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

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

(1.School of Mechanical Engineering,North University of China,Taiyuan 030051,China;2.Jiangsu Yingchuang Power Technology Co.,Ltd,Suzhou 215000,China)

Abstract: 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 undercooling ΔT1,ΔT2,and critical undercooling ΔT* 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.

Key words: rapid solidification;recalescence degree;solidification front;microstructure

Recalescence[1]is the most significant phenomenon during solidification of undercooled melt in the rapid solidification stage,which is mainly shown by the sudden increase of melt brightness during natural cooling.It almost occurs in various alloy systems by using deep undercooling rapid solidification technology[2],such as Ni-Cu system[3],Co-Sn system[4],Fe-BSi system[5],Fe-B system[6],etc.Ni-Si system[7]even shows twice recalescence.In addition to exceeding the hypercooling state[8],the undercooled melt often goes through a near equilibrium solidification stage after the recalescence stage,that is,post-recalescence.The former is related to the formation of metastable phase in alloys,while the latter is generally directly related to the solid state transformation of alloys[9].Therefore,the properties of materials obtained by deep undercooling rapid solidification technology are closely related to the recalescence behavior.And the recalescence degree can indicate the thermal strain intensity of undercooled melt during rapid solidification,and it is also an important indicator of recalescence behavior,which has gradually attracted extensive research[10].

With the development of high-speed photography technology,due to the recalescence behavior of alloy melt in the rapid solidification process,the strong light signal released by the alloy melt can be captured by the high-speed camera,so the solidification process of alloy can now be directly observed in situ.In previous studies[11,12],researchers explored the solidification behavior of alloys,and gradually established a quantitative relationship between the solidification rate and undercooling of various alloys,providing an experimental basis for further improving the solidification theory.In the research on the microstructure of undercooled alloys,it is found that grain refinement can occur in the structures with low undercooling and high undercooling at the same time[7,13,14].Grain refinement is not only important to reduce the defects such as alloy composition segregation and hot cracking,but also important to improve the mechanical properties of alloys[15].Therefore,fine equiaxed grain material is also one of the goals of engineering practice,so it is necessary to explore the formation mechanism of grain refinement.For the research of grain refinement mechanism,there are mainly widely accepted remelting mechanism[13,16],recrystallization mechanism[17,18]and dynamic nucleation theory[19].Other theories,such as critical velocity theory[20],nucleation mechanism[21],and instability of dendrite growth[22],are still in dispute.It is generally believed that the refined grains in the structure with low undercooling are formed under the action of chemical superheating and solid/liquid interface energy.As for the phenomenon of grain refinement in the structure with high undercooling,the theory of stress induced recrystallization proposed by Liu[23]has attracted much attention as one of the possible mechanisms.According to this theory,when the undercooling reaches a certain value,a certain volume of primary solid phase is formed in the process of recalescence of the undercooled melt.With the gradual increase of undercooling,the volume fraction of primary solid phase becomes larger,and dendrite skeleton and dendrite arm are gradually formed.The undercooled melt flows to the solidification front and accumulates stress on the dendrite.Then the accumulated stress makes the dendrite skeleton bend and break through the strength limit of the alloy,and finally the skeleton is broken into crystalline seeds and fragments.The undispersed strain energy stored in the fragments provides a driving force for the recrystallization of the alloy at the post-recalescence stage.

As a widely used engineering material,Cu-based alloys have been trying to improve its comprehensive properties.However,Cu-based alloys with refined grains obtained by deep undercooling rapid solidification technology have such potential.In this paper,Cu55Ni45 alloy with good experimental basis is selected,and comparison between Cu55Ni43Co2 and Cu60Ni38Co2 alloy is obtained by adding a small amount of Co and Cu.The relationship between the recalescence behavior,solidification characteristics,microstructure transformation and undercooling of Cubased alloys was systematically studied,and the mechanism of grain refinement under high undercooling was confirmed.

2.1 Experimental procedure

The above three master alloys were prepared from pure Cu particles (99.99%),pure Ni particles (99.99%)and pure Co particles (99.99%) in a vacuum arc melting furnace under a protective gas atmosphere filled with argon (Ar).The three pure metals should be smelted for at least three times to make their components mix evenly with each other,and about 4 g alloy was cut from the master alloy for experiment.Then the alloy and quartz tube were put in the ultrasonic cleaner for 10 minutes to remove impurities on the surface.After drying the alloy and quartz tube,the alloy and the B2O3purifying agent into the tube,and finally the quartz tube was put into the high-frequency induction coil.The vacuum in the experimental furnace was pumped to 3×10-3Pa,then argon was backfilled to 5×10-2Pa.The infrared thermometer (response time 1ms) was used to record the temperature change of the alloy melt during the natural cooling process,and the high-speed camera(frame rate 39000 fps) was used to capture the solidification image of the alloy melt.The schematic diagram of the undercooling experiment is shown in Fig.1(a).The specific operation of the undercooling experiment were the following methods.First,the temperature was slowly raise to about 750 ℃ for 10 minutes to melt the B2O3powder.Second,the temperature was continued to raise above the alloy melting point for about 200 K for 10 minutes to fully absorb impurities by the B2O3purifier;finally,the undercooled sample was obtained through the cyclic superheating operation,and a solidified sample was obtained every 10 K.

Fig.1 Schematic diagram of experiment and calculation

After cutting,inlaying and polishing,the sample is corroded with analytical pure HNO3solution.Then microstructure morphology was observed under the metallographic microscope (OM,Leica DM 2500M).Then,the selected undercooled samples were polished on a vibration polishing machine for 10 hours for electron backscatter diffraction (EBSD) characterization.The sample with maximum undercooling was made into standard sample through mechanical thinning and ion thinning,and then tested on transmission electron microscope (TEM).

2.2 Experimental principle

The principle of experiment is mainly the purification of purifying agent.B2O3has extremely strong physical adsorption capacity,which can effectively remove the heterogeneous core in the melt,so that it can nucleate uniformly at a lower temperature to obtain undercooling.In addition,the surface tension of molten B2O3is very low,so it is easy to wrap the alloy,which further enhances its purification ability.And the melt also can effectively passivate the heterogeneous substrate by holding it at a higher temperature for a long time.Undercooling (ΔT) is the difference between liquidus temperature (TL) and nucleation temperature(TN) of the alloy equilibrium phase diagram,i e,ΔT=TL-TN.Recalescence degree (ΔTR) is the difference between the maximum recalescence temperature (TR)and the nucleation temperature,i e,ΔTR=TR-TN,and its calculation diagram is shown in Fig.1(c).

3.1 Recalescence behavior

The temperature change of the alloy melt in the undercooling experiment is monitored in real time by an infrared thermometer with a reaction time of 1ms and an error of 3 K.The recorded alloy cooling temperature curve shows that the alloy melts of Cu55Ni45,Cu55Ni43Co2,and Cu60Ni38Co2 have undergone recalescence behavior,as shown in Fig.2.In addition,the process of obvious increasing of melt brightness was also photographed during the experiment,as shown in Figs.4-6.By observing the alloy cooling curve,the recalescence behavior of the melt with low undercooling is not obvious because of low recalescence degree,as shown in Figs.2(a)-2(b).Moreover,the rapid solidification time and the period of post-recalescence time are relatively long,and the slope of recalescence curve is relatively low.With the increase of undercooling,the rapid solidification time of alloy melts with moderate undercooling and high undercooling is not different,but the time of post-recalescence is greatly reduced,as shown in Figs.2(g)-2(h) (the black dot on the curve indicates the completion time of near equilibrium solidification).In addition,when the undercooling is relatively high,the curve of the recalescence stage rises in the form of vertical lines,and the maximum recalescence temperatureTRdecreases gradually with the increase of undercooling.

The recalescence degree in the rapid solidification process can represent the severity of the recalescence behavior.It can be qualitatively seen from Fig.2 that the recalescence degree increases with the increase of the undercooling of the alloy,and the recalescence behavior becomes more and more intense.It can be seen from Fig.3 that within the undercooling range obtained in the experiment,the brightness of the above three alloys increases almost linearly with the increase of undercooling.Among them,Cu55Ni45 alloy was the strongest in recalescence behavior,followed by Cu60Ni38Co2,and Cu55Ni43Co2 was the weakest in recalescence behavior.And it shows that the addition of a small amount of Co weakens the recalescence behavior of Cu55Ni45 alloy,and the thermal strain in the rapid solidification stage is greatly reduced.However,the increase of copper content enhances the recalescence behavior of Cu55Ni43Co2 alloy.The formation of metastable phase is closely related to the recalescence behavior,so the addition of Co may not be conducive to the formation of a large number of metastable phases.

Fig.2 Cooling curves of alloy melt: (a-f) Cu55Ni43Co2 alloy melt cooling curves;(g) Cu55Ni45 alloy melt cooling curve;(h) Cu60Ni38Co2 alloy melt cooling curves

Fig.3 Recalescence degree of alloy: (a) Recalescence degree of Cu55Ni43Co2;(b) Recalescence degree of Cu55Ni45;(c) Recalescence degree of Cu60Ni38Co2;(d) Recalescence degree contrast of alloys

3.2 Solidification characteristics

It can be seen from Fig.2 that Cu55Ni43Co2,Cu55Ni45,and Cu60Ni38Co2 alloys have only experienced one recalescence process,and there is no multiple recalescence phenomenon.The light signal released during the recalescence process can be easily captured by the high-speed camera,so the high-speed camera can record the solidification process of the alloy melt and the migration information of the solid/liquid interface.In addition,the alloy melt can only emit light signals when the primary solid phase is formed during the recalescence process,so the pictures taken from the high-speed camera can easily distinguish the solid/liquid phase region.

Fig.4 Solidification front of Cu55Ni43Co2 alloy

Fig.5 Solidification front of Cu55Ni45 alloy

Fig.6 Solidification front Cu60Ni38Co2 alloy

The dark area in Figs.4-6 is undercooled melt(marked “L” in the figure),and the highlighted part is solid phase (marked “S” in the figure).The pictures of alloy solidification front under all undercooling were systematically analyzed,and representative solidification characteristics of alloy melts with similar undercooling was selected to express.The morphology of solidification front has the following characteristics with the change of undercooling.At low undercooling,the solidification front of alloy melt is a plane with low angle,and the solidification interface migrates from one side to the other,as shown in Figs.4(a)-6(a).At medium undercooling,the alloy melt solidifies forward with sharp front.And with the increase of undercooling,the initial solidification morphology of the melt gradually presents a smooth surface.When the undercooling is high,the solidification front of the alloy melt is almost a smooth arc shape,as shown in Figs.4(c)-6(c).Because the nucleation position of the undercooled melt is uncertain,but due to the strong interface interaction between B2O3and the alloy melt and the adsorption of many impurities by B2O3,and the undercooled melt always tends to nucleate at the interface between B2O3and the melt.Therefore,in most cases,the alloy melt is always solidified from one side to the other.However,nucleation also occurs in the melt with few heterogeneous substrates.At this time,the solidification interface moves from the center to the periphery,and the melt solidifies in a radial form.Moreover,the following differences are found when calculating the solidification rate of alloy melt in Figs.4-6.The alloy melt with low undercooling is solidified in the plane form with low angle,so the solidification rate varies little.This is due to the incomplete effect of B2O3purifying agent and some impurities in the melt.At medium undercooling,the solidification rate at the sharp front is the highest,and there is little difference in the solidification rate at other locations,but the solidification rate is significantly increased compared with that at low undercooling.Due to the ideal effect of the purifying agent,the alloy melt under high undercooling has a very fast solidification speed at the interface,and the solidification rate is uniform at each position.However,the solidification rate of the above alloy melts is very slow at the initial nucleation position (especially at the interface),but the solidification rate is accelerated in the melt,and then it becomes slower at the end of solidification.This phenomenon is related to the interface effect between B2O3and the melt.

3.3 Microstructure transformation

The metallography of three alloys was systematically analyzed,taking Cu55Ni43Co2 as an example,it is found that there are four significant transformation processes of microstructure morphology.

ΔT≤79 K,the coarse dendrite is filled in the microstructure of Cu55Ni43Co2 alloy,and the dendrite structure is clearly visible,as shown in Fig.7(a).The dendrites as a whole show no specific growth direction,but some dendrites grow in a certain direction.And in the microstructure with ΔT=79 K,a large number of secondary dendrite arms on the dendrite trunk disappear,which is the effect of dendrite remelting,and even there are a few round grains in the residual dendrite trunk.

79 K<ΔT <142 K,in the undercooling range,the microstructure of Cu55Ni43CO2 alloy is refined for the first time.Dendrites are basically replaced by fine equiaxed crystals,and equiaxed grain boundaries are bent,as shown in Fig.7(c).According to a large number of previous studies[24-26],this is due to the fact that the dendrites in the primary solid phase of the alloy melt are remelted into many fine grains during the recalescence.In Figs.2(c),2(d),the maximum recalescence temperatureTRexceeds the solidus temperature of the alloy,which is the most direct evidence that the dendrite is remelted after recalescence.With the further increase of undercooling,ΔT=142 K,dendrites begin to appear in the alloy microstructure,as shown in Fig.7(d).

142 K≤ΔT <225 K,the microstructure of Cu-55Ni43Co2 alloy shows dendrite morphology.The appearance of dendrites at this time is quite different from that at low undercooling.First,dendrites at this undercooling have extremely strong directional growth characteristics and become stronger and stronger with the increase of undercooling,as shown by the red line in Fig.7(e).The growth direction is consistent in both local areas and the whole,and there is no chaotic growth.Second,the dendrite density in this undercooling range is much higher than that of the coarse dendrite under low undercooling,and it is a well-developed fine dendrite network with the distance between dendrites and dendrites shortened by about 1/4.

Fig.7 Microstructures of Cu55Ni43Co2 under different undercoolings

ΔT≥225 K,in the undercooling range,the microstructure of Cu55Ni43Co2 alloy shows the second grain refinement,as shown in Fig.7(f).The grain boundaries of the refined equiaxed grains within this undercooling range are relatively straight,and most equiaxed grains are polygonal.What is more,a large number of annealing twins (marked by yellow circle in Fig.7(f)) also appear in the microstructure,which is the most significant difference from the grain refined for the first time.The appearance of annealing twins and polygonal grains with flat grain boundaries indicates that the solidification structure of high undercooling alloy has recrystallized.

The microstructure transformation of Cu55Ni45 alloy and Cu60Ni38Co2 alloy is shown in Fig.8.At low undercooling,the microstructures of the two alloys are coarse dendritic morphology,as shown in Figs.8(a),8(e).With the increase of undercooling,dendritic dendrites are replaced by refined equiaxed crystals,as shown in Figs.8(b),8f At medium undercooling,the microstructures of the two alloys are also extremely fine oriented fine dendrite networks.In the microstructure of the alloy with high undercooling,there are a large number of annealed twins and polygonal equiaxed grains.

Fig.8 Microstructures of Cu55Ni45 alloy and Cu60Ni38Co2 alloy under different undercooling: (a-d) Cu55Ni45 alloy microstructure;(e-h) Cu60Ni38Co2 alloy microstructure

It can be concluded that the microstructure morphology of Cu55Ni43Co2,Cu55Ni45,and Cu60Ni-38Co2 alloys goes through the transformation process of “coarse dendrite-equiaxed grains-oriented fine crystal-equiaxed grains” with the increase of undercooling,but the characteristic undercooling (the undercooling of alloy microstructure morphology transformation)of the three alloys ΔT1,ΔT2and critical undercooling ΔT* is different,as shown in Fig.9(a).It can be seen that the addition of Co and the increase of Cu content improve the characteristic undercooling ΔT1and ΔT2.The undercooling range of coarse dendrite morphology has been greatly expanded,and the undercooling range of the first grain refinement has also been widened,and the undercooling range of directional fine dendrite morphology has been greatly reduced.Critical undercooling ΔT* of three alloys the difference of is not significant.And the undercooling of the second grain refinement is above 220 K,which indicates that the increase of Cu content or the addition of Co has little effect on the undercooling of the second grain refinement,but it is obviously beneficial to homogenizing the grain size,as shown in Figs.9(b)-9(d).

Fig.9 Characteristic undercooling and average grain size of alloys: (a) Characteristic undercooling;(b) Grain size of Cu55Ni45 alloy;(c)Grain size of Cu55Ni43Co2 alloy;(d) Grain size of Cu60Ni38Co2 alloy

The microstructures of Cu55Ni43Co2,Cu55Ni45 and Cu60Ni38Co2 alloys have gone through two grain refining processes with the change of undercooling.The grain refinement under low undercooling is due to the dendrite remelting caused by chemical superheating.The most direct evidence is that the maximum recalescence temperatureTRof Cu55Ni43Co2 alloy melt observed in this experiment exceeds the solidus temperature of the alloy during rapid solidification.The heat released by the alloy melt in the rapid solidification stage causes the temperature of the system to rise sharply,eventually exceeding the solidus temperature of the alloy,so that the dendrites in the primary solid phase are remelted into fine grains.However,the maximum recalescence temperatureTRin the solidified structure with high undercooling has been greatly reduced,as shown in Figs.2(e),2(f).Therefore,dendrite remelting is no longer the main factor for grain refinement in the structure with high undercooling.However,typical recrystallization characteristics were found in the grain refining structures of the above three alloys with large undercooling,so it is likely that recrystallization has refined the grains of the undercooled structures.Next,EBSD is used to characterize the grain refinement structure of the Cu55Ni43Co2 alloy at ΔT=250 K,Cu55Ni45 alloy at ΔT=284 K,and Cu60Ni-38Co2 alloy at ΔT=259 K to confirm the mechanism of the second grain refinement phenomenon.

According to the definition in crystallography,grain boundaries with an orientation difference of less than 15° between adjacent grains are considered as low angle grain boundaries (LAGBs),while those with an orientation difference of more than 15° are considered as high angle grain boundaries (HAGBs).In Fig.10(b),the red line represents the LAGBs,the green line represents the HAGBs,and the blue line represents Σ3 twin grain boundaries (TBs) in HAGBs.The meanings in Figs.11(b),12(b) are the same as above.It is obvious from Figs.10(b)-12(b) that the proportion of LAGBs is very low,while that of HAGBs is very high.The distribution of grain boundary misorientation of the three alloys obtained through statistical calculation Figs.10(d)-12(d) show that the LAGBs in grain refinement structure of Cu55Ni43Co2 alloy,Cu55Ni45 alloy and Cu60Ni38Co2 alloy are 11.5%,9.3%,and 7.7% respectively,and the HAGBs account for 88.5%,90.7%,and 92.3%,while TBs are 20.6%,25.2%,27.1%.The high angle grain boundaries accounts for almost 90% of the grain refinement structure of the three alloys under high undercooling,and the Σ3 twin grain boundaries accounts for more than 20%.It is basically consistent with the recrystallization zone in Figs.10(e)-12(e).The recrystallization proportion of the alloy microstructure is about 95%,and the HAGBs is mainly formed after the recrystallization of the alloy structure.In Figs.10(f)-12(f),the local misorientation of the microstructures of the three alloys is concentrated below 0.5°,and the color of the area within the structure is almost blue.The above fully indicates that the internal deformation degree of the alloy microstructure under high undercooling is very low,which is consistent with that the deformed grains only account for 1% in Figs.10(e)-12(e).In Figs.10(c)-12(c),grains with different colors indicate different spatial orientations,while grains with similar or same colors indicate the same spatial orientations.The disordered color of the grains indicates that the spatial orientation of the grains is random,and there is no preferred orientation.In the polar fingures 10-12 g,no higher strength texture is also observed.

Fig.10 EBSD characterization of grain refinement structure of Cu55Ni43Co2 alloy at ΔT=250 K: (a) Alloy microstructure mapping;(b) Grain boundary;(c) Grain orientation;(d) Misorientation distribution;(e) Recrystallization area;(f) Local misorientation;(g) Polar figure

Fig.11 EBSD characterization of grain refinement structure of Cu55Ni45 alloy at ΔT=284 K: (a) Alloy microstructure mapping;(b) Grain boundary;(c) Grain orientation;(d) Misorientation distribution;(e) Recrystallization area;(f) Local misorientation;(g) Polar figure

Fig.12 EBSD characterization of grain refinement structure of Cu60Ni38Co2 alloy at ΔT=259 K: (a) Alloy microstructure mapping;(b) Grain boundary;(c) Grain orientation;(d) Misorientation distribution;(e) Recrystallization area;(f) Local misorientation;(g) Polar figure

From the EBSD characterization of the grain refinement structure of Cu55Ni45Co2,Cu55Ni45,and Cu60Ni38Co2 alloys with high undercooling,it can be preliminarily confirmed that the typical recrystallization behaviors such as high proportion of large angle grain boundaries and large proportion of annealing twins lead to the refinement of alloy grains.A wide recovery behavior occurred in the post-recalescence period of alloy.The newly formed deformed matrix consuming the deformed matrix around makes fine equiaxed grains formed,and a large number of equiaxed grains swallow each other to produce low angle grain boundaries.The low angle grain boundary rapidly migrates to form the large angle grain boundary.The grain boundary becomes more and more straight from bending,and the spatial orientation of grains also becomes random,eventually leading to the formation of fine grains in the alloy structure.The driving force causing alloy recovery and recrystallization comes from the stress accumulated in the rapid solidification process of alloy melt,which is proposed by Liu[23].He believed that when the undercooled alloy melt reached a certain undercooling,the undercooled melt would gradually form the primary solid phase with dendritic structure in the rapid solidification stage.And the flow of the undercooled melt to the solidification front would produce stress on the dendrites.With the increase of undercooling,the stress accumulated on the dendrite skeleton would break through the strength limit of the alloy,and the strain energy stored in the fragments formed after the dendrite collapse will provide energy for the subsequent recovery and recrystallization behavior.According to this theory,there should be fewer defects in the grain refinement structure with high undercooling.The following TEM is used to characterize the grain refinement structure with high undercooling of three alloys.

The green triangle in Fig.13(a) and Fig.14(a)represents the selected area of low-density dislocation network,and the yellow circle in Figs.13(c),14(c)represents the selected area of high-density dislocation network.From the high magnification area of Figs.13(b),14(b),there are still many dislocations in the low-density dislocation area of the two alloys.There are obviously more dislocations in the high-density dislocation area (Fig.13(d) and Fig.14(d)),but there are fewer high-density dislocation network areas.The dislocation areas are almost all distributed in the grains,and a low amount of dislocation areas are gathered at the grain boundary,while the dislocation areas at the twin grain boundary are more distributed in Fig.15(a).This is because the recrystallization of the alloy consumes most of the energy,thus reducing the dislocation density.However,the formation of twin grain boundaries causes serious dislocation of surrounding atoms,which forms a dislocation area with high density near the twin grain boundaries.The above is the effective evidence of Liu’s theory.So far,combined with BCT model[27],the transformation mechanism of microstructure morphology of Cu55Ni43Co2,Cu55Ni45,and Cu60Ni38Co2 alloys with the increase of undercooling can be described as follows.At low undercooling,the growth of dendrite is mainly affected by the diffusion of solute,and the alloy melt is limited to grow in a very narrow area around,eventually forming a coarse dendrite structure.With the increase of undercooling,the dendrite remelting effect is enhanced,and the dendrites are remelted into fine equiaxed grains,resulting in the first grain refinement of the alloy.In the medium undercooling range,the growth of dendrites is replaced by thermal diffusion from solute diffusion,and dendrites grow along the direction of thermal diffusion to produce directional characteristics,and stress is gradually accumulated on the dendrites.When the undercooling reaches a certain value,the stress accumulated in the dendrite during the recalescence of the alloy melt will break through the strength limit of the alloy,and finally the dendrite skeleton will collapse to form many crystal seeds and dendrite fragments.The strain energy stored by dendrite fragments provides a driving force for the subsequent recovery and recrystallization behavior,so that the second grain refinement occurs at high undercooling.

Fig.13 Selected area electron diffraction (SAED) and selected area bright field images of Cu55Ni43Co2 alloy at ΔT=250 K

Fig.14 Selected area electron diffraction (SAED) and selected area bright field images of Cu55Ni45 alloy at ΔT=284 K

Fig.15 Selected area electron diffraction (SAED) and selected area bright field images of Cu60Ni38Co2 alloy at ΔT=259 K

In this paper,the evolution of the recalescence behavior,solidification characteristics and microstructure morphology of Cu55Ni43Co2,Cu55Ni45,and Cu-60Ni38Co2 alloys with the change of undercooling was systematically studied,and the grain refinement mechanism of the high undercooling refinement structure was analyzed.The main conclusions are as follows:

a) The three alloy melts undergo remarkable recalescence behavior during rapid solidification.In the range of undercooled alloy samples obtained,the recalescence behavior of Cu55Ni45 alloy is the most intense,followed by Cu60Ni38Co2 alloy,and Cu55Ni43Co2 alloy is the lowest.And the recalescence degree of the three alloys increases almost in a linear form with the undercooling change.

b) The three alloys have similar morphology transformation processes at the solidification front during rapid solidification.The alloy melt with low undercooling presents a low angle plane shape at solidification stage,while the alloy melt with medium undercooling presents a sharp edge shape.However,the alloy melt with high undercooling shows a smooth arc shape during solidification.

c) The microstructure morphology of the three alloys has the same transformation process.After the evolution process of “coarse dendrite-equiaxed grainoriented fine dendrite-equiaxed grain”,there are two grain refining phenomena.Through EBSD and TEM characterization,it is confirmed that stress induced recrystallization is the internal mechanism of grain refinement caused by high undercooling structure.

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