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Atom probe studies of spinodal processes in duplex stainless steels and single- and dual-phase FeCrNi alloys
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Atom probe FIM investigations on the intermetallic NiAI phase with B2 superlattice structure Z.G. Liu ~, G. Frommeyer and M. Kreuss .~la~-Planck-lnstitut fi~r Ei.~'enfim~'chungGmbH. Dik~eldorf, Germany Rccci~ed _'~0Jul',~ 1990, accepted for publication ~:-,-7,August 1990 The field evaporation sequence and the imperfection of long range ordered intermetallic NiAI compounds with B2 superlattice structure have been characterized by field ion microscopy and atom probe analysis. An atom probe survey along a (001) direction clearly detected the B2 ordering. However, atom probe analysis of the (111) pole indicates double layer field evaporation, and hence, the superlattice structure cannot be analyzed in detail in this direction. In a nearly stoichiometric NiA1 alloy prepared by hot exlrusion, a considerable contribution of Ni and AI antistructure atoms, as well as Ni or AI rich regions were detected. During annealing the alloy at 1000°C, the degree of long range order increased remarkably. In a nickel rich NiAI alloy the composition fluctuation with a characteristic length of 1.9 nm \~as revealed. The origin of the imperfections of the B2 superlattice structure has been discussed.
278
Surface Science 246 (1991 ) 278--284 North-Holland
Atom probe field-ion microscopy study of ageing behaviour of a Co-free maraging steel W. Sha, A. Cerezo and G.D.W. Smith Department of Materials, University of Oxford, Parks Road, Oxford OXI 3PH, UK Received 1 June 1990: accepted for publication 27 August 1990 This paper reports an atom probe field-ion microscopy (APFIM) study of the ageing reactions in a Co-free (T-300) maraging steel (Fe-18.0%Ni-2.4%Mo-2.2%Ti-0.2%AI-0.2%Si-0.1%C (at%)), using both the conventional A P F I M and the position-sensitive atom probe (POSAP). Two families of intermetallic phases (Ni 3Ti and FevMo6 types) have been found to contribute to age-hardening. The composition and morphology of these precipitates were studied in steel aged at different temperatures (450-610 o C) for different runes (5 min to 360 h) to investigate the ageing sequence. The observation of the FeTM % if-phase is supported by thermochemical calculations. No austenite reversion has been found. The results are compared with previous results from a Co-bearing (C-300) maraging steel to investigate the role of Co.
Surface Science 246 (1991) 285 291 North-Holland
285
Atom probe studies of spinodal processes in duplex stainless steels and single- and dual-phase Fe-Cr-Ni alloys J.E. Brown and G.D.W. Smith Department of Materials, Oxford University, Parks Road, Oxford OXI 3PH, UK Received 1 June 1990; accepted for publication 27 August 1990 Duplex stainless steel alloys with chromium contents in the ferrite phase greater than 13% are subject to embrittlement due to hardening of the ferrite phase. The hardening is caused by a chromium rich phase, a', produced by either nucleation and growth or spinodal decomposition, and secondary mechanisms including the precipitation of carbides and G-phase. The activation energy of the embrittlement process in CF3 castings has previously been measured by mechanical testing and has been found to be close to the activation energy governing chromium diffusion in the ferrite, although one series of alloys has consistently given much lower activation energies for embrittlement. The reason for this is not known but precipitation of carbides and G-phase has been proposed to interfere with the kinetics of the spinodal in some alloys. In an attempt to simplify the situation by minimising the precipitation of other phases, a series of high purity FeCrNi alloys modelling the ferrite phase of CF3 castings were examined in the field ion microscope and atom probe. No carbide or G-phase precipitation is to be expected in such alloys so the only process occurring on ageing should be the spinodal. Single phase (ferrite) and
A175 dual phase (austenite and ferrite) alloys were produced by employing two different solution treatment temperatures. Ageing between 300 ° and 450°C has been studied for times up to 10000 hours. The final amplitude of the spinodal fluctuations was found to decrease with increasing Ni content although the rate of evolution of the spinodal increased with increasing Ni content. In the case of single phase alloys, the rate of increase of the spinodal amplitude was much slower than in the case of CF3 castings, although the activation energy for hardening was very similar. However, those alloys heat treated to produce a small amount of austenite were found to have an accelerated rate of amplitude increase of the spinodal, producing behaviour similar to that in CF3 castings.
292
Surface Science 246 (1991) 292-298 North-Holland
Characterization of the Tishomingo meteorite K.F. Russell, E.A. Kenik and M.K. Miller Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376, USA Received 30 July 1990; accepted for publication 27 August 1990 A preliminary microstructural characterization of the Tishomingo meteorite has been performed with the combined techniques of atom probe field ion microscopy, analytical electron microscopy, scanning electron microscopy, optical microscopy, and nanoindentation. Eighty percent of this meteorite appears to have undergone a martensitic transformation; the remaining 20% being taenite ('t). Field ion microscopy and transmission electron microscopy of the martensitically transformed region revealed a mixture of a face centered cubic phase, a body centered cubic phase and some small face centered cubic precipitates. The composition of these precipitates was found to be between 51 and 57 at% Ni and the surrounding matrix approximately 20% Ni.
Surface Science 246 (1991) 299-303 North-Holland
299
In-situ phase transformation in the field ion microscope M.K. Miller and K.F. Russell Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376, USA Received 30 July 1990; accepted for publication 6 August 1990 Many materials undergo an athermal martensitic transformation when cooled. This transformation has been observed in the Tishomingo meteorite during cooling to cryogenic temperatures. The meteorite is unstable when cooled to the cryogenic temperatures (40-85 K) suitable for field ion imaging since the martensite start (Ms) temperature of this material (Fe-32.5wt%Ni) is approximately 235 K. In many cases, the in-situ martensitic transformation destroyed the field ion specimen prior to imaging. It was also found that the martensite to taenite transformation could be repeatedly induced at cryogenic temperatures by cycling the voltage and hence applying stress to the specimen.
304
Surface Science 246 (1991) 304-314 North-Holland
Measurement of the amplitude of a spinodal M.G. Hetherington a, J.M. Hyde a, M.K. Miller b and G.D.W. Smith a a Department of Materials, University of Oxford, Oxford, UK b Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 30 July 1990; accepted for publication 6 August 1990 One of the strengths of the atom-probe is its ability to measure the amplitude of composition fluctuations on a very fine scale. In previous papers, calculation of the amplitude of a spinodal using a sinusoidal composition distribution has been reported. In this paper, a comparison is made between the fit of experimental data from the atom-probe to a sinUsoidal distribution and also to the amplitude of the composition variations expected from non-linear theories of the spinodal decomposition. It is shown that, in general, the Langer, Bar-on and Miller (LBM) non-linear theory provides better fits to the data. In particular, the non-linear theory is able to describe the asymmetry of the distribution functions for mean compositions far from the critical composition.
"Z,
rh-t{ollaJ ,
Atom probe FIM investigations on the intermetallic NiAI phase with B2 superlattice structure Z.G. Liu ~, G. Frommeyer and M. Kreuss .~la~-Planck-lnstitut fi~r Ei.~'enfim~'chungGmbH. Dik~eldorf, Germany Rccci~ed _'~0Jul',~ 1990, accepted for publication ~:-,-7,August 1990 The field evaporation sequence and the imperfection of long range ordered intermetallic NiAI compounds with B2 superlattice structure have been characterized by field ion microscopy and atom probe analysis. An atom probe survey along a (001) direction clearly detected the B2 ordering. However, atom probe analysis of the (111) pole indicates double layer field evaporation, and hence, the superlattice structure cannot be analyzed in detail in this direction. In a nearly stoichiometric NiA1 alloy prepared by hot exlrusion, a considerable contribution of Ni and AI antistructure atoms, as well as Ni or AI rich regions were detected. During annealing the alloy at 1000°C, the degree of long range order increased remarkably. In a nickel rich NiAI alloy the composition fluctuation with a characteristic length of 1.9 nm \~as revealed. The origin of the imperfections of the B2 superlattice structure has been discussed.
278
Surface Science 246 (1991 ) 278--284 North-Holland
Atom probe field-ion microscopy study of ageing behaviour of a Co-free maraging steel W. Sha, A. Cerezo and G.D.W. Smith Department of Materials, University of Oxford, Parks Road, Oxford OXI 3PH, UK Received 1 June 1990: accepted for publication 27 August 1990 This paper reports an atom probe field-ion microscopy (APFIM) study of the ageing reactions in a Co-free (T-300) maraging steel (Fe-18.0%Ni-2.4%Mo-2.2%Ti-0.2%AI-0.2%Si-0.1%C (at%)), using both the conventional A P F I M and the position-sensitive atom probe (POSAP). Two families of intermetallic phases (Ni 3Ti and FevMo6 types) have been found to contribute to age-hardening. The composition and morphology of these precipitates were studied in steel aged at different temperatures (450-610 o C) for different runes (5 min to 360 h) to investigate the ageing sequence. The observation of the FeTM % if-phase is supported by thermochemical calculations. No austenite reversion has been found. The results are compared with previous results from a Co-bearing (C-300) maraging steel to investigate the role of Co.
Surface Science 246 (1991) 285 291 North-Holland
285
Atom probe studies of spinodal processes in duplex stainless steels and single- and dual-phase Fe-Cr-Ni alloys J.E. Brown and G.D.W. Smith Department of Materials, Oxford University, Parks Road, Oxford OXI 3PH, UK Received 1 June 1990; accepted for publication 27 August 1990 Duplex stainless steel alloys with chromium contents in the ferrite phase greater than 13% are subject to embrittlement due to hardening of the ferrite phase. The hardening is caused by a chromium rich phase, a', produced by either nucleation and growth or spinodal decomposition, and secondary mechanisms including the precipitation of carbides and G-phase. The activation energy of the embrittlement process in CF3 castings has previously been measured by mechanical testing and has been found to be close to the activation energy governing chromium diffusion in the ferrite, although one series of alloys has consistently given much lower activation energies for embrittlement. The reason for this is not known but precipitation of carbides and G-phase has been proposed to interfere with the kinetics of the spinodal in some alloys. In an attempt to simplify the situation by minimising the precipitation of other phases, a series of high purity FeCrNi alloys modelling the ferrite phase of CF3 castings were examined in the field ion microscope and atom probe. No carbide or G-phase precipitation is to be expected in such alloys so the only process occurring on ageing should be the spinodal. Single phase (ferrite) and
A175 dual phase (austenite and ferrite) alloys were produced by employing two different solution treatment temperatures. Ageing between 300 ° and 450°C has been studied for times up to 10000 hours. The final amplitude of the spinodal fluctuations was found to decrease with increasing Ni content although the rate of evolution of the spinodal increased with increasing Ni content. In the case of single phase alloys, the rate of increase of the spinodal amplitude was much slower than in the case of CF3 castings, although the activation energy for hardening was very similar. However, those alloys heat treated to produce a small amount of austenite were found to have an accelerated rate of amplitude increase of the spinodal, producing behaviour similar to that in CF3 castings.
292
Surface Science 246 (1991) 292-298 North-Holland
Characterization of the Tishomingo meteorite K.F. Russell, E.A. Kenik and M.K. Miller Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376, USA Received 30 July 1990; accepted for publication 27 August 1990 A preliminary microstructural characterization of the Tishomingo meteorite has been performed with the combined techniques of atom probe field ion microscopy, analytical electron microscopy, scanning electron microscopy, optical microscopy, and nanoindentation. Eighty percent of this meteorite appears to have undergone a martensitic transformation; the remaining 20% being taenite ('t). Field ion microscopy and transmission electron microscopy of the martensitically transformed region revealed a mixture of a face centered cubic phase, a body centered cubic phase and some small face centered cubic precipitates. The composition of these precipitates was found to be between 51 and 57 at% Ni and the surrounding matrix approximately 20% Ni.
Surface Science 246 (1991) 299-303 North-Holland
299
In-situ phase transformation in the field ion microscope M.K. Miller and K.F. Russell Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376, USA Received 30 July 1990; accepted for publication 6 August 1990 Many materials undergo an athermal martensitic transformation when cooled. This transformation has been observed in the Tishomingo meteorite during cooling to cryogenic temperatures. The meteorite is unstable when cooled to the cryogenic temperatures (40-85 K) suitable for field ion imaging since the martensite start (Ms) temperature of this material (Fe-32.5wt%Ni) is approximately 235 K. In many cases, the in-situ martensitic transformation destroyed the field ion specimen prior to imaging. It was also found that the martensite to taenite transformation could be repeatedly induced at cryogenic temperatures by cycling the voltage and hence applying stress to the specimen.
304
Surface Science 246 (1991) 304-314 North-Holland
Measurement of the amplitude of a spinodal M.G. Hetherington a, J.M. Hyde a, M.K. Miller b and G.D.W. Smith a a Department of Materials, University of Oxford, Oxford, UK b Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 30 July 1990; accepted for publication 6 August 1990 One of the strengths of the atom-probe is its ability to measure the amplitude of composition fluctuations on a very fine scale. In previous papers, calculation of the amplitude of a spinodal using a sinusoidal composition distribution has been reported. In this paper, a comparison is made between the fit of experimental data from the atom-probe to a sinUsoidal distribution and also to the amplitude of the composition variations expected from non-linear theories of the spinodal decomposition. It is shown that, in general, the Langer, Bar-on and Miller (LBM) non-linear theory provides better fits to the data. In particular, the non-linear theory is able to describe the asymmetry of the distribution functions for mean compositions far from the critical composition.