Microstructural Evolution in a 17-4 PH Stainless Steel after Aging at ...

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(a) (b) G phase Cu precipitate 020 fcc 011 2nm Fig. 11 (a) HREM image taken at the [111] zone axis of the martensite phase in the alloy aged at 400°C for 5000 h. (b) micro-diffraction pattern obtained from the region with the Moire fringe. file obtained by the conventional atom probe (Figure 9), Ni, Si and Mn atoms appear to be partitioned to the Cu enriched particle indicated by the dashed lines on the left side of the figure. However, this result is probably an artifact caused by the convoluting effect of the probe hole covering both Cu and G-particles because Ni, Si and Mn enrichment is not observed at the other Cu enriched region in Figure 9. In the latter case, the probe hole covered only the Cu particle and missed the G-phase which was adjacent to the Cu particle. These results demonstrates that employment of the 3DAP technique provides more accurate data for characterizing the morphological feature of fine precipitates embedded in the matrix. Figure 11 (a) shows an HREM image taken along the [111] zone axis of the martensite phase in the alloy aged at 400°C for 5000 hours. In this image, fringe contrast having a periodicity of two (011) planes is ob- bcc served. This is consistent with the fringe contrast expected from the G-phase (Ni X Si , X=Fe, Mn, 16 6 7 Si, Fm3m , a=0.406 nm). Furthermore, a small particle is observed adjacent to the G-phase, which is believed to be a Cu precipitate. A microdiffraction pattern taken from the region with the Moire fringes is shown in Figure 11 (b). The [111] microdiffraction pattern shows Published in Metall. Mater. Trans. A. Vol. 30A, pp. 345-353. 1999 the extra reflections near the {110} bcc reflections. The δ-spacing calculated from the extra reflections is ~0.179 nm which corresponds to the spacing of the {020} fcc planes in fcc-Cu (0.180 nm). The orientation relationship (OR) between the particle and the martensite almost matches with the K-S relationship. IV. DISCUSSION This study has clarified evolution of microstructure in a 17-4 PH stainless steel during long term aging at 400°C. Emphasis is given to characterization of chemical features of the precipitates which appear in the martensite phase after prolonged aging. The ε-Cu precipitates have been found in the δ-ferrite after the solution heat treatment. The appearance of these precipitates in the δ-ferrite was unaffected by subsequent aging. Rack and Kalish [3] reported similar microstructural feature in δ-ferrite, but they attributed them to NbC precipitates. In this study, NbC were observed at martensite lath boundaries with much larger size as shown in Figure 1, and we believe that the fine precipitate in the δ-ferrite observed in the previous study as well as in this study are the ε-Cu. Precipitation of Cu in the Fe-Cu binary system has been a subject of numerous studies [14-18] , and it is well established that the initial Cu-enriched precipitate is perfectly coherent with the bcc matrix, while large overaged precipitates have an fcc structure with the K-S OR. However, binary alloys were all solution heat-treated in the single phase ferrite region around 900°C, while the 17-4 PH stainless steel is solution heat-treated at much higher temperature around 1050°C. Diffusivity of Cu in the d-ferrite at this temperature (D Cu ~ 1 x 10 -9 cm 2 /s) is more than three orders of magnitude higher than that in the austenite (D Cu ~ 4 x 10 -12 cm 2 /s), thus we believe that Cu precipitated out from the δ-ferrite during cooling after the solution heat treatment. Many Cu particles were observed along dislocations, and this would have ease the strain contrast for precipitation of the fcc ε-Cu. On the other hand, no indication of presence of precipitates are recognized in as-quenched martensite. The solubility of Cu in the austenite phase is up to 7 at. pct at 1100°C. However, the diffusivity of Cu in the austenite is orders of magnitude small than that in the ferrite phase. Thus, the precipitation kinetics are much slower in the austenite phase and Cu can be quenched in the martensite phase from the solution heat-treatment temperature. After tempering at 580°C for 4 hours, fine Curich precipitates were detected in the martensite phase using 3DAP. The Cu content in the Cu-rich precipitates is significantly lower than the equilibrium value for ε- Cu. They reported that the average copper concentration of small precipitates is approximately 50 at. pct in the earliest stage of the precipitation. The copper concentration in the fine, Cu-rich particles which precipitate during tempering is in good agreement with the early study by Goodman et al. [16] . Unlike in the d-fer-
ite, these Cu-rich precipitates observed in the martensite phase after tempering are bcc-Cu. The martensite phase decomposes into Fe-rich α and Cr-enriched α′ phases by the spinodal mechanism. In addition, fine precipitates of the G-phase has been found in the martensite phase in direct contact with the Cu precipitates after 5000hours aging at 400°C. Concentration of Cr in the α′ phase is approximately 25 at. pct after 100 hours, then reach 40 at. pct after 5000 hours aging. This concentration is significantly lower than that expected from the binodal line and is thought to be still in the decomposition process. Since no precipitation of the G-phase was observed after 100 hours aging, the increase in yield strength up to 100 hours is attributed to the spinodal decomposition rather than the formation of the G-phase. G-phase precipitation was reported in type 308 stainless steels [9, 19] and maraging steels [20] as a grain boundary phase. In recent years, it was shown that the G-phase exists in various Fe-Cr-Ni alloys as a dispersed phase in the interior of the grains [21-27] . For example, Auger et al. [27] reported phase separation and precipitation of the G-phase in the ferrite phase of duplex stainless steel. They concluded that the nucleation of the G-phase takes place at the ‘interface’ between Cr-rich a¢ and Fe-rich a. However, in the case of the 17-4 PH stainless steel, we have found that G-phase precipitation occurs in intimate contact with ε-Cu precipitates after the decomposition of the martensite have progressed. This result indicates that the Cu precipitates provides heterogeneous nucleation sites for Gphase formation. One reason for this is that the Cu/ martensite interface is a more suitable site for nucleation of the G-phase than the α/α’ interface. The dark field image of the long-term aged martensite phase shows the G-phase is dispersed uniformly in the grain rather than grain boundary as commonly observed in other stainless steels. This suggests that the G-phase precipitation itself does not contribute to the embrittlement. In fact, 80 pct of total yield strength increase occurs before precipitation of the G-phase is observed (100 hours aging at 400°C). Our atom probe results have shown that the atomic ratio of Ni to Si in the G-phase is 6:4. In addition, Mn and Fe are also contained in the G-phase. The ternary silicide designated as the G-phase is known to be fcc having 116 atoms in a unit cell. The lattice parameter is ~1.12 nm. The structure of this phase is isotypic with Th Mn (structure type D8a, space group 6 23 Fm3m ), and the ideal composition was proposed as X Ni Si , where 6 16 7 X is typically Ti, but can be substituted for other transition element. Considering the qualitative nature of the atom probe result, the determined composition (55 at. pct Ni, 25 at. pct Si, 20 at. pct Fe and some Mn) is reasonably close to the ideal stoichiometry of X Ni Si 6 16 7 when one allows for substitution of Mn and Fe for X [28] . In this study, partitioning of Ni in the α phase after Published in Metall. Mater. Trans. A. Vol. 30A, pp. 345-353. 1999 spinodal decomposition of the martensite was not observed. This result is in contrast to the APFIM results by Danoix et al. [26] which reported that nickel is rejected from Cr-enriched α’ phase and partitioned into the Fe-rich α phase in the aged ferrite phase of duplex stainless steel. In the equilibrium condition, the solubility limit of Ni in α and α’ were estimated to be 4.1 and 0.01 at. pct, respectively, by the ThermoCalc software. Thus, Ni should partition into the α’phase in 17- 4 PH as well. This suggests Ni partitioning does not occur until decomposition progresses further and the driving force for the partitioning reaction increases. Similar delayed partitioning behavior was reported for Al in an Fe-Cr based alloy [29] . As the Ni concentration of the duplex stainless steel was higher than that in 17- 4 PH, the driving force for Ni partitioning is much higher in the duplex stainless steel. Thus the absence of concurrent partitioning of Ni with spinodal decomposition in 17-4 PH is reasonable. V. CONCLUSIONS The microstructural features of 17-4 PH stainless steel at various stages of heat-treatment has been investigated by APFIM and TEM. The main results are as follows: 1. The fcc Cu-rich particles precipitate in the δ-ferrite grains during cooling after the solution heat treatment. The precipitates have the K-S orientation relationship with the δ-ferrite matrix. 2. Tempering at 580°C for 4 hours results in precipitation of coherent bcc particles in the martensite phase. The composition of the precipitates is approximately 60 at. pct Cu and Cr, Ni and Si are rejected from the bcc-Cu. The Cr concentration is homogeneous in the martensite phase at this stage. 3. After 100 hours aging at 400°C, evidence for spinodal decomposition of the martensite phase into Fe-rich a and Cr-enriched a’ phase is found. The Cr concentration in the a’ phase is approximately 25 at. pct, significantly lower than the equilibrium value. The concentration of Cu in the Cu-enriched precipitate is approximately 70 at. pct at this stage. 4. Spinodal decomposition of the martensite phase progresses further after 5000 hours aging. The Cr concentration of the α’ phase is 40 at. pct at this stage. The structure of the coarsened Cu precipitates is fcc and their concentration is almost 100 at. pct Cu. Fine particles of G-phase containing approximately 60 at. pct Ni, 25 at. pct Si and some Fe and Mn are present in direct contact with the Cu precipitates, suggesting that G-phase is heterogeneously nucleated at the martensite/Cu interface. 5. The increase in hardness and yield strength of the 17-4 PH stainless steel after aging at 400°C is mostly caused by spinodal decomposition of the martensite phase. G-phase precipitation does not appear to make
Page 1: I. INTRODUCTION PRECIPITATION-harde
Page 5 and 6: ~26nm (a) Cr (b) Cu ~16nm ~26nm age
Page 7: ~15nm Ni + Si Cu (a) Cr (b) Cu, Ni

Microstructural Evolution in a 17-4 PH Stainless Steel after Aging at ...

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