| TiAl based alloys have aroused widely concern on the grounds that they possess low density,good high temperature strength,and favorable oxidation and creep resistance.TiAl based alloys should be canned in the forging process by the virtue of the intrinsic brittleness and narrow hot working window,and this gives rise to complicated procedure and high cost.Based on the newly developed ?solidifying high Nb containing TiAl alloy with only the(?+?)phases(Ti-44Al-8Nb1.5Mn-1.0Cr-0.2B-0.2Y),the hot working characterisitics,finite element simulation,and process optimization were systematically investigated,and the forging without canning was achieved for this alloy.The B2/? phase was completely eliminated and uniform fully lamellar microstructure was reaped via investigating the elimination mechanism of the B2/? phase and lamellae formation mechanism,and regulating heat treatment processes.In addition,oxidation behavior was investigated.The innovations achieved are as follows:(1)The as-cast microstructure of the studied ?-solidifying high Nb containing TiAl alloy only consists of the y and B2/? phases,and the content of the B2/(3 phase is approximately 13.9%.The a2 phase is absent.With the increase in deformation temperature and decrease in strain rate,the flow stress decreases and the dynamic recrystallization increases.The strain iteration method is utilized to establish the constitutive equation and hot processing maps,overcoming the low precision of the method based on peak stress.With the increase in temperature,decrease in strain rate and increase in strain,the efficiency of power dissipation increases.The instability zone is located in the region of low temperature and high strain rate.The hot working temperature should not lower than 1000? along with a strain rate of below 0.1 s-1.Furthermore,the artificial neural network modeling is constructed by the machine learning method,and it exhibits very high accuracy for predicting flow stress.(2)The results of finite element simulation of forging without canning show that when the ratio of height and diameter is 1,the temperature drops large.When the ratio of height and diameter is less than 1.5,the stress is high and it is difficult to deform.When the ratio of height and diameter exceeds 2,the damage factor appears two peaks at the bulging area and deformation inhomogeneity increases,indicating that the workpiece has double bulges.The suitable ratio of height and diameter is 1.5?2 for the studied alloy.At a constant pre-heated temperature,with the increase in strain rate,the distribution uniformity of temperature field is improved,the stress and hard deformation zone are reduced,and the overall deformation uniformity is enhanced.The deformation uniformity increases with the temperature at a certain strain rate.The multi-directional biaxial forging can improve the distribution uniformity of stress and strain,and reduce the hard deformation zones at both ends,improving the deformation uniformity effectively.The multi-directional triaxial forging can achieve more uniform deformation without hard deformation zones.(3)The results regarding the forging without canning show that it is easy to cause cracking for the billet when the temperature of forging dies is low.With the decrease in forging temperature and increase in heigh reduction,the cracking tendency increases.The suitable forging temperature is 1280? along with a strain rate of 0.01?0.05 s-1,the pancakes without cracks can be obtained,indicating forging without canning is successfully achieved.The maximum height reduction of unidirectional multi-step forging and unidirectional single-step forging can reach 70%and 80%,respectively.With the increase in forging temperature and height reduction,the as-forged micro structure only consists of the y and ? phases,the a2 phase is absent,and the content of the ? phase increases from 14.5%to 19.9%.The increase of the temperature and height reduction weaks the elongation of the y and(3 phase,the y grains are dispersed,and the microstructure becomes more uniform.After multi-directional forging,the microstructure uniformity is further improved.The deformation resistance of as-forged alloy is significantly lower than that of ascast alloy.During hot deformation,the ? phase undergoes continuous dynamic recrystallization,and the y phase mainly undergoes discontinuous dynamic recrystallization.Different forging parameters have little effect on the hardness of as-forged alloy.The ductile brittle transition temperature of as-forged alloy is 800?850?.The fracture toughness of the alloy decreases with the increase in forging temperature.(4)The results respecting the microstructure control via heat treatment show that the B2/? phase can be eliminated by ??? in the holding process,and the temperature should not be lower than 1250?.The a phase is formed by ??? a and??? in the holding process.The lamellar structure formed in the furnace cooling stage and the initial formation temperature is approximately 30?40? lower than the annealing temperature.Two primary approaches are identified to form the lamellar structure:nucleated at the grain boundary then grows into grains and precipitated then grows in the grain interior.The oxygen content in the heat treatment process has an important influence on the phase transformation,especially for the annealing in ?+? phase field,and the depth of the lamellar structure increases with the oxygen content.Heat treatment in the ? single phase field promotes the diffusion of the ? stabilizing elements,and increases the?/?interfaces to provide more nucleation sites for the a phase.The following heat treatments in the ?+? phase field can control the transition from the ? phase to the a phase,leading to obtaining a uniform full lamellar structure.The suitable annealing process is that heating to 1450? and holding for 2 h,followed by furnace cooling to 1350? and holding for 9 h,and then furnace cooling to room temperature.(5)The results of oxidation behavior show that the oxidation degree is very low at 700? for the as-cast and as-forged alloys,and the weight gain are 0.075 mg/cm2 and 0.066 mg/cm2,respectively.The scratches on the surface are clearly visible.When oxidized at 800?,the weight gain of as-cast and as-forged alloys are 0.473 mg/cm2 and 0.413 mg/cm2,respectively.The ? phase depresses with the formation of TiO2 particles on its surface,and many holes are formed at the y phase.When oxidized at 900?,the weight gain of as-cast and as-forged alloys are 1.866 mg/cm2 and 1.846 mg/cm2,respectively.There is a mixed product of the Al2O3 and TiO2 on the surface,and the TiO2 grows in the outer layer of Al2O3,but the Al2O3 layer is not completely covered by TiO2.The structure of the oxide film is TiO2 layer/Al2O3 layer/TiO2+Al2O3 layer/Nb+Mn+Cr riched layer.During the oxidation process,the ? phase in the sub-surface layer decomposes.O2 diffuses into the alloy and promotes the phase transformation of ???.The morphology and distribution of the ? phase limits the diffusion of O2 into the alloy,and the more uniform grain distribution makes the distribution of oxide particles more uniform,consequently enhancing the oxidation resistance of as-forged alloy. |
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