| Beta fleck(?-fleck)is a local defect of the enrichment of ?-stabilizing elements in titanium alloys.In this thesis,due to the addition of 4 wt%of the easy-segregating alloying element Cr,TC17 alloy(Ti-5Al-2Sn-2Zr-4Cr-4Mo)was used as the material for this research.The characteristics of ?-flecks in the industrial-scale ingot,the industrial-scale forged bar,the directional solidified sample and the experimental-scale ingot were systematically investigated.The effects of the solidification conditions and process parameters for the ?-fleck formation were investigated.Based on the above,the formation mechanisms of the ?-flecks during the solidification process were discussed.By the research in this thesis,we provided enough experimental and theoretical bases for avoiding ?-fleck segregation defects during the titanium alloy melting process.Main characteristics of the ?-flecks in the industrial-scale ingot and the industrial-scale forged bar were investigated.The results showed that the ?-flecks in TC17 alloy were the local areas where were rich in Cr and Zr,and depleted in Mo,causing a 15-35? decrease of Tp in the local areas.The ?-flecks with similar shapes appearing as irregular spot-like structure were found in the center of the industrial-scale ingot head After hot working process,the ?-flecks which were characterized as the abnormal ?grains in the forged bar,exhibited different morphologies from them in the ingot.However,all ?-flecks appears the same chemical profiles from the matrix to the ?-fleck center.It can be concluded that the ?-flecks in the forged bar should be evolved from the ingot and survived through the hot working process.Combined with the analysis for the TC17-X phase diagrams,it can be concluded that the ?-flecks were induced by the enrichment of k<1 element(Cr and Zr)and the depletion of k>1 element(Mo)during the solidification process.TC17 alloy were melted and directionally solidified at a constant temperature gradient(G=2×104 K/m)at different solidification rates(R).The aim for the directional solidification experiment was to make clear the variation of solute distribution and evolution of solid-liquid interface for TC17 alloy during solidification process.The results showed that when the solidification rate increased from 3 mm/h to 150 mm/h,the shape of the solid-liquid interface transited from planar through cellular to dendritic structure.The critical rate at which the planar interface collapsed completely was about 15 mm/h and the transition rate from cellular to dendritic structure was about 30 mm/h.At the same time,the macro-segregation which was in the final solidified areas transited to the local segregation which was in the intercellular or interdendritic areas.And two kinds of ?-flecks were found respectively,which were the macro-segregation ?-flecks and the channel-like ?-flecks.In order to clarify the ?-fleck formation during vacuum arc remelting(VAR)process,an experimental-scale TC17 ingot was produced by VAR process without electromagnetic stirring.The results showed that when the melt flow was weak during VAR process,three typical zones,which were the equiaxed grain zone,the columnar grain zone,and the hot top zone,could be can be clearly distinguished at low magnification.Irregular(?-flecks were found in the equiaxed grain zone and channel-like ?-flecks were found in the hot top zone respectively,while no ?-flecks existed in the columnar grain zone.It can be concluded that G and R dominated the ?-fleck formation during the solidification process with a relatively weak melt flow.Low R promoted the accumulation of segregated solute into the local residual liquid,which evolved into the ?-flecks in the solidified microstructure.The critical solidification rate was about 4.3×10-4 m/s for the ?-fleck formation.And G controlled the final morphology of the ?-flecks.The effects of the electromagnetic stirring and the remelting times during the VAR process on the ?-fleck formation were investigated.The results showed that the electromagnetic stirring accelerated the melt flow in the molten pool.During the VAR process,the strong melt flow broke the correspondence between the grain structure and the appearance of ?-flecks in the solidified microstructure.The electromagnetic stirring promoted the formation of the channel flow with the segregated liquid in the solidification front,which were solidified as the block-like ?-flecks in the columnar to equiaxed transition(CET)zone in the VAR ingot.The hot top process could effectively suppress the formation of the irregular ? flecks in the center of the ingot head.With the increase of the remelting time and the ingot size,the depth of the molten pool increased and the cooling rate decreased,which resulted in that the ?-flecks tended to form and grow up at the same time.The solidification conditions directly influenced the distribution,morphology,and composition of the ?-flecks in the VAR ingots.Based on the different morphological characteristics of the ?-flecks under the various solidification conditions,all ?-flecks found in this research can be summarized into four types:the macro-segregation ?-flecks,the channel-like ?-flecks,the irregular ?-flecks,and the block-like ?-flecks.And the possible formation process of the four kinds of ?-flecks were discussed,which were proposed and verified through the corresponding the theoretical models and the typical experiments.By this thesis,it has the important theoretical significance and application value for understanding and controlling the ?-flecks in TC17 alloy. |
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