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Epitaxial growth and refined substructures of pure iron, nickel and their alloys after laser treatment
Volume 7. number 12
March 1989
MATERIALS LETTERS
EPITAXIAL GROWTH AND REFINED SUBSTRUCTURES OF PURE IRON, NICKEL AND THEIR ALLOYS AFTER LASER TREATMENT M. JIANG, Insiitutc
J.S. ZHANG,
qf.Melal
X.P. JIANG,
Research. Academra
Sinica,
J.G. HUANG,
110015 Shenyang,
X.F. SUN, Y.L. GE and Z.Q. HU
Chrna
Received 3 I October 1988: in final form 6 January 1989
Pure iron. nickel and their alloys have been treated by a pulsed laser. The results show that epitaxial grains have been formed in the molten zones. but in the alloys, there exist refined dendrites, Fine subgrains composed of dislocation cells or tangles have been formed both in pure metals and their alloys. Our results also show that in the rapidly solidified regions, the epitaxial grains have not been relined. but the dendritic structures and subgrains have been refined obviously. whtch are related to the mcdilication of the surface properties.
1. Introduction Laser irradiation techniques such as glazing, cladding, alloying, etc., can obviously modify the surface microstructures of metallic materials [ l-3 ] and markedly improve the wear, corrosion and oxidation resistant properties [ 4,5 1. Many researches show that the improvement of the surface properties by laser rapid solidification processing is related to the refined microstructures in the molten zone [ 6-81. Up to now, the refining mechanism of the laser treatment has been rarely investigated systematically. In our present work, we have studied the microstructures in the laser molten zone and the refining mechanism of the substructure is discussed.
2. Experimental
procedure
Pure metals (iron and nickel) and their alloys (20# steel and a nickel-base superalloy) were selected as the test materials. Samples are 20 x 20 x 5 mm in size. Laser melting treatment was conducted on a Nd glass solid pulsed laser apparatus with energy input of 80100 J/pulse, 3-8 ms pulse duration, 1.0-1.5 mm beam spot diameter and 0.2-0.4 mm melt depth. The cooling rate was estimated to be 2 105’ C/s. The laser treated samples were ground, polished mechanically and etched for the metallographical observation. 0167-577x/89/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
Samples for SEM examination were prepared by deep etching. The thin foils for TEM observation were first ground to about 30 urn and then perforated on a MPT-1 type jetting equipment. The microstructures were examined by optical and SEM techniques and the substructures were studied by a Philips-420 transmission electron microscope.
3. Results and discussion 3.1. Microstructures in the molten zones qf pure metals To simplify the subject and avoid the effect of constitutional supercooling in alloy, we first studied the microstructures in the laser glazed zones of pure iron and nickel. as shown in fig. 1. The original equiaxed
a
_.:.
-
Fig. I. Metallographs of the molten zones tn Iron (a) and mckel (bl. The length of the marker is 100 urn.
B.V.
449
Volume 7, number
12
MATERIALS
grain structures are ferrite (a-Fe) in iron and austenite (y) with some twins in nickel respectively. After laser radiation, new grains grow epitaxially along the heat flow direction, e.g. the grains nucleate heterogeneously at the molten zone boundary within the original grains and grow along the heat flow direction. In iron, there exists a heat affecting zone in which incomplete a-y allomorphous transition takes place. The epitaxial growth begins from this heat affecting zone. In nickel, there is no such an obvious heat affecting zone and the elongated epitaxial grains grow almost directly from the original grains. For those grains whose orientations are not along the preferential heat flow direction, their growth has been restrained. It must be pointed out that the grains formed in the molten zones have not been refined obviously, while the TEM observation (figs. 2 and 3) shows that the subgrains have been markedly refined. In the original structures, there exist a bit of dislocations in iron (fig. 2a) and some dislocation networks and twins in nickel (fig. 3a). After laser radiation, fine subgrains have been formed by dense cellular or tangled dislocations (figs. 2b and 3b). There is no constitutional supercooling in the solidification of pure metals, so there is no dendrite formed in the molten zone. During the laser melting
Fig. 2. TEM observation of the microstructures in iron. (a) Original structure; (b) subgrains formed in the molten zone. The length of the marker is 0.5 pm.
b
March
1989
process, strong heterogeneous nucleation still takes place even at such a high solidification rate because of the low interface energy and the lack of thermal resistance at the interface between molten zone and substrate. This characteristic can explain the obvious epitaxial growth behavior in the molten zone. The grain size in the molten zone is mainly determined by the original grains, e.g. the grains formed in the molten zone are not refined obviously. As for the fined subgrains, they are formed by the thermal stress caused by the rapid heating and cooling in the molten zone. These line substructures will be helpful to the modification of the surface properties of metals. 3.2. Microstructures in the laser molten zones in alloys Fig. 4 shows the metallographs of a steel and a nickel-base alloy treated by laser processing. The original structures are hypoeutectoid pearlite and ferrite in the steel and y’ precipitate in y matrix and y/y’ eutectics in the nickel-base alloy respectively. In the laser molten zones, typical epitaxial growth can be seen. As in the pure metals, there is no obvious grain refinement. A specific feature in this case is that very tine dendrites have been formed in the elongated epitaxial grains. TEM observation shows that there exist fine substructures in the molten zone (figs. 5 and 6). In the steel, the very tine dendrites are composed of lath martensites (figs. Sa and 5b) which are the subgrains with low angle grain boundaries. Within the lathes, there are dense dislocation tangles (fig. 5b) and some thin twins (fig. 5~). The lath martensites are formed according to the following mechanism: During rapid cooling after laser radiation, carbon in the pearlite
,-
Fig. 3. TEM observation of the microstructures in nickel. (a) Original structure; (b) subgrains formed in the molten zone. The length of the marker is 0. I pm.
450
LETTERS
Fig. 4. Metallographs of the laser molten zones in 20# steel (a) and nickel-base alloy (b). The length of the marker is 100 pm in (a)and20umin (b).
Volume 7. number
12
Fig. 5. TEM observation of the microstructures sites: (c) the twins formed in the martensites.
MATERIALS
LETTERS
March
1989
in 20# steel. (a) The lath martensites: (b) dislocation substructures within the martenThe length ofthe marker is 0.5 urn in (a). 0. I urn in (b) and I urn m (c).
vious grain refinement in the molten zone. The formation of dendrites is caused by the constitutional supercooling during the alloy solidification process. Under rapid solidification conditions, diffusion of the solute atoms is restrained to some extent so that very line dendrites with arm spacing (about 2 urn) have been formed. Like pure metals, very line and dense substructures or subgrains have been formed in the alloys after laser radiation but the dislocation configuration is more complicated in this case. These fine substructures will be beneficial to the strengthening of the materials. 4. Conclusion Fig. 6. TEM observation of the microstructures in nickel-base alloy. (a) Subgrains formed by dislocation tangles; (b) pile-up dislocation; (c) dislocation pairs formed in the matrix; (d) diffraction pattern of the molten zone. The length of the marker is 0.1 urn in (a) and (b) and0.5 urn in (c).
colonies has no time to diffuse for a long distance so that martensite forms in the local carbon-rich regions. The formation of fine martensites, high density of dislocations and thin twins will increase the deformation resistance of the steel. In the nickel-base alloy, there also exist a lot of fine subgrains within dendrites. These subgrains are also formed by dense dislocation tangles (fig. 6a), and sometimes some pile-up dislocations have been observed (fig. 6b). Electron diffraction analysis proves that the structure of the molten zone is austenite (y) with some y’ precipitates indicated by the superlattice diffraction (fig. 6d). Accordingly, some dislocation pairs in the alloy matrix have been observed (fig. 6~). From the above results, it can be seen that because of the strong epitaxial growth feature, there is no ob-
There is a strong tendency of the epitaxial grain growth in the laser glazed zones in pure iron and nickel as well as their alloys. These grains nucleate heterogeneously at the boundaries between the original grains and the molten zones and grow along the heat flow direction, but no obvious grain refinement has been obtained in the molten zones. Only the substructures such as dendrites and subgrains have been markedly relined, which is related to the modilication of the surface properties. The subgrains are formed by dense dislocation tangles which are formed by thermal stress during rapid melting and cooling, while the very line dendrites are formed by constitutional supercooling and high solidification rate in the molten zones. References
[ 1I B.H. Kear and C.M. Banas, Phys. Today 29, no. I I ( 1976) p. 44. [21 T.R. Anthony
and H.E. Cline. J. Appl. Phys. 48 (1977)
3888.
4.51
Volume 7, number
12
MATERIALS
[3] B.H. Kear, E.M. Breinan and L.E. Greenwald, Met. Technol. NY 6 (1979) 4. [4] C.C. Irons, Weld. J. (Dec. 1978) pp. 29-32. [ 51 B.M. Astashkevich, S.S. Voinov and E.A. Shur, Metalloved. Term. Obrab. Met. 4 (1985) 12. [6] Z.Q. Hu, Y.L. Ge, M. Jiang and C.X. Shih, Proceedings of the National Conference on Laser Heat Treatment ( 1986)
pp. 1-5.
452
LETTERS
March
1989
[ 7 ] Y.L. Ge, Z.Q. Hu, W. Gao and C.X. Shi, Acta Metall. Sinica 20A ( 1984) 7 1, in Chinese. 181 Y.L. Ge, Q. An, Z.Q. Hu and C.X. Shi, J. Chinese Prot. 4 ( 1984) 2 18, in Chinese.
Corros.
March 1989
MATERIALS LETTERS
EPITAXIAL GROWTH AND REFINED SUBSTRUCTURES OF PURE IRON, NICKEL AND THEIR ALLOYS AFTER LASER TREATMENT M. JIANG, Insiitutc
J.S. ZHANG,
qf.Melal
X.P. JIANG,
Research. Academra
Sinica,
J.G. HUANG,
110015 Shenyang,
X.F. SUN, Y.L. GE and Z.Q. HU
Chrna
Received 3 I October 1988: in final form 6 January 1989
Pure iron. nickel and their alloys have been treated by a pulsed laser. The results show that epitaxial grains have been formed in the molten zones. but in the alloys, there exist refined dendrites, Fine subgrains composed of dislocation cells or tangles have been formed both in pure metals and their alloys. Our results also show that in the rapidly solidified regions, the epitaxial grains have not been relined. but the dendritic structures and subgrains have been refined obviously. whtch are related to the mcdilication of the surface properties.
1. Introduction Laser irradiation techniques such as glazing, cladding, alloying, etc., can obviously modify the surface microstructures of metallic materials [ l-3 ] and markedly improve the wear, corrosion and oxidation resistant properties [ 4,5 1. Many researches show that the improvement of the surface properties by laser rapid solidification processing is related to the refined microstructures in the molten zone [ 6-81. Up to now, the refining mechanism of the laser treatment has been rarely investigated systematically. In our present work, we have studied the microstructures in the laser molten zone and the refining mechanism of the substructure is discussed.
2. Experimental
procedure
Pure metals (iron and nickel) and their alloys (20# steel and a nickel-base superalloy) were selected as the test materials. Samples are 20 x 20 x 5 mm in size. Laser melting treatment was conducted on a Nd glass solid pulsed laser apparatus with energy input of 80100 J/pulse, 3-8 ms pulse duration, 1.0-1.5 mm beam spot diameter and 0.2-0.4 mm melt depth. The cooling rate was estimated to be 2 105’ C/s. The laser treated samples were ground, polished mechanically and etched for the metallographical observation. 0167-577x/89/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
Samples for SEM examination were prepared by deep etching. The thin foils for TEM observation were first ground to about 30 urn and then perforated on a MPT-1 type jetting equipment. The microstructures were examined by optical and SEM techniques and the substructures were studied by a Philips-420 transmission electron microscope.
3. Results and discussion 3.1. Microstructures in the molten zones qf pure metals To simplify the subject and avoid the effect of constitutional supercooling in alloy, we first studied the microstructures in the laser glazed zones of pure iron and nickel. as shown in fig. 1. The original equiaxed
a
_.:.
-
Fig. I. Metallographs of the molten zones tn Iron (a) and mckel (bl. The length of the marker is 100 urn.
B.V.
449
Volume 7, number
12
MATERIALS
grain structures are ferrite (a-Fe) in iron and austenite (y) with some twins in nickel respectively. After laser radiation, new grains grow epitaxially along the heat flow direction, e.g. the grains nucleate heterogeneously at the molten zone boundary within the original grains and grow along the heat flow direction. In iron, there exists a heat affecting zone in which incomplete a-y allomorphous transition takes place. The epitaxial growth begins from this heat affecting zone. In nickel, there is no such an obvious heat affecting zone and the elongated epitaxial grains grow almost directly from the original grains. For those grains whose orientations are not along the preferential heat flow direction, their growth has been restrained. It must be pointed out that the grains formed in the molten zones have not been refined obviously, while the TEM observation (figs. 2 and 3) shows that the subgrains have been markedly refined. In the original structures, there exist a bit of dislocations in iron (fig. 2a) and some dislocation networks and twins in nickel (fig. 3a). After laser radiation, fine subgrains have been formed by dense cellular or tangled dislocations (figs. 2b and 3b). There is no constitutional supercooling in the solidification of pure metals, so there is no dendrite formed in the molten zone. During the laser melting
Fig. 2. TEM observation of the microstructures in iron. (a) Original structure; (b) subgrains formed in the molten zone. The length of the marker is 0.5 pm.
b
March
1989
process, strong heterogeneous nucleation still takes place even at such a high solidification rate because of the low interface energy and the lack of thermal resistance at the interface between molten zone and substrate. This characteristic can explain the obvious epitaxial growth behavior in the molten zone. The grain size in the molten zone is mainly determined by the original grains, e.g. the grains formed in the molten zone are not refined obviously. As for the fined subgrains, they are formed by the thermal stress caused by the rapid heating and cooling in the molten zone. These line substructures will be helpful to the modification of the surface properties of metals. 3.2. Microstructures in the laser molten zones in alloys Fig. 4 shows the metallographs of a steel and a nickel-base alloy treated by laser processing. The original structures are hypoeutectoid pearlite and ferrite in the steel and y’ precipitate in y matrix and y/y’ eutectics in the nickel-base alloy respectively. In the laser molten zones, typical epitaxial growth can be seen. As in the pure metals, there is no obvious grain refinement. A specific feature in this case is that very tine dendrites have been formed in the elongated epitaxial grains. TEM observation shows that there exist fine substructures in the molten zone (figs. 5 and 6). In the steel, the very tine dendrites are composed of lath martensites (figs. Sa and 5b) which are the subgrains with low angle grain boundaries. Within the lathes, there are dense dislocation tangles (fig. 5b) and some thin twins (fig. 5~). The lath martensites are formed according to the following mechanism: During rapid cooling after laser radiation, carbon in the pearlite
,-
Fig. 3. TEM observation of the microstructures in nickel. (a) Original structure; (b) subgrains formed in the molten zone. The length of the marker is 0. I pm.
450
LETTERS
Fig. 4. Metallographs of the laser molten zones in 20# steel (a) and nickel-base alloy (b). The length of the marker is 100 pm in (a)and20umin (b).
Volume 7. number
12
Fig. 5. TEM observation of the microstructures sites: (c) the twins formed in the martensites.
MATERIALS
LETTERS
March
1989
in 20# steel. (a) The lath martensites: (b) dislocation substructures within the martenThe length ofthe marker is 0.5 urn in (a). 0. I urn in (b) and I urn m (c).
vious grain refinement in the molten zone. The formation of dendrites is caused by the constitutional supercooling during the alloy solidification process. Under rapid solidification conditions, diffusion of the solute atoms is restrained to some extent so that very line dendrites with arm spacing (about 2 urn) have been formed. Like pure metals, very line and dense substructures or subgrains have been formed in the alloys after laser radiation but the dislocation configuration is more complicated in this case. These fine substructures will be beneficial to the strengthening of the materials. 4. Conclusion Fig. 6. TEM observation of the microstructures in nickel-base alloy. (a) Subgrains formed by dislocation tangles; (b) pile-up dislocation; (c) dislocation pairs formed in the matrix; (d) diffraction pattern of the molten zone. The length of the marker is 0.1 urn in (a) and (b) and0.5 urn in (c).
colonies has no time to diffuse for a long distance so that martensite forms in the local carbon-rich regions. The formation of fine martensites, high density of dislocations and thin twins will increase the deformation resistance of the steel. In the nickel-base alloy, there also exist a lot of fine subgrains within dendrites. These subgrains are also formed by dense dislocation tangles (fig. 6a), and sometimes some pile-up dislocations have been observed (fig. 6b). Electron diffraction analysis proves that the structure of the molten zone is austenite (y) with some y’ precipitates indicated by the superlattice diffraction (fig. 6d). Accordingly, some dislocation pairs in the alloy matrix have been observed (fig. 6~). From the above results, it can be seen that because of the strong epitaxial growth feature, there is no ob-
There is a strong tendency of the epitaxial grain growth in the laser glazed zones in pure iron and nickel as well as their alloys. These grains nucleate heterogeneously at the boundaries between the original grains and the molten zones and grow along the heat flow direction, but no obvious grain refinement has been obtained in the molten zones. Only the substructures such as dendrites and subgrains have been markedly relined, which is related to the modilication of the surface properties. The subgrains are formed by dense dislocation tangles which are formed by thermal stress during rapid melting and cooling, while the very line dendrites are formed by constitutional supercooling and high solidification rate in the molten zones. References
[ 1I B.H. Kear and C.M. Banas, Phys. Today 29, no. I I ( 1976) p. 44. [21 T.R. Anthony
and H.E. Cline. J. Appl. Phys. 48 (1977)
3888.
4.51
Volume 7, number
12
MATERIALS
[3] B.H. Kear, E.M. Breinan and L.E. Greenwald, Met. Technol. NY 6 (1979) 4. [4] C.C. Irons, Weld. J. (Dec. 1978) pp. 29-32. [ 51 B.M. Astashkevich, S.S. Voinov and E.A. Shur, Metalloved. Term. Obrab. Met. 4 (1985) 12. [6] Z.Q. Hu, Y.L. Ge, M. Jiang and C.X. Shih, Proceedings of the National Conference on Laser Heat Treatment ( 1986)
pp. 1-5.
452
LETTERS
March
1989
[ 7 ] Y.L. Ge, Z.Q. Hu, W. Gao and C.X. Shi, Acta Metall. Sinica 20A ( 1984) 7 1, in Chinese. 181 Y.L. Ge, Q. An, Z.Q. Hu and C.X. Shi, J. Chinese Prot. 4 ( 1984) 2 18, in Chinese.
Corros.