and the unbalance composition on the two sides trigger the unilateral thickening.