Solidification,Characteristics,and,Microstructure,Evolution,of,Rapidly,Solidified,Cu,Alloys

KONG Huaye,TANG Cheng,HE Xibin,YANG Jin’e,XIE Jinpeng,WANG Hongfu

(1.Shanxi Institute of Technology,Yanquan 045000,China;2.College of Mechanical Engineering,North University of China,Taiyuan 030051,China;3.Jiangsu Yingchuang Power Technol Co.Ltd,Suzhou 215000,China)

Abstract: Cu60Ni40 alloy was taken as the research object.The alloy was undercooled by fluxing method and circulation superheating method,and the solidified samples with different undercoolings were obtained by natural cooling method.The solidification process of undercooled melt was photographed by highspeed camera,and the transformation of solidification front morphology was studied.The microstructure and morphology of all undercooled samples were analyzed.It is found that the microstructure and morphology of Cu60Ni40 alloy change through the evolution process of "coarse dendrite-equiaxed crystal-oriented fine dendrite-equiaxed crystal" with the undercooling.Finally,the electron backscatter diffraction characterization of 261 K grain refinement structure shows that recrystallization occurs in the grain refinement structure with high undercooling,such as high proportion of high angle grain boundaries,random orientation of equiaxed grains and a large number of annealing twins found in the microstructure.

Key words: undercooling;solidification front;microstructure;recrystallization

The recalescence[1]is the most remarkable phenomenon in the solidification of the alloy liquid by using deep undercooling and rapid solidification technology,which is characterized by the sudden regrow of the undercooled melt and the increase of brightness.The reason for this phenomenon is that the solidification latent heat rate released during the rapid solidification of the undercooled melt is far greater than the heat rate radiated from the system to the outside.Therefore,the recalescence phenomenon during the cooling process of the alloy liquid is the rapid solidification stage of the undercooled melt.After recalescence,the alloy often goes through the slow solidification stage.But when the alloy liquid reaches hypercooling state[2],the slow solidification will not occur,and the solidification has been completed in the recalescence stage.Therefore,the properties and microstructure of the materials obtained by this technology have a great relationship with the recalescence stage and the post-recalescence stage.

Deep undercooling and rapid solidification technology has been developed so far,mainly including the following technologies: molten glass purification method (fluxing method)[3],circulation superheating method[4],drop tube method[5],emulsification[6],etc.Among them,fluxing method and circulation superheating method are widely used in the laboratory because of their simple operation,which can prepare large volume materials.Many scholars also use this method to further study and improve the rapid solidification theory[7-12].Due to the significant nonequilibrium effect of undercooled melt during rapid solidification,deep undercooling and rapid solidification technology has also been used to prepare various kinds of crystalline metastable materials such as nanocrystals and ultrafine crystals[7,13-15].

However,the time of the undercooled melt in the rapid solidification stage is very short,and the solidification process is difficult to directly observe.Therefore,there were few studies on the solidification front morphology of the alloy liquid in the recalescence stage in the past[16,17].With the development of high-speed photography technology,the solid/liquid interface migration of undercooled melt in the rapid solidification stage can be recorded by high-speed camera.According to the current research,the solidification front mostly presents a plane or smooth arc,and another part presents a sharp shape[18].However,there is no systematic study on the relationship between the morphology of solidification front and undercooling,nor is it combined with the solidification structure morphology of corresponding undercooling.Therefore,there are still many aspects[19]worth exploring for the morphology of solidification front and corresponding microstructure morphology of undercooled melt.

In this paper,the single phase Cu60Ni40 alloy,which is not easy to burn and oxidize and has a good experimental basis,was specially selected for research.The relationship between the characteristics of solidification front and undercooling is studied,and the evolution process of its microstructure morphology was determined,which provided some experimental evidences for the further deep undercooling and rapid solidification process and theory.

Cu particles and Ni particles with an atomic ratio of 60:40 and a purity of 99.99% were used to prepare Cu60Ni40 master alloy in a vacuum arc melting furnace under a protective environment filled with argon (Ar).The two pure metals were melted repeatedly at least 3 times to ensure that their components were mixed evenly with each other,and then about 4 g alloy is cut from the parent alloy for undercooling experiment.Then,the alloy and quartz test tubes were put in an ultrasonic cleaner for 10 minutes to remove impurities on the surface.After drying the alloy and quartz test tube,the alloy and the prepared B2O3purification agent were put into the test tube,and finally the quartz test tube put into the high-frequency induction coil.The vacuum in the experimental furnace is pumped to 3 × 10-3Pa,and argon backfilled to 5 × 10-2Pa.The infrared thermometer with a response time of 1 ms was used to record the temperature change during natural cooling,and images of the solidification front were taken with a high speed camera.The subsequent operation is as follows: firstly,the temperature was slowly raised to about 750 ℃ and keep it for 10 minutes to melt B2O3powder and fully cover the alloy;secondly,the temperature was continued to raise to about 200 K above the melting point of the alloy and keep it for 10 minutes to fully absorb impurities by B2O3purifier;finally,all undercooled samples were obtained by circulation superheating operation,and the undercooling interval is 10 K.After cutting,inlaying and polishing,the samples were corroded with analytical pure HNO3solution.Then the microstructure was observed under a metallographic microscope (OM,Leica DM 2500M) and pictures were taken.Then,the selected undercooled samples were polished on a vibration polishing machine for 10 hours for electron backscatter diffraction (EBSD) characterization.The schematic diagram of the whole undercooling test is shown in Fig.1.

Fig.1 Schematic diagram of the experiment

3.1 Analysis of solidification characteristics

When the alloy liquid in the undercooled state solidifies,due to the fast solidification rate,the release rate of the latent heat of solidification is much greater than the heat dissipation rate of the system to the outside world,and finally the temperature of the system appears to rise again,that is,the temperature reheats[1].The high-speed camera can capture the light signal during the recalescence process,and the picture of the rapid solidification process of the undercooled melt can be obtained by matching software.

The solidification front of Cu60Ni40 alloy is shown in Fig.2,where the dark area represents the undercooled melt,and the bright area represents the solid part of the alloy.There is a great difference in the shape of the solidification front of the alloy under different undercoolings: the undercooled melt solidifies forward with a small angle interface under low undercooling,as shown in Fig.2(a).This is because the solidification speed is very slow,and the it is mainly affected by the diffusion of the solute.However,at moderate undercooling,the undercooled melt advances with an angular front (as shown in Fig.2(b)),and the speed at the solidification interface is not uniform.With the further increase of undercooling,the solidification front morphology of the undercooled melt is more and more close to the smooth surface (as shown in Fig.2(d)),and the solidification speed is extremely fast.What is more,since the nucleation position of the alloy liquid is random,the solidification direction of the alloy liquid is uncertain.There are many impurities in the B2O3purification solution and the nucleation effect is obvious.Therefore,in most cases,the alloy solution tends to preferentially nucleate at the interface between the purification solution and the undercooled melt.In this case,the solidification direction of the undercooled melt always solidifies from one side to the other.The undercooled melt also uniformly nucleates inside the undercooled melt,so that the solidification direction of the molten alloy is to solidify in the form of radiation,as shown in Fig.2(c).

Fig.2 Solidification front morphologies of Cu60Ni40 alloy under different undercoolings: (a) 88 K;(b) 145 K;(c) 260 K;(d)300 K

3.2 Microstructure analysis

By analyzing the microstructure of all the alloy solidification samples,it is found that there are four morphologic transformation processes:

When the ∆T<62 K,for example ∆T=40 K and∆T=62 K,the microstructure of the alloy is coarse dendrite and contains a large number of secondary dendrite arms.It is clear that dendrites do not have a specific growth direction.In this undercooling range,because the solute diffusion controls the dendrite growth,and coarse dendrites are formed in the alloy[20].The solidification front of the alloy liquid is characterized by small plane morphology.

when 62 K <∆T<114 K,the coarse dendrite disappears gradully,and the microstructure shows a large number of equiaxed crystals.When ∆T=74 K,the alloy structure is almost completely composed of fine equiaxed crystals.The equiaxed crystal boundary is bent,and the grain size is small than coarse dendrite,as shown in Fig.3(c).With the increase of undercooling,the number of equiaxed crystals gradually decreases,and some short dendrites with directional characteristics appear in the microstructure,as shown in Fig.3(d).In this undercooling range,the dendrite growth is dominated by both solute diffusion and thermal diffusion,but the influence of thermal diffusion factor is small,and the solidification front of alloy liquid is also small plane shape.

Fig.3 Microstructures of Cu60Ni40 alloy under different undercoolings

when 114 K <∆T<217 K,with the further increase of the undercooling,the equiaxed grains that appeared in the first grain refinement disappeared,and the microstructure appeared different from the coarse dendrites in the microstructure under low undercooling,which is fine dendrites with strong orientation characteristics,as shown in Fig.3(e).In this undercooling range,the dendrite growth factor controlled by solute diffusion is significantly reduced,and the dendrite growth is controlled by thermal diffusion.Dendrite grows in the direction of thermal diffusion,thus directionality appears.With the increase of undercooling,the larger the undercooling,the more fine dendrite network will well develop,as shown in Fig.3(f).At this time,the shape of solidification front is the front with edges and corners.

When ∆T>217 K,the morphology of the microstructure is no oriented fine dendrite,but fine equiaxed crystal.At this time,the second grain refinement occurs,as shown in Figs.3(g) and 3(h).In this undercooling range,the solidification front of the alloy liquid basically appears as a smooth arc.In addition,different from the first grain refinement phenomenon,the equiaxed crystal boundary is relatively flat,which shows polygonal characteristics.And there are annealing twins that are not found in the first grain refinement phenomenon.The appearance of annealing twins is an important characteristic of recrystallization of solidification structure under high undercooling.

In the microstructure of Cu60Ni40 alloy,the solidified sample with the maximum undercooling of 261 K is selected and tested by electron back scattering diffraction (EBSD) to obtain grain information,which can explain whether recrystallization is the main formation mechanism of grain refinement structure under the high undercooling,as shown in Fig.4.

Fig.4 EBSD characterization of solidification structure of Cu60Ni40 alloy at ∆T=261 K: (a) Grain boundary of alloy microstructure;(b)Grain orientation;(c) Pole figure of b;(d) Distribution of misorientation of a

In crystallography,the grain boundary orientation difference greater than 15° is defined as high angle grain boundaries (HAGBs),and that less than 15° is defined as low angle grain boundaries (LAGBs).The white line represents the low angle grain boundary and the red line represents the high angle grain boundary in Fig.4(a).It can be seen that the high angle grain boundary occupies a very high proportion.By statistical analysis,the distribution of grain boundary misorientation is obtained,as is shown in Fig.4(d).The high angle grain boundary is as high as 85%,and the ∑3 twin grain boundary (<111>60) is 20.7%.The grains of different colors in Fig.4(b) represent different grain orientations.And the disordered and irregular colors show that the grain orientations are random and there is no specific direction.The highstrength texture with main orientation is not observed in the polar figure Fig.4(c).In addition,annealing twins(marked by black circles) are also observed.The high proportion of random orientations,high angle grain boundaries and the appearance of annealing twins are all significant characteristics,which fully indicate that the alloy microstructure has recrystallized under high undercooling.The low angle grain boundary,especially the subboundary lower than 2°,is the result of the recovery of the structure,which also proves that the high undercooling microstructure has undergone extensive recrystallization process.Therefore,there is reason to believe that recrystallization is the main factor for grain refinement of Cu60Ni40 alloy under high undercooling.

Solidification samples of Cu60Ni40 alloy with different undercoolings were obtained by fluxing method and circulation superheating technique.The microstructure of Cu60Ni40 alloy under different undercoolings was systematically studied,and the solidification process of the alloy was photographed by high-speed camera.Finally,the following conclusions are obtained:

a) In the whole undercooling range,Cu60Ni40 alloy undergoes two grain refinement,but the grain characteristics of the two refinements are different.The grain size of refined grain is relatively large under low undercooling,and the grain boundary of refined grain is curved under low undercooling.But the grain boundary of refined grain under high undercooling is relatively flat,and the proportion of grain with high angle and random orientation in the refined structure is very high.And there are a large number of annealing twins.The above characteristics indicate that the refined structure of high undercooling grain has recrystallized.

b) The microstructure of the Cu60Ni40 alloy goes through the evolution process of “coarse dendrite-equiaxed crystal-oriented fine dendrite-equiaxed crystal”.

c) The solidification of undercooled melt different undercoolings has the following characteristics: the undercooled melt with low undercooling presents the shape of small angle plane during solidification,the undercooled melt with medium undercooling presents the shape of angular front during solidification,while the alloy liquid with high undercooling presents the shape of smooth surface during solidification.