T. Waitz et al. Acta Materialia 52(2004)137-147 of 340C for 5 h followed by cooling to RT and quenching to -25C. Fig. 4(c) shows a specimen with S=7.3 annealed at a higher temperature(450C)for 1 h. In all cases the crystallization is complete since the diffraction patterns do not contain diffuse rings any more corresponding to the amorphous phase. Sharp and rather flat grain boundaries were observed both by TEM mean 30nm bright field and HRTEM images. It is important to point out that within the grains almost no dislocations were observed and most of the grains show only weak strain contrast Additional tem bright and dark field taken to measure the size distribution of the grains; the results are summarized in Fig. 5. In the case of S= 6.7 020406080100120140 annealed at 340C the grains have a diameter in the Grain size [nm] range of about 5-90 nm; only few of them are larger than about 50 nm(as shown in Fig. 5(a). In the case of S=7.3 annealed at 340C there is a broad range of grain diameters ranging from 5 to 140 nm(cf Fig. 5(b) Their distribution seems to be non-uniform since fre zza small grains quently areas are observed mainly containing grains of mean 25nm small diameters(mean diameter of about 25 nm, e.g. near S in Fig 4(b))whereas other areas that are adjacent large grains to them contain mainly larger grains(mean diameter mean 70nm about 70 nm, e.g. near M in Fig. 4(b)). In the case of S=7.3 and an annealing temperature of 450C most of LL 10 the grains are larger than about 100 nm(cf Fig. 5(c) Sa diffraction patterns were taken to analyze the H tt crystalline phases occurring in the grains of different di ameter. It is interesting to note that grains smaller than 020406080100120140 about 15 nm show reflections of the B2 phase only. When Grain size [nm the grain size is between 15 and 60 nm reflections of the B2 phase and the r-phase were observed whereas no reflections corresponding to the martensite were en- countered. When the grains are larger than about 60 nm 25 they contain R-phase(cf. the grain marked by R in Fig 4(b)and b19 martensite(cf. the area near M in Fig 4(b)) and in this case the B2 phase is hardly observed d15 mean 160nm Finally, grains larger than about 150 nm contain mainly martensite(cf. Fig. 4(c)). The volume fraction trans- formed to martensite by quenching to -25C was esti- mated to be less than about 30%(cf fig 4(b)and more than about 80%(cf Fig 4(c))in the specimens having a maximum grain size of about 140 and 350 nm(cf Figs. r (b)and (c), respectively. It should be noted that no 050100150200250300350 martensite could be detected in grains smaller than about 60 nm even when the specimens were quenched in liquid (c) Grain size [nm] nitrogen. In this case the volume fraction of R-phase Fig. 5. Histograms of the size distributions of the grains after crystalli seems to increase and only very small grains (less than zation (cf Fig 4(a).)S=6.7 after annealing at 340 C for 5 h is leading about 15 nm diameter)contain residual austenite. The to a mean grain size of 30 nm (b)s=7.3 after annealing at 340C for 5 results of this analysis are summarized in Table I h. The dashed bars correspond to areas containing mainly smaller and 3.3. The martensitic transformations in the nanostructures and the b19 martensite were analyzed in detail using In the specimens annealed at 340C for 5 h followed both SA diffraction and HRTEM methods. As illus- by cooling to RT and quenching to -25C the R-phase trated in Fig. 6 the grains containing the R-phase andof 340 C for 5 h followed by cooling to RT and quenching to )25 C. Fig. 4(c) shows a specimen with S ¼ 7:3 annealed at a higher temperature (450 C) for 1 h. In all cases the crystallization is complete since the diffraction patterns do not contain diffuse rings any more corresponding to the amorphous phase. Sharp and rather flat grain boundaries were observed both by TEM bright field and HRTEM images. It is important to point out that within the grains almost no dislocations were observed and most of the grains show only weak strain contrast. Additional TEM bright and dark field images were taken to measure the size distribution of the grains; the results are summarized in Fig. 5. In the case of S ¼ 6:7 annealed at 340 C the grains have a diameter in the range of about 5–90 nm; only few of them are larger than about 50 nm (as shown in Fig. 5(a)). In the case of S ¼ 7:3 annealed at 340 C there is a broad range of grain diameters ranging from 5 to 140 nm (cf. Fig. 5(b)). Their distribution seems to be non-uniform since fre￾quently areas are observed mainly containing grains of small diameters (mean diameter of about 25 nm, e.g. near S in Fig. 4(b)) whereas other areas that are adjacent to them contain mainly larger grains (mean diameter about 70 nm, e.g. near M in Fig. 4(b)). In the case of S ¼ 7:3 and an annealing temperature of 450 C most of the grains are larger than about 100 nm (cf. Fig. 5(c)). SA diffraction patterns were taken to analyze the crystalline phases occurring in the grains of different di￾ameter. It is interesting to note that grains smaller than about 15 nm show reflections of the B2 phase only. When the grain size is between 15 and 60 nm reflections of the B2 phase and the R-phase were observed whereas no reflections corresponding to the martensite were en￾countered. When the grains are larger than about 60 nm they contain R-phase (cf. the grain marked by R in Fig. 4(b)) and B190 martensite (cf. the area near M in Fig. 4(b)) and in this case the B2 phase is hardly observed. Finally, grains larger than about 150 nm contain mainly martensite (cf. Fig. 4(c)). The volume fraction trans￾formed to martensite by quenching to )25 C was esti￾mated to be less than about 30% (cf. fig 4(b)) and more than about 80% (cf. Fig. 4(c)) in the specimens having a maximum grain size of about 140 and 350 nm (cf. Figs. 5(b) and (c)), respectively. It should be noted that no martensite could be detected in grains smaller than about 60 nm even when the specimens were quenched in liquid nitrogen. In this case the volume fraction of R-phase seems to increase and only very small grains (less than about 15 nm diameter) contain residual austenite. The results of this analysis are summarized in Table 1. 3.3. The martensitic transformations in the nanostructures In the specimens annealed at 340 C for 5 h followed by cooling to RT and quenching to )25 C the R-phase and the B190 martensite were analyzed in detail using both SA diffraction and HRTEM methods. As illus￾trated in Fig. 6 the grains containing the R-phase and Fig. 5. Histograms of the size distributions of the grains after crystalli￾zation (cf. Fig. 4(a)).) S ¼ 6:7 after annealing at 340 C for 5 h is leading to a mean grain size of 30 nm. (b) S ¼ 7:3 after annealing at 340 C for 5 h. The dashed bars correspond to areas containing mainly smaller and larger grains (mean 25 and 70 nm), respectively. (c) S ¼ 7:3 after an￾nealing at 450 C for 1 h. Almost all grains are larger than 100 nm. T. Waitz et al. / Acta Materialia 52 (2004) 137–147 141