| TiAl-based alloy is a new type of high temperature material which may replace some Ni-based alloys below 1000? used in Aeroengine Blades and exhaust valves because of its low density,high elastic modulus,excellent high temperature creep resistance and oxidation resistance.However,the problems of low plasticity at room temperature and difficulty in forming become the shackles of its widespread usage.In this paper,the microstructure evolution and mechanical properties of Ti-44Al-9Nb-1Cr-(W,Y)alloy are studied.The effects of W,Y alloying and drawing rate on the directional solidification process,microstructure orientation and high temperature mechanical properties of the alloy are analyzed by the improved liquid metal cooling directional solidification method.The main factors affecting directionally solidified Ti-Al alloy and the mechanism of improving microstructure uniformity and columnar crystal growth stability are discussed.The main research results are as follows:The Ti-44Al-9Nb-1Cr-(W,Y)alloy which solidified in beta phase is designed.The phase transformation diagram and the element composition of each phase of Ti-44Al-9Nb-1Cr alloy are obtained by thermodynamic calculation.The phase transformation path of the alloy is:L??+?????+?2??2+laves??+?2+?.There is no peritectic transformation in the phase transformation.The high temperature beta phase enriches the beta phase strengthening elements like Nb,Cr and W at room temperature and forms the B2 phase.The CCT curves and TTT curves of the alloys are computed to predict the formation of martensite in the alloys The simulation results of mechanical properties show that the strength of the material is greatly affected by the strain rate at high temperature,and the creep at high temperature causes the strength of the material to decline rapidly.The microstructure and properties of the as-cast alloy with equiaxed grain composition are studied.It is found that the addition of Cr can enhance the resistance of long-term oxidation and refine grain boundaries effectively.The addition of W and Y can refine grain effectively,while Y is superior to W in microstructure optimization.The tensile strength and elongation of the three as-cast alloys are obviously lower than those of the basic compositions due to the insufficient diffusion of W and Y elements,but this can be improved by a short time annealing treatment in the alpha region.The near lamellar microstructure with uniform distribution,low hardness and uniform composition can be obtained by short time annealing or normalizing in the temperature range of the alpha phase.The martensite with uniform fineness and little hardness change can be obtained by quenching at room temperature from no less than 1150?.The cyclic heat treatment refines the microstructure by the forming of small lamellar clusters with different orientation in a large amount at the grain boundaries or grain boundaries.The directional solidification experiment of the designed alloy is performed.The measured temperature gradient is 19?/mm which is good for the growth requirement of the alloy.When W element is added alone,very fine and discontinuous columnar grains can be obtained only at extremely high drawing rate.Microstructure uniformity is poor and the beta segregation is serious.Other alloys alloyed with W and Y have uniform full-lamellar columnar crystals.With the increase of drawing rate,columnar grain size of directionally solidified specimens with different compositions decreases,the B2 phase segregation becomes more serious,and the homogeneity of grains and the quality of casting microstructure also decreases.Y element alloying can remarkably improve the growth stability of directionally solidified grains and slow down the influence of heat flow on the macrostructure of directionally solidified alloys,thus obtaining well-grown directionally solidified microstructure.In addition,W and Y elements can refine the columnar crystals further under the premise of guaranteeing the stability of columnar crystal growth.The columnar crystals with stable growth and no impact of heat flow can be obtained at lower drawing rate,and the segregation of B2 phase in the alloy can also be controlledThe quenched dendritic structure of W and Y co-alloyed materials by directional solidification is studied.It is confirmed that the alloy solidified in beta cooling path.With the increase of drawing rate,the content and range of B2 phase segregation in primary dendrite increase obviously.Meanwhile,Nb,W and other stable elements in beta phase are enriched.and the homogeneity of microstructure decreases.Y element exists in pre-solidified dendrite as nucleation and solution strengthening element,and the corresponding dendritic gap is Y-poor,which indicates that Y element does not affect the directional solidification process of the alloy while optimizing the microstructure of the alloy.Combining with the fact that B2 phase segregation aggravated by XRD analysis,the increase of growth rate can refine the size of columnar grain and grain boundary,but obviously reduce the homogeneity of the alloy structure.High temperature tensile tests on the specimens with stable growth and directional solidification at the corresponding rates show that:when the pulling rate increases,the angle between the lamellar orientation and the tensile direction increases.Meanwhile,the preferred orientation of the lamellar decreases,thus the corresponding axial tensile properties of the alloy decrease. |
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