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Microstructural investigations of iron implanted with nitrogen ions at the temperature of liquid nitrogen
316
Nuclear Instruments and Methods in Physics Research B23 (1987) 316-322 North-Helled, Amsterdam
MICROSTRUCTURAL INVESTIGATIONS OF IRON IMPLANTED WITH NITROGEN IONS AT THE TEMPERATURE OF LIQUID NITROGEN Part I: Imphntation b the dose range 1 X 1O’6 to 1 X lOI7 N +-ionsfern B. RAUSCHENBACH
I), K. HOHMUTH
‘) and G. tiSTNER
2,
‘) Akademie der Wissenschojten der DDR, Zentrafinstitut ftir Kernforschung Rossendorf, 8OSI Dresden, PF 19, CDR zi Institut fGr ~estk~r~erph~~.s~k und ~~ektronenm~krosko~~~, 402 Haile. ~ei~ber~eg
2, PF 250, GDR
Received 9 September 1986 and in revised form 17 November 1986
The microstructure of iron implanted with nitrogen ions to fluences between 1 X 10" and 1~10’~ NC-ions/cm2 at the temperature of liquid nitrogen (- 796O C) have been investigated by high-voltage electron microscopy and transmission high-energy electron diffraction. Substitutional nitrogen-atom clusters are formed after implantation with nitrogen ions between 1 XlO”e and 5-7~10’~ Ni-ions/cm2, where the nitrogen atoms on the octahedral sites stabilii the b&structure. These clusters can be considered as a pre-pr~ipitatio~ stage prior to formation of other bet-phases. A molecular solid nitrogen phase (/3-N,) was found above an implantation dose of 4-5 x 10n’ N+-ions/cm*. The implantation-induced microstructure after nitrogen ion implantation at the temperature of hquid nitrogen differs from that after implantation at room temperature.
It is well established that ~~-~uen~e ion implantation improves the wear resistance, the fatigue lifetime and corrosion resistance of pure metals and alloys. The tribological and anticorrosion properties are mainly determined by the formation of compounds and their interaction with lattice defects. The understanding of these fundamental mechanisms is essential to the effective use of high-fluence ion implantation for applications. Microst~ctural investigations of iron and steels after nitrogen ion implantation at room temperature and post-annealing have been the object of many experimental studies. In practice, temperature effects during implantation are very important. Freliminary studies have shown that nitrogen implantation in steels [l-S] and iron [1,6] at higher temperatures (loo-350 ’ C) and ion current densities (- 50-200 p A/cm2) respectively lead to implantation profiles and to impl~tation-induced microstructures, which differ from those of implantation at room temperature (RT) and post-annealing. No investigations into the influence of nitrogen implantation at low temperatures f < room temperature) on the microstructure of iron, steels or Fe-based alloys are known. In the present experiments, we have implanted nitrogen ions into pure iron at liquid nitrogen temperature (LNT) and have investigated the nitrogen distribution and microstructure with respect to the implantation dose. In this paper we present the results ~I68-583X/g7/$03.50 0 Flsevier Science Publishers (North-Holl~d Physics Publishing Division)
B.V.
after implantation up to a dose of 1 X lOI7 N+ions/cm=. In a second part, we have reported our results after implantation with higher doses f 2 7. x 10” N ‘+-ions/cm2) [7].
Polycrystalline layers of iron were deposited on freshly cleaved NaCl, KC1 or silicon in a high vacuum of 10m4 Pa. The thickness of the iron layers varied between 100 nm and 1 pm. These thin films were implanted with nitrogen ions (vacuum 10e5 Pa) with energies from 30 to 60 keV in the fluence range 1 X 10t6-1 X 1017 N+-ions/cm’. A cooiing system in the isotope separator chamber was used in order to implant at the temperature of liquid nitrogen (- 196’ C) or at higher temperatures [6]. The carbon concentration after nitrogen ion implantation at LNT is very high. Measurements using Auger electron spectroscopy have shown that in the near-surface region (= up to 20 nm) the carbon concentration is about 30-35 at.% and that the carbon concentration is higher than the concentration of the implanted nitrogen in this region [6]. The temperature during implantation was measured with a thermocouple in contact with the sample. The crystallographic structure and mo~hology of the iron layers before and after the implantation were investigated by high voltage electron microscopy (HVEM) and transmission high-energy electron diffrac-
B. Rauschenhuch et al. / Microstructure
tion (THEED). Identification of implantation-induced phases took place on the basis of calibrated selected area diffraction diagrams. The calibration of SAD-patterns is generally straightforward but can be facilitated by using and internal standard (unimplanted iron). Computer programs are used to calculate the interplanar spacings and intensities, and to design the synthetic electron diffraction spectra (for comparison with the experimental determined spectra). Calculations are based on application of sphne functions and a leastsquares anaIysis (for details see ref. [8]). It is possible to determine the positions and intensities of the overlapped electron diffraction peaks with this method. The goodness-of-fit of the calculated synthetic spectra to the observed diffraction spectra is better than 9.5%. (for discussion see refs. 171 and [g]).
3. Results and discussion Recently we reported on the analysis of iron nitride phases after nitrogen ion im~I~tation in the fluenee range From t X 1016 to 1 X 10’s ions/cm2 at room temperature [Y-11]. The occurrence of the iron nitride phases and solid solutions y-austenite, a’-martensite, r-Fe,N, _--5 and a” - Fe,,Nz was shown to be fluence dependent. Fig. 1 shows a large bright field micrograph of u~~l~t~ iron. The bee-reffections of unimplanted iron are used for calibration of the implanted iron. In table 1 are summarized the lattice parameters and some crystallographical data of the analysed iron nitride phases. The interaction of the iron surface with oxygen forms a thin magnetite layer (Fe&, a = 0.8397 nm). Electron diffraction reflections of other iron oxides (ruiistite, haematite) were not observed. 3.1. Implantation cm2
fluence: I X10’” to 5~ IO’” N “-ions/
Fig. 1 also shows micrographs of iron implanted with 1 x 10t6, 2.5 x lOi6 and 5 X lOI6 NC-ions/cm’ at LNT. In contrast to the unimplanted iron, the micrographs are characterized by a pronounced contrast of the habit of crystallites. Depending on the implantation
of nitrogen
317
fluence a marked morphological difference can be observed. A recent investigation has confirmed the existence of nitrogen-atom clusters. Ferguson and Jack [12) have shown that quench-ageing of supersaturated nitrogen-ferrite leads to the formation of three-dimensional arrays of nitrogen-atom clusters as platelets. These clusters can be considered as arrangements of nitrogen atoms randomly occupying octahedral interstices in the ferrite matrix. In the present ease, fig. I shows rod-like and disc-like nitrogen-atom clusters after implantation. THIZD investigations have confirmed that the implantation at LNT induces N-atom clusters. Fig. 2 shows an electron diffraction pattern with indexed photodensitometer plot of iron after nitrogen ion implantation of 1 X lOi ions/cm2 at LNT. In addition to the reflectians of iron, bet N-atom clusters were identified. In contrast to implantation at RT, we did not find y-austenite [9,10]. The lattice positions of implanted nitrogen in iron were studied by Swanson et al. [13]. They have measured that the majority of implanted nitrogen atoms occupy octahedral or near-octahedral sites after im~l~tation at 35 K. It is known that the octahedral intersitial nitrogen stabilizes the bet structure of iron [14]. Fig. 3 shows the evolution of the N-atom clusters as a function of the implantation fluence (nitrogen concentration). The electron scattering intensity is plotted as a function of the electron scattering angle parameter s (s = 47rsin v/he, v = Bragg angle, h, = relativistic wavelength of electrons) for selected peaks after background correction and smoothing (for details see ref. [S]). The size of the N-atom clusters (see also Fig. 1) increases with increasing implantation Iluence. After implantation with 5 X 10 I6 N’-ions/cm’ the electron reflections (lOI), (002) and (020) of the N-atom cluster precipitates have a higher intensity than in the case of implantation with 1 X 1016 Nf-ions/cm2 (fig. 3). Fig. 3 shows also that implantation of 1 x 1Or7 N”-ions/cm2 does not lead to formation of N-atom clusters. A homogeneous dispersion of nitrogen-atom clusters could be found in the fluence range between 1 x lOI and 5-7 X lOI N’-ions/cm*, These clusters are very unstable against thermal treatment [15]. The chemical state of nitrogen implanted iron at different temperatures was
Table 1 Crystallographicaldata of the analysed crystal phases and solid solutions and their lattice parameters. Bravais lattice
Lattice p‘wameter [nm]
Space group
No. of space group
a-Fe, ferrite N-atom duster
bee bet
Im3m 14/mmm
229 139
molecular &N,
cubic
u = 0.28664 If = 0.28644- O.wo1X a> c/a = I i 0.0091 a) c1= 0.61641
Pm3m
221
Phase
‘) At.% N.
Refs.
318
B. Rauschenbach
qt al. / Microstructure
of nitrogen
Fig. 1. HVEM bright field micrographsof unimplantediron and iron implanted with 50 keV nitrogen ions to a fluence of 1 XlOl”, 2.5 x lOI and 5 x 1Oz6ions/cm’ at LNT.
analysed by using the X-ray photoelectron spectroscopy 1151. The XPS-spectrum for N,, (at a depth = the mean projected range of the nitrogen ions) shows two peaks at 398.2 eV and 405.5 eV after impl~tation at LNT and two peaks at 395.0 eV and 399.6 eV after implantation at 300°C. The peak at 403.5 eV indicates formation of solid nitrogen while the signal at 398.2 eV and 395.0 eV corresponding to nitrides and the peak at 399.0 eV corresponding to free or weakly bonded nitrogen (for discussion see ref. [15]). This means that the solid nitrogen state is unstable against thermal treatment and that only a fraction of the implanted nitrogen is in the nitride bonding state.
Ferguson and Jack [12] have found that the tetragonality continually increases due to cluster formation and that N-atom clusters contain 5 0.3-0.4 nitrogen atoms per 100 iron atoms. A linear relationship exists between the lattice parameters a and c respectively and the concentration of nitrogen [12,16,17]. Starting from these linear relations, it is possible to derive the nitrogen content with the aid of experimentally determined interplanar spacings. This approach is demonstrated for iron after nitrogen ion impl~tation at room temperature [lO,ll]. ‘The measured interplanar spacings dent, depend on uexp and cexp. Using a special computer program (see refs. [lO,ll]) the values of dexp were fitted
B. Rauschenbach
et al. / Mtcrostructure
of nttrogen
319
-
a-t210
-
alwl-Iml
-
a-l!nol
~
SCATTERINGANGLE PARAMETER Fig.
2.
s fnm-‘1
Transmission electron diffraction pattern (SAD) of iron implanted with nitrogen ions to a fluence of 1 X
10’”
ions/cm’
at
LNT.
to the theoretical values of d,,,_,_ This calculation gives the values of a_, and c,_,. It was estimated, that the concentration varies between 1 and 3 N-atoms per 100 iron atoms depending on the implantation fluence. The formation of N-atom clusters can be considered as a pre-precipitation stage prior to formation of the bet metastable phases a’-martensite and a”-Fe16N2 [7].
Fig. 4 shows a low-magnification micrograph of iron after implantation with 7.5 x 10’” N+-ions/cm* at LNT. Large areas of the iron sample exhibit a streak-like structure. The observed THEED-patterns (fig. 4), can be interpreted as originating from condensed nitrogen gas. The diffraction patterns indicate that this solid nitrogen is the molecular /3-N, phase (see table 1). The solid B-N, phase has a molecular lattice of the Pm3m
INTERPLANAR
SPACING
dexp
[nm]
Fig. 3. Details of microphotometer curves of the strongest electron diffraction reflections after background subtraction and smoothing for iron implanted with 1 X 10i6, 5 x 1016 and 1 x 10” N ‘-ions/cm2 at LNT.
B. Raurchenbuch
320
et ul. / Microstructure
of nitrogen
PLOT
E-SOkeV
Fig. 4. Bright
field micrograph
(BF), selected
area diffraction (SAD) and indexed 7.5 X 1016 N+-ions/cm* at LNT.
space group (8 molecules per unit cell) [18]. The centers of the nitrogen molecules are arranged in such a way that they are shifted slightly from the primitive cubic
Fig. 5. HVEM bright
field micrograph
photometer
plot (PLOT)
of iron implanted
with
positions. The high-pressure behaviour of solid nitrogen appears to be quite complex. However, the molecular character persists for a pressure below 10 GPa (studies
of iron implanted
with 1 X 10”
N f-ions/cm*
at LNT.
B. Rauxhenbach et al. / Microstructure of nitrogen
321
Table 2 Summary of the most frequent experimentally determined interplanar spacings dexp and intensities I_, of iron implanted with 7.5 x 10’6 and 2.5 x 10” N+-ions/cm’. The calculated values of dtheo and Itheo are compared with the experimentally determined values of molecular N-atom clusters. Miller
Theoretical interplanar spacing
indices
d theo [MI] and intensity Ithe,,
Measured interplanar spacing & [nm] and intensity lexp for two fluences
d thee
(7.5 X lOi6 N+/cm’)
h
k
I
2 0 0 2 1 0 2 1 1 2 2 2 4 0 0
0.30821 0.27567 0.25165 0.17794 0.15410
I thee
88.6 100.0 75.4 6.5 9.5
(2.5 X 10” N+/cm’)
d ev
I exP
d exP
I
0.305 0.270 0.250 0.177 0.154
75 100 60 10 10
0.307 0.275 0.253 0.177 0.154
90 100 70 10 15
with a diamond anvil cell) [19,20]. Table 2 shows a comparison of experimentally and theoretically determined values of the interplanar spacings and intensities. The reflections of molecular p-N2 are clearly separated from the reflections of iron. A hexagonal structure has been suggested for solid nitrogen [19], but such lattice structure has not been observed. The molecular solid /3-N, is formed between 4-5 x 1016 and 2-6 x 10” N+-ions/cm* after implantation at LNT [7]. Fig. 5 shows a high-magnification micrograph of iron after nitrogen implantation to a fluence of 1 X 10” N+-ions/cm* at LNT. In addition to the streak-like structure, particles with a sphere-like morphology can be observed. The higher stress and the higher nitrogen concentration in the high fluence implanted layer might lead to another morphology. It can be assumed that the formation of solid nitrogen is possible in connection with a low temperature during implantation and a high lateral stress induced by implantation. Hartley [21] and Eer Nisse and Picraux [22] have demonstrated that the internal stress in pure metals and steel after implantation is very high (= theoretical yield stress) and after noble gas implantation it is connected with the formation of blisters. Recently, Madakson [23] reported that the implantation-induced stress in aluminium increases linearly with fluence up to about 5 x 1016 N+-ions/cm* and while it relieved at higher fluences. In th present case, it is assumed that a high stress (= 1 GPa) can explain the phenomenon of the stability of molecular nitrogen in iron. Recently, solid noble gas precipitates were detected after noble gas implantation [24]. These precipitations are stable for pressures 2 1 GPa. This result is in good agreement with our results. The high lateral stress during implantation stabilizes the solid p-N2 phase (minimization of the precipitation energy), and the low temperature promotes this transformation. Presumably, both factors are necessary because a solid nitrogen phase does not occur after implantation at RT.
exP
4. Summary The formation of phases and solid solutions of iron after implantation with nitrogen ions at LNT have been investigated by high-voltage electron microscopy and transmission high-energy electron diffraction. Some important results are summarized as follows: (1) Substitutional nitrogen-atom clusters are formed in the fluence range between 1 x 1016 and 7 x 1016 N+-ions/cm*, where about l-3 nitrogen atoms per 100 iron atoms on octahedral sites stabilize the bet structure. (2) Formation of the N-atom clusters can be considered as pre-precpitation stage prior to the formation of other bet phases. is found above an im(3) Molecular solid @itrogen plantation fluence of 5 X 1016 N+-ions/cm*. Presumably, the implantation-induced stress and the low temperature during the implantation are necessary conditions for this phase to be stable. In the second part of this paper [7], the microstructure of iron and the distribution of nitrogen after implantation at LNT (fluence >_1 X 10” N+-ions/cm’) are presented and discussed. The authors would like to thank Dr. J. Schoneich and Mr. J. Altmann for performing the implantations.
References
VI P.D. Goode and I.J.R. Baumvol, Nucl. Instr. and Meth. 189 (1981) 161. 121 E. Ramous, G. Principi, L. Giodano, S. LoRusso and C. Tosello, Thin Solid Films 102 (1983) 97. 131 N. Moncoffre, G. Hollinger, H. Jaffrezic, G. Marest and J. Tousset, Nucl. Instr. and Meth. B7/8 (1985) 177. 141 Th. Bamavon, H. Jaffrezic, G. Marest, N. Moncoffre, J. Tousset and S. Fayeulle, Mater. Sci. Eng. 69 (1985) 531.
322
B. Ruuschenbuch
et ul. / Mmmtructure
[S] N. Moncoffre, G. Marest, S. Hiadsi and J. Tousset, Nucl. Instr. and Meth. B15 (1986) 620. [6] B. Rauschenbach, Nucl. Instr. and Meth. B15 (1986) 756. [7] B. Rauschenbach, Nucl. Instr. and Meth. B18 (1987) 34. [8] B. Rauschenbach, J. Mater. Sci. 21 (1986) 395. [9] K. Hohmuth, B. Rauschenbach, A. Kolitsch and E. Richter, Nucl. Instr. and Meth. 209/210 (1983) 249. [lo] B. Rauschenbach and A. Kolitsch, Phys. Status Solidi A80 (1983) 211. [ll] B. Rauschenbach, A. Kohtsch and K. Hohmuth, Phys. Status Solidi A80 (1983) 471. [12] P. Ferguson and K.H. Jack, Philos. Mag. A50 (1984) 221 and A52 (1985) 509. [13] M.L. Swanson, L.M. Howe, J.A. Jackman, T.E. Jackman, K. Griffiths and A.F. Quenneville, Nucl. Instr. and Meth. B7/8 (1985) 85. [14] N. DeCristofaro and R. Kaplow, Metal1 Trans. 8A (1977) 35.
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
ofnitrogen
B. Rauschenbach and G. Leonhardt, to be published. K.H. Jack, Proc. R. Sot. A208 (1951) 200. R.C. Ruhl and M. Morris, Trans. AIME 245 (1969) 241. D.T. Cromer, R.L. Mills, D. Schiferl and L.A. Schwalbe, Acta Crystall. B37 (1981) 8. E.M. Horl and L. Morton, Acta Crystall. 14 (1961) 11 and T.A. Scott, Phys. Rep. 27C (1976) 89. R. Reichlin, D. Schiferl, S. Martin, C. Vanderborgh and R.L. Mills, Phys. Rev. Lett. 55 (1985) 1464. N.E.W. Hartley, J. Vat. Sci. Technol. 12 (1975) 485. E.P. EerNisse and S.T. Picraux, J. Appl. Phys. 48 (1977) 9. P.B. Madakson, J. PHys. D: Appl. Phys. 18 (1985) 531. R.C. Birtcher and W. Jager, J. Nucl. Mater. 135 (1985) 274; J.H. Evans and R.J. Mazey, J. Nucl. Mater. 138 (1986) 176; C. Templier, H. Garem and J.P. Riviere, Philos. Mag. A53 (1986) 607.
Nuclear Instruments and Methods in Physics Research B23 (1987) 316-322 North-Helled, Amsterdam
MICROSTRUCTURAL INVESTIGATIONS OF IRON IMPLANTED WITH NITROGEN IONS AT THE TEMPERATURE OF LIQUID NITROGEN Part I: Imphntation b the dose range 1 X 1O’6 to 1 X lOI7 N +-ionsfern B. RAUSCHENBACH
I), K. HOHMUTH
‘) and G. tiSTNER
2,
‘) Akademie der Wissenschojten der DDR, Zentrafinstitut ftir Kernforschung Rossendorf, 8OSI Dresden, PF 19, CDR zi Institut fGr ~estk~r~erph~~.s~k und ~~ektronenm~krosko~~~, 402 Haile. ~ei~ber~eg
2, PF 250, GDR
Received 9 September 1986 and in revised form 17 November 1986
The microstructure of iron implanted with nitrogen ions to fluences between 1 X 10" and 1~10’~ NC-ions/cm2 at the temperature of liquid nitrogen (- 796O C) have been investigated by high-voltage electron microscopy and transmission high-energy electron diffraction. Substitutional nitrogen-atom clusters are formed after implantation with nitrogen ions between 1 XlO”e and 5-7~10’~ Ni-ions/cm2, where the nitrogen atoms on the octahedral sites stabilii the b&structure. These clusters can be considered as a pre-pr~ipitatio~ stage prior to formation of other bet-phases. A molecular solid nitrogen phase (/3-N,) was found above an implantation dose of 4-5 x 10n’ N+-ions/cm*. The implantation-induced microstructure after nitrogen ion implantation at the temperature of hquid nitrogen differs from that after implantation at room temperature.
It is well established that ~~-~uen~e ion implantation improves the wear resistance, the fatigue lifetime and corrosion resistance of pure metals and alloys. The tribological and anticorrosion properties are mainly determined by the formation of compounds and their interaction with lattice defects. The understanding of these fundamental mechanisms is essential to the effective use of high-fluence ion implantation for applications. Microst~ctural investigations of iron and steels after nitrogen ion implantation at room temperature and post-annealing have been the object of many experimental studies. In practice, temperature effects during implantation are very important. Freliminary studies have shown that nitrogen implantation in steels [l-S] and iron [1,6] at higher temperatures (loo-350 ’ C) and ion current densities (- 50-200 p A/cm2) respectively lead to implantation profiles and to impl~tation-induced microstructures, which differ from those of implantation at room temperature (RT) and post-annealing. No investigations into the influence of nitrogen implantation at low temperatures f < room temperature) on the microstructure of iron, steels or Fe-based alloys are known. In the present experiments, we have implanted nitrogen ions into pure iron at liquid nitrogen temperature (LNT) and have investigated the nitrogen distribution and microstructure with respect to the implantation dose. In this paper we present the results ~I68-583X/g7/$03.50 0 Flsevier Science Publishers (North-Holl~d Physics Publishing Division)
B.V.
after implantation up to a dose of 1 X lOI7 N+ions/cm=. In a second part, we have reported our results after implantation with higher doses f 2 7. x 10” N ‘+-ions/cm2) [7].
Polycrystalline layers of iron were deposited on freshly cleaved NaCl, KC1 or silicon in a high vacuum of 10m4 Pa. The thickness of the iron layers varied between 100 nm and 1 pm. These thin films were implanted with nitrogen ions (vacuum 10e5 Pa) with energies from 30 to 60 keV in the fluence range 1 X 10t6-1 X 1017 N+-ions/cm’. A cooiing system in the isotope separator chamber was used in order to implant at the temperature of liquid nitrogen (- 196’ C) or at higher temperatures [6]. The carbon concentration after nitrogen ion implantation at LNT is very high. Measurements using Auger electron spectroscopy have shown that in the near-surface region (= up to 20 nm) the carbon concentration is about 30-35 at.% and that the carbon concentration is higher than the concentration of the implanted nitrogen in this region [6]. The temperature during implantation was measured with a thermocouple in contact with the sample. The crystallographic structure and mo~hology of the iron layers before and after the implantation were investigated by high voltage electron microscopy (HVEM) and transmission high-energy electron diffrac-
B. Rauschenhuch et al. / Microstructure
tion (THEED). Identification of implantation-induced phases took place on the basis of calibrated selected area diffraction diagrams. The calibration of SAD-patterns is generally straightforward but can be facilitated by using and internal standard (unimplanted iron). Computer programs are used to calculate the interplanar spacings and intensities, and to design the synthetic electron diffraction spectra (for comparison with the experimental determined spectra). Calculations are based on application of sphne functions and a leastsquares anaIysis (for details see ref. [8]). It is possible to determine the positions and intensities of the overlapped electron diffraction peaks with this method. The goodness-of-fit of the calculated synthetic spectra to the observed diffraction spectra is better than 9.5%. (for discussion see refs. 171 and [g]).
3. Results and discussion Recently we reported on the analysis of iron nitride phases after nitrogen ion im~I~tation in the fluenee range From t X 1016 to 1 X 10’s ions/cm2 at room temperature [Y-11]. The occurrence of the iron nitride phases and solid solutions y-austenite, a’-martensite, r-Fe,N, _--5 and a” - Fe,,Nz was shown to be fluence dependent. Fig. 1 shows a large bright field micrograph of u~~l~t~ iron. The bee-reffections of unimplanted iron are used for calibration of the implanted iron. In table 1 are summarized the lattice parameters and some crystallographical data of the analysed iron nitride phases. The interaction of the iron surface with oxygen forms a thin magnetite layer (Fe&, a = 0.8397 nm). Electron diffraction reflections of other iron oxides (ruiistite, haematite) were not observed. 3.1. Implantation cm2
fluence: I X10’” to 5~ IO’” N “-ions/
Fig. 1 also shows micrographs of iron implanted with 1 x 10t6, 2.5 x lOi6 and 5 X lOI6 NC-ions/cm’ at LNT. In contrast to the unimplanted iron, the micrographs are characterized by a pronounced contrast of the habit of crystallites. Depending on the implantation
of nitrogen
317
fluence a marked morphological difference can be observed. A recent investigation has confirmed the existence of nitrogen-atom clusters. Ferguson and Jack [12) have shown that quench-ageing of supersaturated nitrogen-ferrite leads to the formation of three-dimensional arrays of nitrogen-atom clusters as platelets. These clusters can be considered as arrangements of nitrogen atoms randomly occupying octahedral interstices in the ferrite matrix. In the present ease, fig. I shows rod-like and disc-like nitrogen-atom clusters after implantation. THIZD investigations have confirmed that the implantation at LNT induces N-atom clusters. Fig. 2 shows an electron diffraction pattern with indexed photodensitometer plot of iron after nitrogen ion implantation of 1 X lOi ions/cm2 at LNT. In addition to the reflectians of iron, bet N-atom clusters were identified. In contrast to implantation at RT, we did not find y-austenite [9,10]. The lattice positions of implanted nitrogen in iron were studied by Swanson et al. [13]. They have measured that the majority of implanted nitrogen atoms occupy octahedral or near-octahedral sites after im~l~tation at 35 K. It is known that the octahedral intersitial nitrogen stabilizes the bet structure of iron [14]. Fig. 3 shows the evolution of the N-atom clusters as a function of the implantation fluence (nitrogen concentration). The electron scattering intensity is plotted as a function of the electron scattering angle parameter s (s = 47rsin v/he, v = Bragg angle, h, = relativistic wavelength of electrons) for selected peaks after background correction and smoothing (for details see ref. [S]). The size of the N-atom clusters (see also Fig. 1) increases with increasing implantation Iluence. After implantation with 5 X 10 I6 N’-ions/cm’ the electron reflections (lOI), (002) and (020) of the N-atom cluster precipitates have a higher intensity than in the case of implantation with 1 X 1016 Nf-ions/cm2 (fig. 3). Fig. 3 shows also that implantation of 1 x 1Or7 N”-ions/cm2 does not lead to formation of N-atom clusters. A homogeneous dispersion of nitrogen-atom clusters could be found in the fluence range between 1 x lOI and 5-7 X lOI N’-ions/cm*, These clusters are very unstable against thermal treatment [15]. The chemical state of nitrogen implanted iron at different temperatures was
Table 1 Crystallographicaldata of the analysed crystal phases and solid solutions and their lattice parameters. Bravais lattice
Lattice p‘wameter [nm]
Space group
No. of space group
a-Fe, ferrite N-atom duster
bee bet
Im3m 14/mmm
229 139
molecular &N,
cubic
u = 0.28664 If = 0.28644- O.wo1X a> c/a = I i 0.0091 a) c1= 0.61641
Pm3m
221
Phase
‘) At.% N.
Refs.
318
B. Rauschenbach
qt al. / Microstructure
of nitrogen
Fig. 1. HVEM bright field micrographsof unimplantediron and iron implanted with 50 keV nitrogen ions to a fluence of 1 XlOl”, 2.5 x lOI and 5 x 1Oz6ions/cm’ at LNT.
analysed by using the X-ray photoelectron spectroscopy 1151. The XPS-spectrum for N,, (at a depth = the mean projected range of the nitrogen ions) shows two peaks at 398.2 eV and 405.5 eV after impl~tation at LNT and two peaks at 395.0 eV and 399.6 eV after implantation at 300°C. The peak at 403.5 eV indicates formation of solid nitrogen while the signal at 398.2 eV and 395.0 eV corresponding to nitrides and the peak at 399.0 eV corresponding to free or weakly bonded nitrogen (for discussion see ref. [15]). This means that the solid nitrogen state is unstable against thermal treatment and that only a fraction of the implanted nitrogen is in the nitride bonding state.
Ferguson and Jack [12] have found that the tetragonality continually increases due to cluster formation and that N-atom clusters contain 5 0.3-0.4 nitrogen atoms per 100 iron atoms. A linear relationship exists between the lattice parameters a and c respectively and the concentration of nitrogen [12,16,17]. Starting from these linear relations, it is possible to derive the nitrogen content with the aid of experimentally determined interplanar spacings. This approach is demonstrated for iron after nitrogen ion impl~tation at room temperature [lO,ll]. ‘The measured interplanar spacings dent, depend on uexp and cexp. Using a special computer program (see refs. [lO,ll]) the values of dexp were fitted
B. Rauschenbach
et al. / Mtcrostructure
of nttrogen
319
-
a-t210
-
alwl-Iml
-
a-l!nol
~
SCATTERINGANGLE PARAMETER Fig.
2.
s fnm-‘1
Transmission electron diffraction pattern (SAD) of iron implanted with nitrogen ions to a fluence of 1 X
10’”
ions/cm’
at
LNT.
to the theoretical values of d,,,_,_ This calculation gives the values of a_, and c,_,. It was estimated, that the concentration varies between 1 and 3 N-atoms per 100 iron atoms depending on the implantation fluence. The formation of N-atom clusters can be considered as a pre-precipitation stage prior to formation of the bet metastable phases a’-martensite and a”-Fe16N2 [7].
Fig. 4 shows a low-magnification micrograph of iron after implantation with 7.5 x 10’” N+-ions/cm* at LNT. Large areas of the iron sample exhibit a streak-like structure. The observed THEED-patterns (fig. 4), can be interpreted as originating from condensed nitrogen gas. The diffraction patterns indicate that this solid nitrogen is the molecular /3-N, phase (see table 1). The solid B-N, phase has a molecular lattice of the Pm3m
INTERPLANAR
SPACING
dexp
[nm]
Fig. 3. Details of microphotometer curves of the strongest electron diffraction reflections after background subtraction and smoothing for iron implanted with 1 X 10i6, 5 x 1016 and 1 x 10” N ‘-ions/cm2 at LNT.
B. Raurchenbuch
320
et ul. / Microstructure
of nitrogen
PLOT
E-SOkeV
Fig. 4. Bright
field micrograph
(BF), selected
area diffraction (SAD) and indexed 7.5 X 1016 N+-ions/cm* at LNT.
space group (8 molecules per unit cell) [18]. The centers of the nitrogen molecules are arranged in such a way that they are shifted slightly from the primitive cubic
Fig. 5. HVEM bright
field micrograph
photometer
plot (PLOT)
of iron implanted
with
positions. The high-pressure behaviour of solid nitrogen appears to be quite complex. However, the molecular character persists for a pressure below 10 GPa (studies
of iron implanted
with 1 X 10”
N f-ions/cm*
at LNT.
B. Rauxhenbach et al. / Microstructure of nitrogen
321
Table 2 Summary of the most frequent experimentally determined interplanar spacings dexp and intensities I_, of iron implanted with 7.5 x 10’6 and 2.5 x 10” N+-ions/cm’. The calculated values of dtheo and Itheo are compared with the experimentally determined values of molecular N-atom clusters. Miller
Theoretical interplanar spacing
indices
d theo [MI] and intensity Ithe,,
Measured interplanar spacing & [nm] and intensity lexp for two fluences
d thee
(7.5 X lOi6 N+/cm’)
h
k
I
2 0 0 2 1 0 2 1 1 2 2 2 4 0 0
0.30821 0.27567 0.25165 0.17794 0.15410
I thee
88.6 100.0 75.4 6.5 9.5
(2.5 X 10” N+/cm’)
d ev
I exP
d exP
I
0.305 0.270 0.250 0.177 0.154
75 100 60 10 10
0.307 0.275 0.253 0.177 0.154
90 100 70 10 15
with a diamond anvil cell) [19,20]. Table 2 shows a comparison of experimentally and theoretically determined values of the interplanar spacings and intensities. The reflections of molecular p-N2 are clearly separated from the reflections of iron. A hexagonal structure has been suggested for solid nitrogen [19], but such lattice structure has not been observed. The molecular solid /3-N, is formed between 4-5 x 1016 and 2-6 x 10” N+-ions/cm* after implantation at LNT [7]. Fig. 5 shows a high-magnification micrograph of iron after nitrogen implantation to a fluence of 1 X 10” N+-ions/cm* at LNT. In addition to the streak-like structure, particles with a sphere-like morphology can be observed. The higher stress and the higher nitrogen concentration in the high fluence implanted layer might lead to another morphology. It can be assumed that the formation of solid nitrogen is possible in connection with a low temperature during implantation and a high lateral stress induced by implantation. Hartley [21] and Eer Nisse and Picraux [22] have demonstrated that the internal stress in pure metals and steel after implantation is very high (= theoretical yield stress) and after noble gas implantation it is connected with the formation of blisters. Recently, Madakson [23] reported that the implantation-induced stress in aluminium increases linearly with fluence up to about 5 x 1016 N+-ions/cm* and while it relieved at higher fluences. In th present case, it is assumed that a high stress (= 1 GPa) can explain the phenomenon of the stability of molecular nitrogen in iron. Recently, solid noble gas precipitates were detected after noble gas implantation [24]. These precipitations are stable for pressures 2 1 GPa. This result is in good agreement with our results. The high lateral stress during implantation stabilizes the solid p-N2 phase (minimization of the precipitation energy), and the low temperature promotes this transformation. Presumably, both factors are necessary because a solid nitrogen phase does not occur after implantation at RT.
exP
4. Summary The formation of phases and solid solutions of iron after implantation with nitrogen ions at LNT have been investigated by high-voltage electron microscopy and transmission high-energy electron diffraction. Some important results are summarized as follows: (1) Substitutional nitrogen-atom clusters are formed in the fluence range between 1 x 1016 and 7 x 1016 N+-ions/cm*, where about l-3 nitrogen atoms per 100 iron atoms on octahedral sites stabilize the bet structure. (2) Formation of the N-atom clusters can be considered as pre-precpitation stage prior to the formation of other bet phases. is found above an im(3) Molecular solid @itrogen plantation fluence of 5 X 1016 N+-ions/cm*. Presumably, the implantation-induced stress and the low temperature during the implantation are necessary conditions for this phase to be stable. In the second part of this paper [7], the microstructure of iron and the distribution of nitrogen after implantation at LNT (fluence >_1 X 10” N+-ions/cm’) are presented and discussed. The authors would like to thank Dr. J. Schoneich and Mr. J. Altmann for performing the implantations.
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