Periodic layered structure in Ni 3 Si/Zn diffusion couples Meng He, Xuping Su, * Fucheng Yin, Jianhua Wang and Zhi Li Institute of Materials Research, School of Mechanical Engineering, Xiangtan University, Hunan 411105, China Key Laboratory of Materials Design and Preparation Technology of Hunan Province, Xiangtan University, Hunan 411105, China Received 14 March 2008; revised 9 April 2008; accepted 11 April 2008 Available online 20 April 2008 During the reaction in Ni 3 Si/Zn ternary diffusion couples at temperatures between 390 and 450 °C the formation of a periodic layered structure was observed. The periodic layered structure was determined by using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray diffraction. It consists of parallel alternating layers: a ternary phase T (Ni 2 Zn 3 Si) layer and the c (NiZn 3 ) phase layer. A model describing the formation of such a layered structure in the Ni 3 Si/Zn system is presented. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Periodic layered structure; Ternary diffusion couples; Reactive diffusion zone The formation of periodic layered structures during solid-state reactions was first discovered by Osinski et al. [1] in 1982. In recent years, interesting periodic layered structures have been observed in some ternary diffusion couples such as Fe 3 Si/Zn, Co 2 Si/Zn, NiCo/Mg, SiC/ Ni, SiC/Pt and SiO 2 /Mg [1–7]. Four types of morphology that may develop in a ternary system during a solid-state reaction have been described: (1) simple-layered struc- ture; (2) rod-aggregate structure; (3) interwoven-aggre- gate structure; (4) periodic layered structure. Many different explanations [1–11] for the periodic layered structure have been proposed, and a long-stand- ing controversy exists over the reaction mechanism lead- ing to the formation of these interesting structures. In this present paper we systematize the experimental re- sults obtained from Ni 3 Si/Zn diffusion couples in which this periodic layered structure was observed, and discuss a possible explanation for this interesting phenomenon in the Ni 3 Si/Zn system. The materials used to prepare the diffusion couples were nickel rod (99.99%), silicon powders (99.99%) and zinc bulk (99.9%). The Ni 3 Si alloy was prepared by repeated argon arc melting. The alloy and the zinc bulk prepared for solid–solid couples were cut to 3 Â 3 Â 2 mm. Before clamping the couple halves, the slices were mechanically ground and polished. The cou- ples were then clamped and heated in sealed evacuated silica capsules at 390 °C for appropriate times. The alloy prepared for liquid–solid couples was cut to 8 Â 4 Â 2 mm. The liquid–solid diffusion couples were made by packing the mechanically ground Ni 3 Si slice with appropriate zinc bulk. The couples were then heated in sealed evacuated silica capsules at 450 °C for appropriate times. In all experiments the temperature was controlled to within ±2 °C. After the reaction the diffusion couples were quenched and subsequently mounted, ground and pol- ished. After etching in a 4% Nital solution (HNO 3 in alcohol) they were examined by scanning electron microscopy (SEM) and energy dispersive X-ray spec- troscopy (EDS). In some cases the reaction zone was studied with X-ray diffraction (XRD). Figure 1 shows an example of the layer morphology in solid–solid diffusion couples annealed at 390 °C for 144 h. The fine periodic layered structure appears in the reactive diffusion zone. The morphology is charac- terized by a regular array of thin parallel bands with a very small spacing in the diffusion zones. The spacing of the fine periodic layered structure is about 1 lm. It can be seen from the micrographs that the band spacing slightly increases towards the Ni 3 Si alloy of the diffusion zone. By using EDS and XRD, the existence of a ternary phase T (Ni 2 Zn 3 Si) and the c phase (NiZn 3 ) in the diffu- sion reaction zone were confirmed. 1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.04.015 * Corresponding author. Address: Institute of Materials Research, School of Mechanical Engineering, Xiangtan University, Hunan 411105, China. Tel.: +86 732 8292060; fax: +86 732 8292210; e-mail: [email protected]Available online at www.sciencedirect.com Scripta Materialia 59 (2008) 411–413 www.elsevier.com/locate/scriptamat
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Available online at www.sciencedirect.com
Scripta Materialia 59 (2008) 411–413
www.elsevier.com/locate/scriptamat
Periodic layered structure in Ni3Si/Zn diffusion couples
Meng He, Xuping Su,* Fucheng Yin, Jianhua Wang and Zhi Li
Institute of Materials Research, School of Mechanical Engineering, Xiangtan University, Hunan 411105, China
Key Laboratory of Materials Design and Preparation Technology of Hunan Province, Xiangtan University, Hunan 411105, China
Received 14 March 2008; revised 9 April 2008; accepted 11 April 2008Available online 20 April 2008
During the reaction in Ni3Si/Zn ternary diffusion couples at temperatures between 390 and 450 �C the formation of a periodiclayered structure was observed. The periodic layered structure was determined by using scanning electron microscopy coupled withenergy dispersive X-ray spectroscopy and X-ray diffraction. It consists of parallel alternating layers: a ternary phase T (Ni2Zn3Si)layer and the c (NiZn3) phase layer. A model describing the formation of such a layered structure in the Ni3Si/Zn system ispresented.� 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Periodic layered structure; Ternary diffusion couples; Reactive diffusion zone
The formation of periodic layered structures duringsolid-state reactions was first discovered by Osinski et al.[1] in 1982. In recent years, interesting periodic layeredstructures have been observed in some ternary diffusioncouples such as Fe3Si/Zn, Co2Si/Zn, NiCo/Mg, SiC/ Ni,SiC/Pt and SiO2/Mg [1–7]. Four types of morphologythat may develop in a ternary system during a solid-statereaction have been described: (1) simple-layered struc-ture; (2) rod-aggregate structure; (3) interwoven-aggre-gate structure; (4) periodic layered structure.
Many different explanations [1–11] for the periodiclayered structure have been proposed, and a long-stand-ing controversy exists over the reaction mechanism lead-ing to the formation of these interesting structures. Inthis present paper we systematize the experimental re-sults obtained from Ni3Si/Zn diffusion couples in whichthis periodic layered structure was observed, and discussa possible explanation for this interesting phenomenonin the Ni3Si/Zn system.
The materials used to prepare the diffusion coupleswere nickel rod (99.99%), silicon powders (99.99%)and zinc bulk (99.9%). The Ni3Si alloy was preparedby repeated argon arc melting. The alloy and the zincbulk prepared for solid–solid couples were cut to
1359-6462/$ - see front matter � 2008 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2008.04.015
* Corresponding author. Address: Institute of Materials Research,School of Mechanical Engineering, Xiangtan University, Hunan411105, China. Tel.: +86 732 8292060; fax: +86 732 8292210; e-mail:[email protected]
3 � 3 � 2 mm. Before clamping the couple halves, theslices were mechanically ground and polished. The cou-ples were then clamped and heated in sealed evacuatedsilica capsules at 390 �C for appropriate times.
The alloy prepared for liquid–solid couples was cut to8 � 4 � 2 mm. The liquid–solid diffusion couples weremade by packing the mechanically ground Ni3Si slicewith appropriate zinc bulk. The couples were thenheated in sealed evacuated silica capsules at 450 �C forappropriate times. In all experiments the temperaturewas controlled to within ±2 �C.
After the reaction the diffusion couples werequenched and subsequently mounted, ground and pol-ished. After etching in a 4% Nital solution (HNO3 inalcohol) they were examined by scanning electronmicroscopy (SEM) and energy dispersive X-ray spec-troscopy (EDS). In some cases the reaction zone wasstudied with X-ray diffraction (XRD).
Figure 1 shows an example of the layer morphologyin solid–solid diffusion couples annealed at 390 �C for144 h. The fine periodic layered structure appears inthe reactive diffusion zone. The morphology is charac-terized by a regular array of thin parallel bands with avery small spacing in the diffusion zones. The spacingof the fine periodic layered structure is about 1 lm. Itcan be seen from the micrographs that the band spacingslightly increases towards the Ni3Si alloy of the diffusionzone. By using EDS and XRD, the existence of a ternaryphase T (Ni2Zn3Si) and the c phase (NiZn3) in the diffu-sion reaction zone were confirmed.
Figure 1. Reactive diffusion zone in solid–solid Ni3Si/Zn diffusioncouples annealed at 390 �C for 144 h: (a) general view (SEM); (b)magnified area close to the Ni3Si/reaction layer interface (BEI).
Figure 2. Reactive diffusion zone in liquid–solid Ni3Si/Zn diffusioncouples annealed at 450 �C for 192 h: (a) general view; (b) magnifiedview.
Figure 3. Schematic diagram showing the formation process of theperiodic layered structure in Ni3Si/Zn.
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Figure 2 shows the diffusion zone of the liquid–soliddiffusion couples annealed at 450 �C for 192 h. The peri-odic layered structure also appears in the reactive diffu-sion zone. The periodic layered structure close to Ni3Sisubstrate is much more ordered; some regions are regu-lar and consist of parallel alternating layers, others areirregular but a certain kind of periodicity can still be ob-served. By using EDS and XRD, the existence of a ter-nary phase T (Ni2Zn3Si) and the c phase (NiZn3) in thediffusion reaction zone were also confirmed.
The morphology in the Ni3Si/Zn diffusion couples at390 and 450 �C exhibits a very interesting feature: theperiodic layered structure. The mobility of Zn, Ni andSi species are very different. Zn is the most mobile com-ponent in the product layer, Ni is slowly mobile, and Siis relatively immobile. The T/c double layer (T1/c1) is animmediate result of the reactions occurring at the inter-face. The c layer (c1) grows by the diffusion of Znthrough the c phase to the T/c interface to react withNi diffusing (short-range diffusion) from the Ni3Si/Tinterface; the T layer (T1) grows by diffusion of Znthrough the reactive diffusion zone to the Ni3Si/T inter-face. The growth rate of the c layer is controlled by thesupply of component Ni, and the growth rate of the Tlayer is controlled by the supply of component Zn.However, with the growth of the T layer the diffusionof Ni element to the T/c interface becomes increasinglydifficult, and the growth of the c1 layer becomes slowerowing to the lack of Ni supply. Since the Ni:Si ratio inthe T phase is not more than 2, while the ratio in Ni3Si
substrate is 3. The diffusion resistance of Ni causes Nielement build up at the Ni3Si/T interface. As soon asthe concentration of Ni at the Ni3Si/T interface reachesa certain critical value required for the nucleation of thec phase, the c phase (c2) may nucleate at the Ni3Si/Tinterface. However, we have also shown (in work to bepublished later) that no stable interface exists betweenNi3Si and the c phase, so the T2/c2 pair will form rapidlyat the Ni3Si/T1 interface. The process is periodic, whichresults in the formation of the periodic layer structure.Figure 3 shows a schematic diagram of this process.The proposed mechanism of the periodic layered struc-ture considers long-range diffusion of Zn atoms throughthe reaction zone to the Ni3Si surface, short-range diffu-sion of Ni atoms from Ni3Si through the adjacent Tphase to the T/c interface, and complete immobility ofSi atoms. It supports the mechanism proposed byGutman et al. [11].
Figure 4. Schematic diagram showing the mass transfer across theperiodically layered reaction zone in Ni3Si/Zn.
M. He et al. / Scripta Materialia 59 (2008) 411–413 413
According to the proposed process description, thegrowth process between Ni3Si and Zn was discussed be-low with the assumption that the process of the old pairwill cease when the new pair of layers has been formed.Figure 4 is a schematic diagram showing the mass trans-fer across the periodically layered reaction zone in theNi3Si/Zn couple, where L is the total thickness of thediffusion zone, Lc is the thickness of the c layer adjacentto the Ni3Si surface, and LT is the thickness of the Tlayer adjacent to the Ni3Si surface.
It is known that the diffusion of Zn through the dif-fusion zone to the Ni3Si surface needs more and moretime as L increases, and that the Ni atoms accumulatedat the Ni3Si surface have more time to diffuse to the cphase. Hence the accumulation of Ni takes longer toreach the critical value required for the nucleation ofthe c phase, which causes the thickness of the c/T pairto increase. In the reactive diffusion zone, the layer mor-phology is probably to be related to the grain orienta-tion of Ni3Si end member. The layer morphology isdifferent with a variety of grain orientations. The quali-tative explanation for this is that the critical values forthe various grain orientations are different. These pro-posals are in good agreement with the experimentalobservations, especially in Figure 1b.
The appearance of a periodic layered morphologyseems to be a general diffusion phenomenon. In theNi3Si/Zn ternary system we conclude that:
1. The formation of a periodic layered structure wasobserved both in the solid–solid diffusion couples at390 �C and the liquid–solid diffusion couples at450 �C. The physical state of Zn has no influence onthe appearance of the bands.2. The proposed mechanism of the periodic layeredstructure considers long-range diffusion of Zn atomsthrough the reaction zone to the Ni3Si surface, short-range diffusion of Ni atoms from Ni3Si through theadjacent T phase to the T/c interface, complete immo-bility of Si atoms, and large difference of mobilitybetween Ni and Zn atoms.3. The growth rate of the c phase is controlled by thesupply of component Ni, and the growth rate of thetotal reaction zone is controlled by the supply of compo-nent Zn.
This work was supported by National Natural Sci-ence Foundation of China (Nos. 50671088, 50771089).
[1] K. Osinski, A.W. Vriend, G.F. Bastin, F.J.J.Z. van Loo,Metallkd. 73 (1982) 258.
