Recalescence Velocity and Microstructure Evolution of Deeplyundercooled Cu_(65)Ni_(35) Alloy

The Cu_(65)Ni_(35) alloy liquid was undercooled by the fluxing method,and the rapid solidification structure was obtained by natural cooling.The solidification interface migration information of Cu_(65)Ni_(35) alloy liquid in rapid solidification stage was photographed with the help of high-speed camera,and the recalescence velocity was calculated.The microstructure evolution of the alloy was systematically studied by observing the microstructure morphology and taking photos on the metallographic microscope.By analyzing the evolution of dendrite grain size and microstructure microhardness with undercoolingand relying on electron backscatter diffraction(EBSD)technology,the grain refinement mechanism of microstructure under high undercooling and low undercooling is finally confirmed.

RECALESCENCES AND SOLIDIFICATION MICROSTRUCTURE OF HIGHLY UNDERCOOLED Ni_(68)B_(21)Si_(11) ALLOY

One,two and three recalescence events in the solidification of highly undercooledNi68B21Si11 alloy melt and the conditions of undercooling together with the effect onsolidification morphologies were revealed.The solidification morphologies of the undercooledalloy may be examined in the light of two aspects,i.e,growth of either Ni3B,Ni6Si2B andNi2B ternary eutectic or dendrite cluster with Ni3B as leading phase.When the degree ofundercooling below 200 or over 310 K,the alloy solidifies to accompany one recalescence only,and the crystals grow according to the first mode of solidification.While undercooling at othertemperatures,two or three events of recalescence may occur and the dendrite cluster may so-lidify.

Recalescence Behavior, Solidification Characteristics and Microstructure Transformation of Rapidly Solidified Undercooled Cu-based Alloys

The undercooled solidification microstructures of Cu55Ni45,Cu55Ni43Co2,and Cu60Ni38Co2 Cu-base alloys were obtained by fluxing method.Using infrared temperature measuring device,the law of the change of the recalescence degree with the increase of the undercooling during rapid solidification was studied.At the same time,high-speed camera was used to capture and photograph the images of solid/liquid interface migration during rapid solidification of undercooled melt,and the morphology evolution of solidification front was discussed.Finally,the microstructure morphology and transformation process of the Cubased alloys were systematically analyzed.It is found that the microstructure morphology of the alloys goes through the same evolution process and appeared two grain refinement phenomena,that is,“coarse dendrite-equiaxed grain-oriented fine dendrite-equiaxed grain”.But its characteristics undercoolingΔT_(1),ΔT_(2),and critical undercoolingΔT^(*)varies.Electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM)were used to characterize the grain refinement structure with high undercooling.EBSD results show that the grain refinement structure with high undercooling presents a very high proportion of high angle grain boundaries,the grain orientation is random and there is no high strength texture,and a large number of annealing twins,which indicates that recrystallization occurs in the structure.TEM results show that dislocation network and stacking fault density are relatively low in most areas of grain refinement structure with high undercooling,which can confirm the theory that stress induces recrystallization of the structure.

Effect of Co on Solidification Characteristics and Microstructural Transformation of Non-equilibrium Solidified Cu-Ni Alloys

Non-equilibrium solidification structures of Cu55Ni45 and Cu55Ni43Co2 alloys were prepared by the molten glass purification cycle superheating method.The variation of the recalescence phenomenon with the degree of undercooling in the rapid solidification process was investigated using an infrared thermometer.The addition of the Co element affected the evolution of the recalescence phenomenon in Cu-Ni alloys.The images of the solid-liquid interface migration during the rapid solidification of supercooled melts were captured by using a high-speed camera.The solidification rate of Cu-Ni alloys,with the addition of Co elements,was explored.Finally,the grain refinement structure with low supercooling was characterised using electron backscatter diffraction(EBSD).The effect of Co on the microstructural evolution during nonequilibrium solidification of Cu-Ni alloys under conditions of small supercooling is investigated by comparing the microstructures of Cu55Ni45 and Cu55Ni43Co2 alloys.The experimental results show that the addition of a small amount of Co weakens the recalescence behaviour of the Cu55Ni45 alloy and significantly reduces the thermal strain in the rapid solidification phase.In the rapid solidification phase,the thermal strain is greatly reduced,and there is a significant increase in the characteristic undercooling degree.Furthermore,the addition of Co and the reduction of Cu not only result in a lower solidification rate of the alloy,but also contribute to the homogenisation of the grain size.

Effect of Static Mgnetic Fields on the Recalescence Behavior of Undercooled Co-Sn Liquid

The recalescence behavior of undercooled Co-Sn alloys under static magnetic fields was investigated using glass slag fluxing technique in a 12T superconducting magnet.Results indicated that the nucleation temperature and the highest temperature after recalescence of undercooled melt can be affected by the static magnetic field.The undercooling for Co-Sn alloys keep unchanged when a strong magnetic field was applied while the recalescence extent was reduced.With the increasing Co content for Co-Sn eutectic systems,the effect of the magnetic field on the recalescence was strengthened.We find that the magnetic field has very limited effect on the nucleation temperature on alloys while the highest temperature after recalescence is affected by the field depending on the magnetization difference of the undercooled melt before and after solidification.

GRAIN REFINEMENT IN BULK UNDERCOOLED Ni-BASED ALLOY

Fluxing of 5 g bulk melt Ni77Si13B10 permits high undercoolings to be attained prior to nu-cleation onset.Investigations of grain refinement in the bulk undercooled alloy as a function ofundercooling,recalescence behavior and cooling rate have been reported.A significant inhomo-geneity of reduction in grain size of a bulk sample is observed,which is caused by the different so-lidification conditions:(1)recalescence process,and(2)the followed plateau in which the heatrelease and extraction rates are equal.It is concluded that the homogeneous refined microstructurecan be achieved if the initial undercooling prior to nucleation,or cooling rate after recalescence isfurther increased.

SOLIDIFICATION MECHANISM OF UNDERCOOLEDSINGLE PHASE ALLOYS

The solidification behavior of bulk undercooled Ni50Cu50 alloy was systematically investigated. Double recalescences were discovered for the first time in a single phase Ni-Cd alloy, in which there is no solid phase transition at elevated temperature. When the alloy melt was undercoolcd below the solidus temperature, the effect of nonequilibrium solidification could make the solid with the norminal composition C0 of the alloy nucleate and grow till tempemture approached To (the temperuture at which the free enengies of solid and liquid with C0 are equal). Following it, the nucleation and growth of the solid with more high melting point component were required, so the secondary recalescence took place. The concentration distribution in crystals was also analysed.

Peritectic solidification under high undercooling conditions

The solidification characteristics of highly undercooled Cu-7.77% Co peritectic alloy has been examined by glass fluxing technique. The obtained undercoolings vary from 93 to 203 K(0.14 T_L). It is found that the a(Co) phase always nucleates and grows preferentially, which is followed by peritectic transformation. This means that the peritectic phase cannot form directly, even though the alloy melt is undercooled to a temperature far below its peritectic point. The maximum recalescence temperature measured experimentally decreases as undercooling increases, which is lower than the thermodynamic calculation result owing to the actual non-adiabatic nature of recalescence process. The dendritic fragmentation of primary α(Co) phase induced by high undercooling is found to enhance the completion of peritectic transformation. In addition, the LKT/BCT dendrite growth model is modified in order to make it applicable to those binary alloy systems with seriously curved liquidus and solidus lines. The dendrite