Hongfu Wang, Hongen An, Shaopei YangCollege of Mechanical Engineering, North University of China, Taiyuan, Shanxi, P. R. China

Grain refinement mechanism of rapidly solidifiednickel alloys

Abstract: The solidification microstructure evolution ofNi-25 at.% Cu alloys under different undercooling degreeswere studied by the cladding method and cyclic superheatingmethod. Two grain refinement phenomena were observed inthe obtained undercooling. In the low undercooling condi-tion, dendrite remelting is the main reason for grain refine-ment in the recalescence process, while in the high under-cooling condition, the stress accumulated in the recalescenceprocess leads to recrystallization in the later stage of recales-cence. Under the condition of high undercooling, the solidifi-cation structure is composed of complete equiaxed grainswith relatively uniform grain size, which indicates that grainboundary migration occurs during grain growth.

Keywords: Undercooling; Grain refinement; Recrystalli-zation; Dendrite remelting

1 Introduction

Compared with the traditional process, the grain refinementin supercooled melt is spontaneous. Because of this, moreand more attention has been paid to this phenomenon sinceWalker [1] obtained the grain refinement structure in super-cooled melt in 1959. Researchers have studied many alloysystems, such as pure Ni [2, 3] and Ni–Zr [4]. At the sametime, the methods used by researchers to obtain undercool-

ing of alloys or metals are also inconsistent (such as meltflux method [5, 6], dropper method [7], and melt spinningmethod [8]). However, molten coating and cyclic super-heating techniques have been widely accepted and used asmethods to obtain three–dimensional bulk alloys [9– 14].

In the studied alloy system, fine grain structure can beobtained under high undercooling, and the formation mech-anism of fine grains has been widely studied. At present, themain mechanisms include growth instability, recrystalliza-tion, dendrite remelting and dendrite fracture induced bycritical growth rate [2]. In single-phase binary alloys, den-drite remelting debris caused by chemical overheating atlow undercooling is the main reason for grain refinement.However, the stress-induced recrystallization mechanismproposed in this paper is used to verify the grain refinementphenomenon at high undercooling. Recrystallization usual-ly occurs in highly deformed metals or alloys. The mainprocess of recrystallization is the nucleation and subsequentgrowth of plastic deformed metals and alloys under anneal-ing conditions. In high undercooling metals, with the in-crease of initial undercooling, stress accumulation occursin the solidification structure, and recrystallization occursspontaneously at the recrystallization temperature, so as toobtain the grain refinement structure.

In this paper, binary single phase Ni-25 at.% Cu alloywas used as the research object, and B2O3 was used as thepurification agent to adsorb impurities. The solidificationstructures under different undercooling were studied sys-

H. Wang et al.: Grain refinement mechanism of rapidly solidified nickel alloys

International Journal ofMATERIALS RESEARCH

Zeitschrift fur METALLKUNDE OOriginal Contributions

682 Int. J. Mater. Res. 112 (2021) 9

tematically. The results show that there are two kinds ofgrain refinement structure in the whole solidification pro-cess. Electron backscattering diffraction (EBSD) imagesof the two kinds of grain refinement samples show that thedislocation angles of the two kinds of grain refinement sam-ples are obviously different, which also confirms the differ-ent mechanism of the two kinds of grain refinement.

2 Experimental procedure

The Ni-25 at.% Cu alloys were prepared by melting purenickel (99.99%) and copper (99.99 %) particles under theprotection of high-purity Ar gas. Before melting,the purenickel and copper were pre-polished with sandpaper to re-move surface oxidants and etched with HCI solution. Be-fore the undercooling experiment, the quartz crucible glassand the sample were first cleaned in an ultrasonic cleaningmachine containing alcohol for at least 6 min to removethe impurities on the surface. The whole undercooling ex-periment was carried out under vacuum condition, and thevacuum pressure was 10–3 Pa. Under the heating of induc-tion heating coil, B2O3 was first heated to the molten stateand kept warm for 10 min to absorb impurities on the sur-face of the material. Then gradually increasing the tempera-ture until the temperature was 100 K to 150 K higher thanthe melting point of the alloy and keeping the temperaturefor about 15 min, each sample will be continuously heatedand cooled by circulation, then turnning off the high-fre-

quency heating power supply, and the undercooled samplewill spontaneously cool to room temperature. Undercoolingwas the liquidus temperature minus the starting temperaturepoint of recalescence (solidification). We used a tempera-ture detector to measure the cooling-recalescence tempera-ture curve of the alloy sample, and the undercooling valuewas calculated by using the liquidus temperature minus thestarting temperature point of recalescence(solidification).In order to get a more detailed rule of solidification micro-structure evolution, the difference of undercooling degreeof each supercooled sample was about 10 K. The wholetemperature change process was collected by an infraredthermometer with an accuracy of ±5 K and a response timeof 10 ms, and transmitted to a computer in real time.

3 Results and discussion

In the range of supercooling, some typical microstructuraltransformation processes were observed. Figure 1 showsthe solidification structure of undercooled Ni-25 at.% Cuunder typical undercooling conditions. The results showthat there are several obvious microstructural transforma-tion stages in the whole solidification process.

(a) Figure 1a shows the solidification structure at 30 Kundercooling. The results show that the whole solidificationstructure is coarse dendrite with large grain size and a largenumber of secondary dendrite arms.

(b) With the increase of undercooling, the morphology ofthe solidified structure changes. It can be seen from the op-tical micrographs that coarse dendrites are replaced by re-fined equiaxed crystals (Fig. 1b). This is the first time to re-fine the grain at low undercooling. The solidificationstructure has smooth grains and wide boundary. This is theresult of dendrite remelting.

(c) With the increase of undercooling, it can be seen fromFig. 1c that dendrites gradually replace refined equiaxedgrains. However, compared with the dendrite under low un-dercooling, the dendrite under undercooling is obviouslysmaller, the dendrite spacing is closer, and the secondarydendrite arm is developed.

(d) With the increase of undercooling, the solidificationstructure was also observed (Fig. 1d and e). It is obvious thatdendrites are replaced by equiaxed grains, which is the sec-ond grain refinement. Compared with the first refinement,the solidification structure presents complete equiaxedgrains with relatively uniform grain size, which indicates thatgrain boundary migration occurs during grain growth. At thesame time, we can see the presence of annealing twins,which is important evidence of recrystallization.

According to the solidification structure diagram inFig. 1, the structures obtained from the two kinds of grainrefinement are different. At low undercooling, the grainsare smooth and the grain boundaries are wide, which iscalled a spherical crystal structure. When the undercoolingis high, the solidification structure is equiaxed, the grainsize is uniform, and there are annealing twins in the grains.It can be seen that the two grain refinement mechanismsare necessarily different.

Fig. 1. Microstructural evolution of undercooled Ni-25 at.% Cu alloysas a function of the initial undercooling (a) 30 K; (b) 50 K; (c) 140 K;(d) 260 K; (e) 280 K.

H. Wang et al.: Grain refinement mechanism of rapidly solidified nickel alloys

Int. J. Mater. Res. 112 (2021) 9 683

The grain refinement at low undercooling can be attribu-ted to the dendrite remelting at the later stage of recales-cence. Dendrite remelting mainly includes dendrite remelt-ing fragments driven by solid–liquid interfacial tensionand dendrite remelting fragments caused by chemical over-heating proposed by Karma [8]. Firstly, a model describingthe dendrite fragmentation at low undercooling was pro-posed. The physical mechanism of grain refinement is de-scribed. Grain refinement occurs when the time of completesolidification of interdendritic melt is longer than the timeof dendrite fracture. At the same time, both of these timesdepend on the undercooling degree, so the grain refinementcorresponds to the critical undercooling degree. In order toobtain fine grain structure, undercooling must reach a cer-tain critical value.

In addition, the effect of chemical superheating on the so-lidification structure also plays an important role. When thetemperature of undercooled melt is lower than the liquidustemperature, the alloy will obtain the driving force of ther-modynamic crystallization, which leads to the rapid growthof nucleating crystals. At the same time, a large amount oflatent heat of crystallization will be released during the soli-dification process of undercooled metal. Because the rate ofrapid solidification is much higher than normal, the re-leased latent heat of crystallization mainly acts on the den-drites, which directly leads to overheating and remelting ofsolidified dendrites, and leads to the transformation of soli-dification morphology of primary dendrites. Figure 2 showsthe relationship between the initial undercooling and the re-melting rate of dendrites. The results show that when theundercooling is less than 50 8C, the remelting rate of den-drites is very high, which proves that the remelting of den-drites has an important effect on the transformation of soli-dification structure at low undercooling. Compared withlow undercooling, the dendrite remelting fraction at highundercooling is smaller, and it is not easy to cause dendriteremelting.

With the increase of initial undercooling, the solidifica-tion rate of undercooled melt also increases. Therefore,under the condition of high undercooling, the transforma-tion rate of undercooled melt to solid will be very high.Because primary dendrites are very fragile, they have noability or strength to resist deformation. When the residu-al undercooled melt acts on the dendrite, there will be acertain amount of stress accumulation. When the accumu-lated stress is greater than the maximum stress that theprimary dendrite can bear, the dendrite will fracture anda large amount of stress will accumulate in the dendrite.The energy is stored in the deformed dendrite fragments

Fig. 3. (a) the grain structure of undercooledNi-25 at.% Cu at 60 K, (b) the misorientationdiagram, and (c) the inverse pole diagram.

Fig. 2. Remelted fraction of the primary dendrite at the highest reca-lescence temperature in highly undercooled Ni-25 at.% Cu alloys.

H. Wang et al.: Grain refinement mechanism of rapidly solidified nickel alloys

684 Int. J. Mater. Res. 112 (2021) 9

in the form of strain energy, which pats the dendrite frag-ments in a metastable thermodynamic state. The forma-tion of dense crystal defects leads to the occurrence anddevelopment of high temperature recovery and recrystal-lization.

Typical grain refinement structures were detected usingthe EBSD technique for both low and high undercoolingconditions. Firstly, we define the orientation difference asthe crystal orientation difference between two adjacentgrains. The frequency of a certain orientation angle is de-fined as the length of the grain boundary with the same ori-entation angle divided by the total length of the grainboundary in the EBSD diagram. We define a low anglegrain boundary as a grain boundary angle less than158and high angle grain boundary with grain boundary an-gle more than 158. Figure 3 shows the EBSD diagram ofNi-25 at.% Cu alloy undercooled at 50 K. The grainboundary is smooth and wide, which indicates that thegrain has been coarsened. As can be seen from the inversepole graph of Fig. 3c, the texture is almost random. Figur-e 3b shows the orientation angle distribution of grainboundaries in alloy Ni-25 at.% Cu undercooled at 50 K. Itcan be seen that the proportion of low angle grain bound-ary is 60.7 %. In addition, at low undercooling, the propor-tion of annealing twins in the whole grain boundary is verysmall. Figure 4 shows the EBSD pattern of Ni-25 at.% Cuundercooled at 260 K, which is a typical grain refinementstructure under high undercooling condition. The orienta-tion angle distribution of grain boundaries in undercooledalloy Ni-25 at.% Cu at 260 K shows that the proportionof high angle grain boundary is about 88.4 %, and the pro-portion of orientation angle is about 60%. The recognition

rate of twin boundary is about 14.5 %. According to theabove data, recrystallization is the main cause of grain re-finement under deep undercooling. As shown in Fig. 4c,high density dislocations can be observed in the specimen.These lattice defects indicate that plastic deformationshould occur during rapid solidification, leading to highlydefective structures.

4 Conclusions

The microstructural evolution of Ni-25 at.% Cu alloy underdifferent degrees of undercooling was systematically stu-died by means of cladding and cyclic superheating, and thetypical solidification structures were detected in detail byEBSD. The main conclusions are as follows

(1) Two kinds of grain refinement were observed in thewhole solidification structure, one occurred in the range oflow undercooling, the other occurred in the range of highundercooling. According to the evolution of the solidifica-tion structure, the whole solidification process can be di-vided into four stages: coarse dendrite?equiaxed crys-tal? fine dendrite?equiaxed crystal.

(2) Through the EBSD analysis of two typical grain re-finement structures, it can be more clearly observed thatthere are differences between the two kinds of grain refine-ment, which also shows that the mechanism of the twokinds of grain refinement is inconsistent

(3) At low undercooling, dendrite remelting is the mainreason for grain refinement. At high undercooling, thestress accumulated during rapid solidification provides thedriving force for the subsequent recrystallization, resultingin grain refinement.

Fig. 4. (a) the grain structure of undercooledNi-25 at.% Cu at 260 K, (b) the inverse polediagram, and (c) typical bright field TEMimages of the dislocation networks.

H. Wang et al.: Grain refinement mechanism of rapidly solidified nickel alloys

Int. J. Mater. Res. 112 (2021) 9 685

Funding: This work was supported by National Natural Science Foun-dation of China (Nos. 51701187). Basic Applied Research Projects inShanxi Province (201801D221151). Key research and developmentand promotion projects in Henan Province (212102210267).

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(Received August 12, 2020; accepted May 17, 2021; onlinesince August 6, 2021)

Correspondence address

Dr Hongfu WangCollege of Mechanical EngineeringNorth University of China030051 TaiyuanShanxiP.R. ChinaTel.: +86 15935627156E-mail: wanghongfu@nuc.edu.cn

BibliographyDOI 10.1515/ijmr-2020-8034Int. J. Mater. Res. 112 (2021) 9; page 682–686ª 2021 Walter de Gruyter GmbH, Berlin/Boston, GermanyISSN 1862-5282 · e-ISSN 2195-8556

H. Wang et al.: Grain refinement mechanism of rapidly solidified nickel alloys

686 Int. J. Mater. Res. 112 (2021) 9