Study On The Solidification Behavior Of Co-Sn Alloys

Solidification is considered to be an important step in metallic material processing,and the microstructure has great influence on the properties of materials.Therefore,understanding the crystal growth behavior during solidification,and realizing to control the solidification microstructure has become an important subject of materials science and engineering.Generally,planar solidification interfaces are unstable,and will continue to branch.If the interface tip remains stable in this process,branching takes place at a certain distance after the tip and dendritic structure is formed.Otherwise,seaweed structure is formed with tip splitting.Dendritic growth is the most common in solidifying alloys.To date,seaweed growth has been observed in the solidification of a few inorganic nonmetallic or organic materials,and is rarely observed in metals and alloys.A lot of issues for seaweed growth in the solidification of metals are still not clear.In this dissertation,firstly,the directional solidification and undercooling solidification of Co-Sn binary alloys were performed,on the basis the solidification behaviors of the alloys with third element Nb addition were studied.The effect of temperature gradient,withdrawal velocity and undercooling on crystal growth morphology were comprehensively analyzed,and the functional mechanism of third element Nb was discussed.The following findings have been achieved:When the Co-Sn alloys containing 22-30 at.%Sn are directionally solidified at low withdrawal velocity,the solidification interface is planar and the microstructure is regular lamellar eutectic.As the withdrawal velocity increases,the composition range for the formation of regular lamellar eutectic in planar solidification is reduced.As for Co₇₆Sn₂₄eutectic alloy,no matter how larger withdrawal velocity is,the regular lamellar eutectic is formed.The coupled zone likes an“inverted cone”in directional solidification,but likes a“pear”in undercooling solidification,out of which the solidification microstructure is composed of primary phase and eutectic.Careful experimental study using Co₇₀Sn₃₀hypereutectic alloy indicates that at a given temperature gradient,the solid-liquid interface morphology transits from planar eutectic to primary?-Co₃Sn₂ dendrite plus eutectic and then to primary?-Co₃Sn₂ seaweed plus eutectic with the increase of withdrawal velocity.Increasing the temperature gradient makes the critical withdrawal velocity corresponding to the primary?-Co₃Sn₂ phase growth pattern transition from dendritic to seaweed decrease.The tip splitting frequency f of?-Co₃Sn₂ seaweed versus growth velocity V exhibits a power function relationshipf?V³².The nominal primary arm spacing d₁ is related to the growth velocity asd₁?28?k₁V^-e1,and the value of exponent e₁ is larger for seaweed growth than dendritic growth.When Co₇₆Sn₂₄ eutectic alloy is directionally solidified at low withdrawal velocity,the solidification interface is planar with coupled growth of two eutectic phases.At 200K/cm and 300 K/cm temperature gradient,the coupled eutectic growth interface becomes cellular only when the withdrawal velocity is larger than 30?m/s and 40?m/s,respectively.The maximum withdrawal velocity for planar eutectic solidification is significantly decreased due to 0.5 at.%Nb added to Co₇₆Sn₂₄ alloy.At a given temperature gradient,the solid-liquid interface morphology transits from planar eutectic to cellular eutectic and then to seaweed eutectic with the increase of withdrawal velocity.However,for the reference alloy of(Ni_81.3Sn_18.7)_99.5Nb_0.5 eutectic alloy,at a given temperature gradient,the solid-liquid interface morphology transits from planar eutectic to cellular eutectic and then to dendritic eutectic with the increase of withdrawal velocity.When(Co₆₇Sn₃₃)_100-xNb_x?x=0,0.5,0.8,1.0?hypereutectic alloys are solidified at low undercooling,the growth pattern of primary?-Co₃Sn₂ phase changes with the Nb content from fractal seaweed?x=0,0.5?into dendrite?x=0.8?and then returns to fractal seaweed?x=1.0?.As for the solidification of(Co₆₇Sn₃₃)_99.2Nb_0.8 alloy at different undercooling,the growth pattern of?-Co₃Sn₂ phase transits from dendritic to fractal seaweed when the undercooling is higher than 32 K.At larger undercooling,the fractal seaweed is replaced by compact seaweed.The growth velocity of?-Co₃Sn₂ phase slightly increases at undercooling lower than 135 K but clearly decreases at larger undercooling due to 0.8 at.%Nb added to the base Co₆₇Sn₃₃ alloy.The growth velocity sharply increases with undercooling as the growth pattern of Co₃Sn₂ phase transits from fractal seaweed to compact seaweed.When(Co₇₆Sn₂₄)_100-xNb_x?x=0,0.5,0.8,1.0?eutectic alloy were solidified at low undercooling,it is revealed that with increasing Nb content,the eutectic interface morphology transits from eutectic seaweed?x=0?to eutectic dendrite?x=0.5?.For x=0.8,because the enrichment of Nb in the liquid ahead of the two eutectic phases causes a significant difference in growth velocity between the?-Co and?-Co₃Sn₂ phases and,as such,Co₃Sn₂ doublons form.For x=1.0,the difference in growth kinetics between the two eutectic phases is so large that divorced eutectic growth takes place on a large scale.Taking(Co₇₆Sn₂₄)_99.5Nb_0.5 eutectic alloys an example to be solidified at different undercoolings,the eutectic interface morphology transits from dendritic eutectic to factual seaweed eutectic as the undercooling prior to solidification increases to 38 K,and then transits to compact seaweed eutectic at more than 181 K undercooling.