Formation Condition and Pattern Evolution of Twinned Dendrites in Al-4.5%Cu Alloy during Bridgman Solidification

Author:Yang Lu Yan

Supervisor:li shuang ming


Degree Year:2018





Feathery grains have attracted considerable research interest since they were first observed in semi-continuous casting of industrial aluminum alloys about 70 years ago.Different from the regular columnar or equiaxed morphologies observed in alloy castings,this special crystal structure is made of elongated twinned dendrites.Unlike usual<100>axes of cubic metals,the twinned dendrite trunks always grow along<110>directions and are split in their centers by a straight coherent(111)twin plane.Secondary dendrite arms grow primarily along<110>,and also sometimes<100>directions,and their impingement of the two neighboring dendrites creates wavy incoherent twin boundaries.Thus,the so-called feathery grains,each consisting of a sequence of twinned columnar dendrites,exhibit the alternant twinned and untwinned lamellae morphology.Numerous previous studies focus on the analysis and characterization of the twinned dendrite growth appearing in direct-chill(DC)or similar semi-continuous casting processes.And the twinned dendrite growth during these solidification processes is believed to occur when the following solidification conditions are met:(i)a relatively high thermal gradient(typically100 K/cm);(ii)a large growth speed(at least 1 mm/s);(iii)the presence of convection in the melt.However,it should be noted that under above experimental conditions the solidification variables could not independently vary with each other.The thermal gradient and growth speed values are in fact intercoupled.This greatly limits the quantitative and physical understanding of the growth behaviors of twinned dendrites during solidification,and some deep and important questions on twinned growth are still open up to now.The Bridgman directional solidification technique is a very efficient method to study fundamental relations between the microstructures and the processing variables.During Bridgman solidification,the thermal gradient at the interface G and the growth speed of the solid V can be separately controlled so that the effect of each experimental variable on the microstructural evolution can be precisely characterized.This technique,however,was previously considered inefficient to produce twinned dendrites for the slight melt convection in small crucibles.In this work,by improving the solidification parameters,we first successfully produced twinned dendrites with high reproducibility in Al-4.5 wt.%Cu alloys at various constant growth speeds(V)of 100,200,500,1000,1500,2000 and 3000μm/s during Bridgman solidification.Under a relatively high thermal gradient(G,around 200 K/cm),the growth speed condition necessary for the twinned dendrite growth has been broadened.Experimental results indicate that the increase in growth speeds has substantially changed the twinned morphologies.Importantly,the isolated twinned dendrite growth was observed at the growth speeds less than1 mm/s(100,200,and 500μm/s).At higher growth speeds,the finer twinned dendrites developed the close-packing appearance of an ordered sequence of alternating twinned and untwinned lamellae.By using electron backscattered diffraction(EBSD)technique,the crystallographic features of these twinned microstructures were analyzed in detail.The G/V value is considered as the critical factor for the isolated or usual twinned dendrite growth.The increase in growth speeds was also found to result in a decrease in the secondary dendrite arm spacing of the twinned dendrites.An intensive analysis of the melt flow confirms that the slight convection during Bridgman solidification is large enough for the twinned dendrite growth.Furthermore,we provide a visualized way to evaluate the velocity of the in-plane twinned extension and its value is typically one third of the growth speed(500μm/s).Based on frequent“penetration”growth phenomena among secondary arms,a possible lateral propagation mechanism of the twinned dendrites has been proposed.By introducing an abrupt decrease in growth speed,the growth stability and pattern selection of twinned dendrites in Al-4.5 wt.%Cu alloy was systematically studied during Bridgman solidification.Based on the twin planes already formed at 3000μm/s,the twinned growth could be produced till the end of solidification after abruptly decreasing the growth speed to 100,50,20,10,5,3,1 and 0.3μm/s,respectively.The dominated twinned structures at low growth speeds have their(111)twin plane and<110>dendritic trunk closest to thermal gradient direction.For the first time,the so-called twinned cell pattern was clearly observed and prevailed in the solidified structures at 3μm/s and lower speeds.The twinned patterns in aluminum alloys are richer than previously expected during a wide range of solidification processes.The corresponding pattern selection map was further revealed.Meanwhile,an interesting orientation rotation phenomenon was verified by EBSD on either side of the coherent twin boundary during twinned cell growth.In high-resolution transmission electron microscopy(HRTEM)views,successive stacking faults(SFs)occurred nearby the twin boundaries(TBs)at 1μm/s,while the distortion around the TBs at 3000μm/s was quite slight.Importantly,distinct SF structures directly connecting two adjacent TBs,with a spacing of about 221nm,provides a direct experimental evidence for the close correlation between the SFs and the twinned dendrite growth in aluminum alloys.Furthermore,the growth advantage of twinned dendrites over regular columnar ones was characterized and analyzed during Bridgman solidification.This twinned growth advantage was found to derive from three essential components:the lateral twin propagation perpendicular to twin plane(R_x),the twin propagation parallel to twin plane(R_y),and the dendrite tip growth(R_z).The lateral extension component R_x played a vital role in the twinned growth advantage and would be limited at a low growth rate of 10μm/s,where the coexisting structure containing both twinned and regular dendrites were obtained.Meanwhile,competitive growth behaviors between the twinned dendrites in different feathery grains were also investigated at 3000μm/s.By analyzing the solute field near grain boundary,the mutual orientation angle between the twin planes of different grains was found to be a key factor.When this angle was relatively large,the lateral twin propagation would keep down the in-plane twin propagation.When this angle was relatively small,the twin lamellae interpenetration phenomena were usually observed and growth of different twinned grains could coexist.