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Laser surface alloying of Ni film on Al-based alloy
248
Applied Surface Science 43 119891 248 2.>5 North-tlolland
LASER SURFACE ALLOYING OF Ni FILM O N AI-BASED ALLOY
E. G A F F E T * ('entre d'Etudes de Chimie M&allurgique/ C,¥RS, 15 Rue G. L/rham, 1<94407 [,'itt3"/ Seine (edev. t'Wance
J.M. P E L L E T I E R and S. B O N N E T - J O B E Z ('ALFETMAT, Biitimenl 4(13, INSA, 1:-69621 Villeurhanne, t'rance
Received 30 May 1989: accepted for publication 20 June 1989
Based on SIMS analyses, X-ray diffraction paUcrns, SEM/EDX and TEM/EI)X investigations, the inechanisms of la,,,er surface alloying of the Ni coating of an Al-based alloy has been investigated (melting and solidification). The formation of AI ~Ni primary dendrites in the case of low thickness of the initial Ni coating is reported. A double dendritic mode i>, observed in the case of thicker coating, AI3Ni 2 followed b', A1~Ni dendrites. A Si surface segregation has been detected and related to the dendritic solidification mode.
1. Introduction There is considerable interest in the improvemen( of the properties of the near surface region which can determine the p e r f o r m a n c e of a material in m a n y technological applications. The use of high energy beams, such as laser beams, allows melting of a pre-deposited layer on a substrate a n d is k n o w n to improve the surface m e c h a n i c a l / chemical properties relatively to the base matrix (for example: a high melting p o i n t film on a low melting p o i n t substrate [1], c h r o m i u m carbide on an Ni alloy [2]). In this work, we report on the Ni alloying by CO. laser surface t r e a t m e n t of an Al-based alloy. Utilizing S E M / E D X , T E M / E D X in investigations, SIMS profiles, the solidification m e c h a n i s m has been investigated.
* To whom correspondence should be senl. 0169-4332/89/$03.50 :i;, Elsevier Science Publishers B.V. (North-Holland)
2. Experimental procedure 2.1. L a s e r surfiwe t r e a t m e n t
The n o m i n a l c o m p o s i t i o n of the AI based alloy corresponds to ( 4 . 5 - 6 wt%) Si, (2.8 3.8 wt%) Cu. Two thicknesses of an electrolytic layer of Ni have been chosen: 25 /*m (alloy 1), and 50 /tin (alloy 11). The c o n d i t i o n s of the CO, laser surface treatm e n t are: i n c i d e n t power density, Q , = 10 5 W / c m : . interaction time of a b o u t 100 ms. The samples (20 × 60 × 4 m m ~) have been fixed on a t r a n s l a t i o n table a n d moved u n d e r the laser beam at various speeds. The surface oxidation is dim i n i s h e d by a protective gas (argon). 2.2. Characterization techniques
(i) The m a c r o s t r u c t u r e has been investigated by X-ray diffraction using the Co K a wavelength (4 = 0.17889 nm). (ii) The morphological and chemical investigations have been p e r f o r m e d using a S E M - Z E I S S
E. Gaffet et al. / Laser surface alloying of Ni film an Al-based alloy
DSM 950, which is equipped with a Si-Li X-ray detector combined with a Tracor microanalyser. The chemical profile has been also investigated using the SIMS method. (iii) The microstructure has been studied using a TEM-JEOL 2000FX, and microanalyses have been performed at a finer scale (less than a /~m). The TEM foils have been directly prepared by ultramicrotomy [3,4].
249
alloy I, (b) alloy II. The identified phases are: matrix: A1, A12Cu , A12Cu3, C u 4 S i , alloy I: A1, AlaNi , NisSi2, A12Cu3, alloy II: A1, A13Ni 2, A13Ni, Ni3Si, NisSi 2. The X-ray peaks which correspond to the A13Ni ~ are located on the following positions: 21 °, 29.6 o, 57.3 ° (20, XCoKc 0.
3.2. Morphological (SEM/EDX)
and chemical
investigation
3. Experimental results
3.1. Structural investigations Fig. 1 exhibits the typical X-ray patterns which are obtained after the laser surface treatment: (a)
3.2.1. Morphologies (i) The melted area exhibits a dendritic solidification structure, the thickness of the melted area is about 200-250 /~m (see figs. 2 and 3). The dendritic orientation is non-uniform. Convection
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Fig. 1. X-ray diffraction pattern: (a) alloy I after laser surface treatment, (b) alloy II after laser surface treatment.
~0
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E. GafJet et al. / Laser surJktce alloying of Ni film an dl-hased allqv
Fig. 2. SEM micrographs of the transverse cut after laser irradiation of alloy I (backscattered electron image mode).
Fig. 3. SEM micrographs of the transverse cut after laser irradiation of alloy I1 (backscattered electron image mode).
E, Gaffet et a L / Laser surface alloying of Ni film an Al-based alloy
105
251
(iii) The matrix is composed of some A1 grains (dark contrast), surrounded by areas enriched in Si and Cu (bright contrast).
At Si
~10 4 ffl
~io 3
Cu
I--
~ 10 2
Ni
ILl I'--
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Matrix
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I
I
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I
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300
400
500
600
~.m Fig. 4. Composition profile along a line perpendicular to the surface (SIMS method, incident ions/oxygen, probe diameter
=8~m). movements of the liquid have perturbed the dendritic solidification by modifying the chemical diffusion process. (ii) A thermal affected area is observed at the limit between the substrate and the melted area. This zone has a small thickness of about 10-30 /zm (see figs. 2 and 3).
3.2.2. Chemical distribution (i) SIMS investigations. Fig. 4 shows elemental profiles parallel to the plane of the surface within the probe resolution of about 8/~m. The Ni content appears to be relatively constant in the melted area. Far from this zone, the Ni content diminishes rapidly (three order of magnitude). The Si and Cu have a more homogeneous distribution in the whole melted area. The concentration in A1 is rather uniform in the entire analyzed area. (ii) Some E D X / S E M analyses and EDX maps have been performed in order to study the composition fluctuations at the/~m scale. Typical EDX maps are presented in figs. 5 and 6. The dendritic arms and the interdendritic spaces are enriched in Ni and in (A1, Si, Cu) respectively. The composition of the laser surface is equal to: alloy I: A184.n_+l.8-Si9.8_+0.9-Ni4.5_+lA-CUl.5+0. 3 (A194.9 ± 1.0-Ni5.1 ± 1.o); alloy II: two areas have to be considered: the middle of a single laser exposure (A) and the surface corresponding to the overlap between two successive laser exposures (B). This leads to the
Fig. 5. EDX cartograph of the melted area of alloy I.
252
E. GafJet et al. / Laser surfiwe alloying of Ni film an A l-based alloy
Fig. 6. EDX cartograph of the melted area of alloy lI. 100
100
95 90
.< 8O 85
80
!
0
70
i
100
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300
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it3
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200
300
Z
a
b
Fig. 7. Chemical composition as a function of the depth ( E D X / S E M analyses): (a) alloy I, (b) alloy 11. (A) AI, ([23)Ni. 11~) Si.
E. Gaffet et al. / Laser surface alloying of Ni film an Al-based alloy
253
surrounded by the interdendritic spacements. The typical SAD patterns indicates the presence of A13Ni (A), pure A1 (B), A1 and Si (C). The chemical composition microanalyses support such structural information, i.e., the dendrites are exclusively A1/Ni with a composition close to A13Ni and the Ni content inside the interdendritic spacements does not exceed 1.3 at%. The Si repartition is heterogeneous at the scale of 0.25/~m inside the interdendritic areas (areas B and C).
4. Discussion
4.1. Ni layer dissolution During the melting, a dissolution of the Ni layer occurs, accelerated by the convection movement. The measured Ni content inside the melted depth corresponds to the one which may be estimated from a simple mixing rule.
4.2. Solidification mechanism
Fig. 8. TEM micrographs (bright field image). The dark areas correspond to the selected area diffraction pattern and EDX microanalyses.
following compositions: (A) A176.0± 0.8-Si 8.1 ± 0.2-Ni14.5 ± 0.6-CUl.4 ± 0.1 (A184.0 ± 0.7-Nil6.0 ± 0.7);
(B) A180.4± 0.4-Si7.5 _+o.t-Nill.0 ± 0.6-Cut.2 ± 0.1 (A188.0 _+0.6-Ni12.0 ± o.6)" The mean composition is equal to: A177.8-Si8. oNil2.9-Cul.4(A185.8-Ni14.2 ). The composition profile of the alloy is obtained on a polished transverse section of the sample which has been cut perpendicular to the laser beam motion. The results are shown in figs. 7a and 7b. (iv) The microstructures and microanalyses have been investigated by T E M (BF images, SAD patterns, EDX microanalyses). The BF image (fig. 8) reveals the presence of some interdendritic areas
Starting from a mean composition of about 10 at% in Ni (alloy I), the solidification develops primary A13Ni dendritic arms. There is a segregation of A1 and S i / C u inside the interdendritic spaces which later solidifies. In the case of the alloy II with a complete dissolution of the Ni layer, the composition is about 20 at% and therefore is beyond the monotectic composition. The primary arms are A13Ni 2. As the solidification evolves, the remaining liquid is enriched in A1, then the next arm to be formed is A13Ni. Such a solidification mechanism is based on the A1-Ni equilibrium phase diagram (fig. 2) and some remarks have to be noted: (i) The influence of the solute Cu and Si on the solidification process is very weak, the overall process is related to the A1 and Ni relative contents. Taking into account such an observation, we may estimate the cooling rate from the measured interdendritic spacements ( - / ~ m ) and some previous reported results [5]: such a dendritic spacement is obtained for a quenching rate of about 10 4 o C / S .
254
E. Gaffer et al. / Laser surjace alloying of Nt film an A l-based alh)v
(ii) the chosen laser conditions do not lead to the formation of a homogeneous Al Ni supersaturated solid solution (or to the obtainment of an amorphous alloy). The maximum Ni content is 1.3 at% and this has been detected for just one area. Such a result is in agreement with the fact that the relative motion of the target under the laser beam is about 1 c m / s . This means, that under stationary conditions, the maximum velocity of the liquid-crystal interface is equal to this value, which is not so high, compared to the one estimated in the case of the amorphization by laser. Furthermore, some previous results have been reported concerning the metastable A I - N i phase diagram [6] which may be obtained using a pulsed electron beam treatment. Solidification rates as high as 7 m / s are required for the attainment of a single phase, A1Ni, instead of the stable intermetallic compounds such as A13Ni and A13Ni 2 as observed in our experiment. The presence of the intermetallic phase A13Ni 2 (alloy II) may be explained by the A I - N i equilibrium phase diagram. The estimated and the mean measured Ni compositions indicate that the first phase which is expected to be formed - under equilibrium conditions - is the intermetallic compound A13Ni 2. In the case of the thicker layer (50/~m), no complete dissolution may be achieved in some part of the heat affected area leading to some particular solidification feature: equiaxial crystal growth, starting from parts of the crystal Ni film.
4.3. Si surface segregation The increase in Si content (correlated to a decrease in Ni content) just below the surface may be considered a result of the dendritic solidification mode: during the cooling, the dendrites are surrounded by a liquid containing the elements A I - S i - C u which have been segregated during the dendrite growth. At the end of the solidification, so in the depth close to the final surface, this liquid will solidify after the last dendrite tips, this leads to segregation in Si and Cu, over the dendrite tips and just below the free surface.
4.4. Relation between the dendritic solidification mode and surface composition The two areas which are observed in the case of alloy II are also related to the dendritic solidification mode; indeed the dendrites which solidify in the middle of the laser passage grow in stationary conditions, i.e. the interface liquid/crystal speed is equal to the relative speed of the laser, and the direction of the growth is nearly parallel to the surface when the interface reaches the free surface [7]. On the edges of the melted area, the dendrites will grow along a direction which is close to the perpendicular to the surface. Then, chemical analyses taken from the latter region, will integrate dendrites and interdendritic spacements, leading to a mean composition value, corresponding to a balance between the dendritic and the interdendritic compositions. The chemical analyses which are performed on the area where the interdendrites and dendrites are finer and parallel to the surface will increase the importance of the dendrite contribution, leading to a higher value in Ni content, in comparison to the former value. 4.5. Melting point influence of the fusion solidifi'cation mode
The evolution of the grains sizes just at the interface between the melted area (containing Ni) and the matrix may be explained as followed: the temperature which is obtained during the laser surface treatment may not be sufficient to melt the Ni layer, but is sufficient to melt the matrix AI Si Cu. During the solidification, there is a growth of A1 grains, with a rejection of Si, leading to the formation of grains surrounded by Si. The width of the AI grains is smaller than the original ones, according to the relatively high cooling and henceforth by the fact the unmelted grains have undergone an annealing during the laser treatment which leads to an increase of the unmelted grains size as reported in some previous results [3,4].
5. Conclusion
After the determination of the optimized laser treatment parameters, surface alloys have been
E. Gaffet et al. / Laser surface alloying, of Ni film an Al-based alloy
elaborated by a surface melting of a Ni-electrodeposited layer on an Al-based substrate. The main results are: (i) The solidification structure is dendritic with a typical wavelength of about 1 ~m which corresponds to a quenching rate of about 1040 C/s. (ii) The structures of the dendritic arms correspond to some of the intermetallic compounds of the A1-Ni equilibrium phase diagram: A13Ni in the case of the Alloy I, A13Ni z and A13Ni for the alloy II. (iii) The Ni content of the interdendritic areas is less than 1.3 at%. The cooling conditions are not sufficient to obtain a homogeneous A1-Ni supersaturated solid solution. Si and Cu are found in the interdendritic areas. The heterogeneity scale is in the order of a/~m. A Si segregation is observed just below the surface (2 ~m), this corresponds to a decrease in the Ni concentration. An increase in the Ni content is observed in the middle of the laser passage (relatively to the border areas), such an observation may be explained by the solidification mechanism.
Acknowledgements The authors thank the Pechiney society and the Minist+re de la Recherche (MRES) for their finan-
255
cial support, Msse Vialle for her technical assistance with laser experiments and G. Deluze and J.L. Pastol ( C E C M / C N R S ) for their help in obtaining the TEM foils prepared by ultramicrotomy and the EDX cartographies respectively. This work has been supported by the Groupement Scientifique/CNRS "Laser de Puissance No. 85" and a research contract M R T No. 88W0983 ( E U R E K A 194).
References [1] K. Takei, S. Fujimori and K, Nagai, J. Appl. Phys. 51 (1980) 2903. [2] J.H.P.C, Megaw, A.S. Bransden, D.N.H. Trafford and T. Bell, in: Proc. 3rd Intern. Colloq. on Welding and Melting by Electrons and LASER Beam, Lyon, 5-9 September 1983, France. [3] E. Gaffer, G. Deluze, G. Martin, J.M. Pelletier and D. Pergue, Mater. Sci. Eng. 98 (1988) 291, [4] E. Gaffer, Thesis, Doctorat Universit6 Paris V! (1988), [5] H. Matiya, B.C. Giessen and N.J. Grant, J. Inst. Metals 96 (1968) 30. [6] D.M. Follstaedt and S.T. Picraux, in: Alloy Phase Diagrams, Eds. L.H. Bennett, B.C. Giessen and T.B. Massalski (Material Research Society, Pittsburgh, 1988) p. 94. [7] W. Kurz and D.J. Fisher, Fundamentals of Solidification (Trans Tech Publications Ltd, Switzerland, 1986).
Applied Surface Science 43 119891 248 2.>5 North-tlolland
LASER SURFACE ALLOYING OF Ni FILM O N AI-BASED ALLOY
E. G A F F E T * ('entre d'Etudes de Chimie M&allurgique/ C,¥RS, 15 Rue G. L/rham, 1<94407 [,'itt3"/ Seine (edev. t'Wance
J.M. P E L L E T I E R and S. B O N N E T - J O B E Z ('ALFETMAT, Biitimenl 4(13, INSA, 1:-69621 Villeurhanne, t'rance
Received 30 May 1989: accepted for publication 20 June 1989
Based on SIMS analyses, X-ray diffraction paUcrns, SEM/EDX and TEM/EI)X investigations, the inechanisms of la,,,er surface alloying of the Ni coating of an Al-based alloy has been investigated (melting and solidification). The formation of AI ~Ni primary dendrites in the case of low thickness of the initial Ni coating is reported. A double dendritic mode i>, observed in the case of thicker coating, AI3Ni 2 followed b', A1~Ni dendrites. A Si surface segregation has been detected and related to the dendritic solidification mode.
1. Introduction There is considerable interest in the improvemen( of the properties of the near surface region which can determine the p e r f o r m a n c e of a material in m a n y technological applications. The use of high energy beams, such as laser beams, allows melting of a pre-deposited layer on a substrate a n d is k n o w n to improve the surface m e c h a n i c a l / chemical properties relatively to the base matrix (for example: a high melting p o i n t film on a low melting p o i n t substrate [1], c h r o m i u m carbide on an Ni alloy [2]). In this work, we report on the Ni alloying by CO. laser surface t r e a t m e n t of an Al-based alloy. Utilizing S E M / E D X , T E M / E D X in investigations, SIMS profiles, the solidification m e c h a n i s m has been investigated.
* To whom correspondence should be senl. 0169-4332/89/$03.50 :i;, Elsevier Science Publishers B.V. (North-Holland)
2. Experimental procedure 2.1. L a s e r surfiwe t r e a t m e n t
The n o m i n a l c o m p o s i t i o n of the AI based alloy corresponds to ( 4 . 5 - 6 wt%) Si, (2.8 3.8 wt%) Cu. Two thicknesses of an electrolytic layer of Ni have been chosen: 25 /*m (alloy 1), and 50 /tin (alloy 11). The c o n d i t i o n s of the CO, laser surface treatm e n t are: i n c i d e n t power density, Q , = 10 5 W / c m : . interaction time of a b o u t 100 ms. The samples (20 × 60 × 4 m m ~) have been fixed on a t r a n s l a t i o n table a n d moved u n d e r the laser beam at various speeds. The surface oxidation is dim i n i s h e d by a protective gas (argon). 2.2. Characterization techniques
(i) The m a c r o s t r u c t u r e has been investigated by X-ray diffraction using the Co K a wavelength (4 = 0.17889 nm). (ii) The morphological and chemical investigations have been p e r f o r m e d using a S E M - Z E I S S
E. Gaffet et al. / Laser surface alloying of Ni film an Al-based alloy
DSM 950, which is equipped with a Si-Li X-ray detector combined with a Tracor microanalyser. The chemical profile has been also investigated using the SIMS method. (iii) The microstructure has been studied using a TEM-JEOL 2000FX, and microanalyses have been performed at a finer scale (less than a /~m). The TEM foils have been directly prepared by ultramicrotomy [3,4].
249
alloy I, (b) alloy II. The identified phases are: matrix: A1, A12Cu , A12Cu3, C u 4 S i , alloy I: A1, AlaNi , NisSi2, A12Cu3, alloy II: A1, A13Ni 2, A13Ni, Ni3Si, NisSi 2. The X-ray peaks which correspond to the A13Ni ~ are located on the following positions: 21 °, 29.6 o, 57.3 ° (20, XCoKc 0.
3.2. Morphological (SEM/EDX)
and chemical
investigation
3. Experimental results
3.1. Structural investigations Fig. 1 exhibits the typical X-ray patterns which are obtained after the laser surface treatment: (a)
3.2.1. Morphologies (i) The melted area exhibits a dendritic solidification structure, the thickness of the melted area is about 200-250 /~m (see figs. 2 and 3). The dendritic orientation is non-uniform. Convection
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X- Ray intensity (a.u.) X. Ray intensity (au.)
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.
.
.
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.
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Fig. 1. X-ray diffraction pattern: (a) alloy I after laser surface treatment, (b) alloy II after laser surface treatment.
~0
250
E. GafJet et al. / Laser surJktce alloying of Ni film an dl-hased allqv
Fig. 2. SEM micrographs of the transverse cut after laser irradiation of alloy I (backscattered electron image mode).
Fig. 3. SEM micrographs of the transverse cut after laser irradiation of alloy I1 (backscattered electron image mode).
E, Gaffet et a L / Laser surface alloying of Ni film an Al-based alloy
105
251
(iii) The matrix is composed of some A1 grains (dark contrast), surrounded by areas enriched in Si and Cu (bright contrast).
At Si
~10 4 ffl
~io 3
Cu
I--
~ 10 2
Ni
ILl I'--
_ZlO~ lO 0 ~ L ~ Surface . . . . . . . . . . . . . . . . . . . . . . . .
0
Matrix
I
I
I
1
I
1O0
2 (30
300
400
500
600
~.m Fig. 4. Composition profile along a line perpendicular to the surface (SIMS method, incident ions/oxygen, probe diameter
=8~m). movements of the liquid have perturbed the dendritic solidification by modifying the chemical diffusion process. (ii) A thermal affected area is observed at the limit between the substrate and the melted area. This zone has a small thickness of about 10-30 /zm (see figs. 2 and 3).
3.2.2. Chemical distribution (i) SIMS investigations. Fig. 4 shows elemental profiles parallel to the plane of the surface within the probe resolution of about 8/~m. The Ni content appears to be relatively constant in the melted area. Far from this zone, the Ni content diminishes rapidly (three order of magnitude). The Si and Cu have a more homogeneous distribution in the whole melted area. The concentration in A1 is rather uniform in the entire analyzed area. (ii) Some E D X / S E M analyses and EDX maps have been performed in order to study the composition fluctuations at the/~m scale. Typical EDX maps are presented in figs. 5 and 6. The dendritic arms and the interdendritic spaces are enriched in Ni and in (A1, Si, Cu) respectively. The composition of the laser surface is equal to: alloy I: A184.n_+l.8-Si9.8_+0.9-Ni4.5_+lA-CUl.5+0. 3 (A194.9 ± 1.0-Ni5.1 ± 1.o); alloy II: two areas have to be considered: the middle of a single laser exposure (A) and the surface corresponding to the overlap between two successive laser exposures (B). This leads to the
Fig. 5. EDX cartograph of the melted area of alloy I.
252
E. GafJet et al. / Laser surfiwe alloying of Ni film an A l-based alloy
Fig. 6. EDX cartograph of the melted area of alloy lI. 100
100
95 90
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80
!
0
70
i
100
200
!
0
300
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100
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200
300
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0
t
0
100
200
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0
300
Z
~
0
it3
1 O0
200
300
Z
a
b
Fig. 7. Chemical composition as a function of the depth ( E D X / S E M analyses): (a) alloy I, (b) alloy 11. (A) AI, ([23)Ni. 11~) Si.
E. Gaffet et al. / Laser surface alloying of Ni film an Al-based alloy
253
surrounded by the interdendritic spacements. The typical SAD patterns indicates the presence of A13Ni (A), pure A1 (B), A1 and Si (C). The chemical composition microanalyses support such structural information, i.e., the dendrites are exclusively A1/Ni with a composition close to A13Ni and the Ni content inside the interdendritic spacements does not exceed 1.3 at%. The Si repartition is heterogeneous at the scale of 0.25/~m inside the interdendritic areas (areas B and C).
4. Discussion
4.1. Ni layer dissolution During the melting, a dissolution of the Ni layer occurs, accelerated by the convection movement. The measured Ni content inside the melted depth corresponds to the one which may be estimated from a simple mixing rule.
4.2. Solidification mechanism
Fig. 8. TEM micrographs (bright field image). The dark areas correspond to the selected area diffraction pattern and EDX microanalyses.
following compositions: (A) A176.0± 0.8-Si 8.1 ± 0.2-Ni14.5 ± 0.6-CUl.4 ± 0.1 (A184.0 ± 0.7-Nil6.0 ± 0.7);
(B) A180.4± 0.4-Si7.5 _+o.t-Nill.0 ± 0.6-Cut.2 ± 0.1 (A188.0 _+0.6-Ni12.0 ± o.6)" The mean composition is equal to: A177.8-Si8. oNil2.9-Cul.4(A185.8-Ni14.2 ). The composition profile of the alloy is obtained on a polished transverse section of the sample which has been cut perpendicular to the laser beam motion. The results are shown in figs. 7a and 7b. (iv) The microstructures and microanalyses have been investigated by T E M (BF images, SAD patterns, EDX microanalyses). The BF image (fig. 8) reveals the presence of some interdendritic areas
Starting from a mean composition of about 10 at% in Ni (alloy I), the solidification develops primary A13Ni dendritic arms. There is a segregation of A1 and S i / C u inside the interdendritic spaces which later solidifies. In the case of the alloy II with a complete dissolution of the Ni layer, the composition is about 20 at% and therefore is beyond the monotectic composition. The primary arms are A13Ni 2. As the solidification evolves, the remaining liquid is enriched in A1, then the next arm to be formed is A13Ni. Such a solidification mechanism is based on the A1-Ni equilibrium phase diagram (fig. 2) and some remarks have to be noted: (i) The influence of the solute Cu and Si on the solidification process is very weak, the overall process is related to the A1 and Ni relative contents. Taking into account such an observation, we may estimate the cooling rate from the measured interdendritic spacements ( - / ~ m ) and some previous reported results [5]: such a dendritic spacement is obtained for a quenching rate of about 10 4 o C / S .
254
E. Gaffer et al. / Laser surjace alloying of Nt film an A l-based alh)v
(ii) the chosen laser conditions do not lead to the formation of a homogeneous Al Ni supersaturated solid solution (or to the obtainment of an amorphous alloy). The maximum Ni content is 1.3 at% and this has been detected for just one area. Such a result is in agreement with the fact that the relative motion of the target under the laser beam is about 1 c m / s . This means, that under stationary conditions, the maximum velocity of the liquid-crystal interface is equal to this value, which is not so high, compared to the one estimated in the case of the amorphization by laser. Furthermore, some previous results have been reported concerning the metastable A I - N i phase diagram [6] which may be obtained using a pulsed electron beam treatment. Solidification rates as high as 7 m / s are required for the attainment of a single phase, A1Ni, instead of the stable intermetallic compounds such as A13Ni and A13Ni 2 as observed in our experiment. The presence of the intermetallic phase A13Ni 2 (alloy II) may be explained by the A I - N i equilibrium phase diagram. The estimated and the mean measured Ni compositions indicate that the first phase which is expected to be formed - under equilibrium conditions - is the intermetallic compound A13Ni 2. In the case of the thicker layer (50/~m), no complete dissolution may be achieved in some part of the heat affected area leading to some particular solidification feature: equiaxial crystal growth, starting from parts of the crystal Ni film.
4.3. Si surface segregation The increase in Si content (correlated to a decrease in Ni content) just below the surface may be considered a result of the dendritic solidification mode: during the cooling, the dendrites are surrounded by a liquid containing the elements A I - S i - C u which have been segregated during the dendrite growth. At the end of the solidification, so in the depth close to the final surface, this liquid will solidify after the last dendrite tips, this leads to segregation in Si and Cu, over the dendrite tips and just below the free surface.
4.4. Relation between the dendritic solidification mode and surface composition The two areas which are observed in the case of alloy II are also related to the dendritic solidification mode; indeed the dendrites which solidify in the middle of the laser passage grow in stationary conditions, i.e. the interface liquid/crystal speed is equal to the relative speed of the laser, and the direction of the growth is nearly parallel to the surface when the interface reaches the free surface [7]. On the edges of the melted area, the dendrites will grow along a direction which is close to the perpendicular to the surface. Then, chemical analyses taken from the latter region, will integrate dendrites and interdendritic spacements, leading to a mean composition value, corresponding to a balance between the dendritic and the interdendritic compositions. The chemical analyses which are performed on the area where the interdendrites and dendrites are finer and parallel to the surface will increase the importance of the dendrite contribution, leading to a higher value in Ni content, in comparison to the former value. 4.5. Melting point influence of the fusion solidifi'cation mode
The evolution of the grains sizes just at the interface between the melted area (containing Ni) and the matrix may be explained as followed: the temperature which is obtained during the laser surface treatment may not be sufficient to melt the Ni layer, but is sufficient to melt the matrix AI Si Cu. During the solidification, there is a growth of A1 grains, with a rejection of Si, leading to the formation of grains surrounded by Si. The width of the AI grains is smaller than the original ones, according to the relatively high cooling and henceforth by the fact the unmelted grains have undergone an annealing during the laser treatment which leads to an increase of the unmelted grains size as reported in some previous results [3,4].
5. Conclusion
After the determination of the optimized laser treatment parameters, surface alloys have been
E. Gaffet et al. / Laser surface alloying, of Ni film an Al-based alloy
elaborated by a surface melting of a Ni-electrodeposited layer on an Al-based substrate. The main results are: (i) The solidification structure is dendritic with a typical wavelength of about 1 ~m which corresponds to a quenching rate of about 1040 C/s. (ii) The structures of the dendritic arms correspond to some of the intermetallic compounds of the A1-Ni equilibrium phase diagram: A13Ni in the case of the Alloy I, A13Ni z and A13Ni for the alloy II. (iii) The Ni content of the interdendritic areas is less than 1.3 at%. The cooling conditions are not sufficient to obtain a homogeneous A1-Ni supersaturated solid solution. Si and Cu are found in the interdendritic areas. The heterogeneity scale is in the order of a/~m. A Si segregation is observed just below the surface (2 ~m), this corresponds to a decrease in the Ni concentration. An increase in the Ni content is observed in the middle of the laser passage (relatively to the border areas), such an observation may be explained by the solidification mechanism.
Acknowledgements The authors thank the Pechiney society and the Minist+re de la Recherche (MRES) for their finan-
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cial support, Msse Vialle for her technical assistance with laser experiments and G. Deluze and J.L. Pastol ( C E C M / C N R S ) for their help in obtaining the TEM foils prepared by ultramicrotomy and the EDX cartographies respectively. This work has been supported by the Groupement Scientifique/CNRS "Laser de Puissance No. 85" and a research contract M R T No. 88W0983 ( E U R E K A 194).
References [1] K. Takei, S. Fujimori and K, Nagai, J. Appl. Phys. 51 (1980) 2903. [2] J.H.P.C, Megaw, A.S. Bransden, D.N.H. Trafford and T. Bell, in: Proc. 3rd Intern. Colloq. on Welding and Melting by Electrons and LASER Beam, Lyon, 5-9 September 1983, France. [3] E. Gaffer, G. Deluze, G. Martin, J.M. Pelletier and D. Pergue, Mater. Sci. Eng. 98 (1988) 291, [4] E. Gaffer, Thesis, Doctorat Universit6 Paris V! (1988), [5] H. Matiya, B.C. Giessen and N.J. Grant, J. Inst. Metals 96 (1968) 30. [6] D.M. Follstaedt and S.T. Picraux, in: Alloy Phase Diagrams, Eds. L.H. Bennett, B.C. Giessen and T.B. Massalski (Material Research Society, Pittsburgh, 1988) p. 94. [7] W. Kurz and D.J. Fisher, Fundamentals of Solidification (Trans Tech Publications Ltd, Switzerland, 1986).