《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Martensitic phase transformations in nanocrystalline NiTi

Availableonlineatwww.sciencedirect.com BCIENCE Acta materialia ELSEVIER Acta Materialia 52(2004)137-147 www.actamat-journals.com Martensitic phase transformations in nanocrystalline Niti studied by tem T. Waitz.v Kazykhanov. H P. Karnthaler Received 16 June 2003: accepted 22 August 2003 Abstract By high pressure torsion(HPT) deformation almost complete amorphization is obtained in bulk Ni-50. 3at %Ti containing B1 martensite. During low temperature annealing tiny crystallites retained after the hpt deformation are acting as nuclei and trigger the nanocrystallization of B2 austenite. It is shown that the density of the nuclei is a function of the HPt strain and determines together with the annealing temperature the grain size of the nanocrystals ranging from 5 to 350 nm. Upon cooling the nano- structures transform to b19 partially since the grain boundaries hinder the autocatalytic formation of martensite. The large transformation strains of B19 are reduced by very fine(00 1) compound twins. With decreasing grain size an increasing energy barrier arises and the martensitic transformation is completely suppressed in grains smaller than 60 nm. The r-phase transformation causing only small transformation strains is observed in grains between 15 and 60 nm. Whereas in grains below 15 nm B2 remains dicating that no transformation occurs at all e 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: High pressure torsion; Bulk amorphous materials; Nanocrystallization; Martensitic phase transformation: High-resolution electron microscopy 1. ntroduction cause of the nanocrystallization in the HPt deformed alloys remained unclear. In the intermetallic compound NiTi the shape mem- As the martensitic transformation occurring in ory effect and superelastic properties are related to a nanocrystalline NiTi is concerned there are contradic martensitic phase transformation being of considerable tory results presented in the literature. In Ni-498at %Ti interest both from a scientific and a technological point subjected to cold rolling crystal refinement and partial of view. In addition, in NiTi amorphization can be in- amorphization was achieved and the martensitic trans duced applying various solid state processes such as formation was suppressed even after cooling to-150C particle irradiation [1] and strong mechanical deforma- [5, 6]. The reason for the observed change in the trans tion by cold rolling [2] and mechanical alloying [3]. formation seems to be unclear; it was proposed that Recently severe plastic deformation by high pressure dislocations induced by the cold rolling may stabilize the torsion(HPT) methods was applied to NiTi achieving parent B2 austenite. Contrary to this, it was reported amorphization; followed by a suitable heat treatment a at the martensite start temperature Ms increases with nanocrystalline phase can obtained in bulk HPT alloys decreasing grain size in a bulk nanocrystalline material by devitrification of the amorphous phase [4]. Still, the obtained by annealing a shock compacted amorphous Ni-49.12at %Ti powder. The increase of Ms seems to be Corresponding author. Fax: +43-1-4277-51316 unclear; it was proposed that enhanced nucleation is E-Jmail address: waitz(@ap univie. acat (T. Waitz) facilitated by internal stresses caused by the nanograins On leave from the Institute of advanced Materials. Ua state or by effects of the shock compression [7 Aviation Technical Univ Since contradictory results of the martensitic trans- Russian Federation 12 K Marksa Street, 450000 Ufa. formation presented in the literature may be attributed to 1359-6454/S30.00@ 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved 036

Martensitic phase transformations in nanocrystalline NiTi studied by TEM T. Waitz *, V. Kazykhanov 1 , H.P. Karnthaler Institute of Materials Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria Received 16 June 2003; accepted 22 August 2003 Abstract By high pressure torsion (HPT) deformation almost complete amorphization is obtained in bulk Ni–50.3at.%Ti containing B190 martensite. During low temperature annealing tiny crystallites retained after the HPT deformation are acting as nuclei and trigger the nanocrystallization of B2 austenite. It is shown that the density of the nuclei is a function of the HPT strain and determines together with the annealing temperature the grain size of the nanocrystals ranging from 5 to 350 nm. Upon cooling the nano￾structures transform to B190 partially since the grain boundaries hinder the autocatalytic formation of martensite. The large transformation strains of B190 are reduced by very fine (0 0 1) compound twins. With decreasing grain size an increasing energy barrier arises and the martensitic transformation is completely suppressed in grains smaller than 60 nm. The R-phase transformation causing only small transformation strains is observed in grains between 15 and 60 nm. Whereas in grains below 15 nm B2 remains indicating that no transformation occurs at all.
2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: High pressure torsion; Bulk amorphous materials; Nanocrystallization; Martensitic phase transformation; High-resolution electron microscopy 1. Introduction In the intermetallic compound NiTi the shape mem￾ory effect and superelastic properties are related to a martensitic phase transformation being of considerable interest both from a scientific and a technological point of view. In addition, in NiTi amorphization can be in￾duced applying various solid state processes such as particle irradiation [1] and strong mechanical deforma￾tion by cold rolling [2] and mechanical alloying [3]. Recently severe plastic deformation by high pressure torsion (HPT) methods was applied to NiTi achieving amorphization; followed by a suitable heat treatment a nanocrystalline phase can obtained in bulk HPT alloys by devitrification of the amorphous phase [4]. Still, the cause of the nanocrystallization in the HPT deformed alloys remained unclear. As the martensitic transformation occurring in nanocrystalline NiTi is concerned there are contradic￾tory results presented in the literature. In Ni–49.8at.%Ti subjected to cold rolling crystal refinement and partial amorphization was achieved and the martensitic trans￾formation was suppressed even after cooling to )150 C [5,6]. The reason for the observed change in the trans￾formation seems to be unclear; it was proposed that dislocations induced by the cold rolling may stabilize the parent B2 austenite. Contrary to this, it was reported that the martensite start temperature Ms increases with decreasing grain size in a bulk nanocrystalline material obtained by annealing a shock compacted amorphous Ni–49.12at.%Ti powder. The increase of Ms seems to be unclear; it was proposed that enhanced nucleation is facilitated by internal stresses caused by the nanograins or by effects of the shock compression [7]. Since contradictory results of the martensitic trans￾formation presented in the literature may be attributed to Acta Materialia 52 (2004) 137–147 www.actamat-journals.com * Corresponding author. Fax: +43-1-4277-51316. E-mail address: waitz@ap.univie.ac.at (T. Waitz). URL: http://www.univie.ac.at/Materialphysik/EM. 1 On leave from the Institute of Advanced Materials, Ufa State Aviation Technical University, 12 K. Marksa Street, 450000 Ufa, Russian Federation. 1359-6454/$30.00
2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2003.08.036

T. Waitz et al. Acta Materialia 52(2004)137-14 stresses caused by lattice defects rather than to the ultra- of 40C/min in flowing nitrogen gas using a Perkin fine grain size it is the aim of the present study to carry out Elmer DSC 7 NiTialloy was investigated containing little lattice strains twin-jet polishing. The thinning was dome EM foils by a systematic investigation. Therefore a nanocrystalline The specimens were used to prepare Tenupol 3 and almost no dislocations and having a broad range of with a solution of 75% CH3oh and 25%HNO3(-22oC grain sizes(from about 5 to 350 nm). In a first step, the and 15 V). The different phases were analyzed by se- NiTi alloy showing the monoclinic B19 martensite phase lected area(SA) diffraction applying different beam di at room temperature(rT) was subjected to HPT defor- rections(BD)in a TEM(Philips CM 200 operating at mation achieving a bulk amorphous alloy. As compared 200 kV). Afterwards a hRTEM analysis was carried out to mechanically amorphized powders no further com- using a Philips CM30 ST(operating at 250 and 300 kv) paction or densification is necessary and contrary to cold equipped with a Gatan slow scan CCD camera rolling almost complete amorphization can be achieved by the HPT method. In a second step, a nanocrystalline microstructure was obtained by annealing the alloy close 3. Experimental results to the crystallization temperature. To investigate the nanocrystallization, two different degrees of HPT defor- 3. 1. The nanostructured amorphous phase after HPT of mation were carried out and the microstructures were different strains analysed carefully prior and after the devitrification. It ddition at rt the transformation induced microstruc Fig. I shows a TEM study of a specimen immediately ture and its dependence on the grain size was investigated after the hPt deformation at a strain of s= 6.7.As seen in detail using both transmission electron microscopy in Fig. 1(a)(bright field image) the specimen contains (TEM) and high resolution transmission electron heterogeneously distributed nanocrystals(spots of dark microscopy(HRTEM) methods contrast) that are embedded in an amorphous matrix and have a size between 5 and 30 nm. As analyzed by HRTEM and sa diffraction methods most of the larger 2. Experimental procedure nanocrystals contain B19 martensite whereas the smaller crystallites have the B2 structure. The contrast is reversed In the present study a binary NiTi alloy with a in the dark held image of Fig. I(b)that was achieved by nominal composition Ni-50.at. %Ti was used. The de- placing an objective aperture over part of the( 110)B2, tails of the alloy preparation and the transformation(11)Big and (020)Big diffraction rings of the nano- temperatures are given by [8]: the initial coarse grained crystals. a band shaped area that contains only a very alloy shows a single step transformation from B2 aus- small volume fraction(<1%)of crystallites that have a tenite to monoclinic B19 martensite(the martensite diameter in the range of 5-15 nm is marked by L; an area finish temperature Mr is in the range from 22 to 45C containing a higher density of mainly larger crystals is depending on the thermo-mechanical treatment prior to marked by H. In the diffraction pattern of Fig. I(c)the The alloy was quenched from 800oC in water below by the rather sharp diffraction rings of the crystallo o he transformation) amorphous phase gives rise to diffuse rings superimpose r and used to prepare HPT discs applying 10 turns at a seen g. 2 almost the entire volume of the pressure of 6 GPa. The as-processed HPT discs had a specimens deformed at the higher strain S=7.3 is diameter of 12 mm and a thickness of about 0. 2 mm. amorphous. A low density (less than about 1%)of From the hPt discs specimens with a diameter of 2.3 nanocrystals is observed in bright and dark field images mm were punched by spark erosion using very low (cf. Figs. 2(a) and(b)). Their analysis shows that the power to avoid any heating. To study the influence of nanocrystals are heterogeneously distributed; areas are strain on the amorphization specimens were taken at observed containing a higher density of nanocrystal two different distances from the centre of the hPt discs:(e.g. near h) whereas other areas are almost free of them about 2.5 and 4.3 mm; this corresponds to true loga-(e.g. near L). The crystallites have a size of less than rithmic strains S of about 6.7 and 7.3, respectively, at the about 15 nm and using HRTEM methods some of them ntral area of the TEM specimens. To achieve different with a diameter of only 3 nm were analyzed. In the grain sizes two different heat treatments were carried diffraction pattern(see Fig. 2(c))diffuse rings are ob- out: the specimens were annealed either at 340C for 5 h served; the radius of the inner, bright ring correspond or at 450C for I h under vacuum. The heat treatment to 4.7 nm and therefore to the reflection gllo of the was followed by cooling to RT and by quenching either NiTi B2 lattice; additional rings being weak and broad into methanol at a temperature of -25C or into liqu correspond to g211 and g220(as indicated in Fig. 2(c)) nitrogen. The peak temperature Tp and the onset tem- The analysis of several diffraction patterns shows that perature Tx of the crystallization were measured by frequently the first diffuse ring is superimposed by weak differential scanning calorimetry(DSC)at a heating rate(1 10)B2 reflection spots of the nanocrystals In addition

stresses caused by lattice defects rather than to the ultra- fine grain size it is the aim of the present study to carry out a systematic investigation. Therefore a nanocrystalline NiTi alloy was investigated containing little lattice strains and almost no dislocations and having a broad range of grain sizes (from about 5 to 350 nm). In a first step, the NiTi alloy showing the monoclinic B190 martensite phase at room temperature (RT) was subjected to HPT defor￾mation achieving a bulk amorphous alloy. As compared to mechanically amorphized powders no further com￾paction or densification is necessary and contrary to cold rolling almost complete amorphization can be achieved by the HPT method. In a second step, a nanocrystalline microstructure was obtained by annealing the alloy close to the crystallization temperature. To investigate the nanocrystallization, two different degrees of HPT defor￾mation were carried out and the microstructures were analysed carefully prior and after the devitrification. In addition, at RT the transformation induced microstruc￾ture and its dependence on the grain size was investigated in detail using both transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) methods. 2. Experimental procedure In the present study a binary NiTi alloy with a nominal composition Ni–50.3at.%Ti was used. The de￾tails of the alloy preparation and the transformation temperatures are given by [8]: the initial coarse grained alloy shows a single step transformation from B2 aus￾tenite to monoclinic B190 martensite (the martensite finish temperature Mf is in the range from 22 to 45 C depending on the thermo-mechanical treatment prior to the transformation). The alloy was quenched from 800 C in water below Mf and used to prepare HPT discs applying 10 turns at a pressure of 6 GPa. The as-processed HPT discs had a diameter of 12 mm and a thickness of about 0.2 mm. From the HPT discs specimens with a diameter of 2.3 mm were punched by spark erosion using very low power to avoid any heating. To study the influence of strain on the amorphization specimens were taken at two different distances from the centre of the HPT discs: about 2.5 and 4.3 mm; this corresponds to true loga￾rithmic strains S of about 6.7 and 7.3, respectively, at the central area of the TEM specimens. To achieve different grain sizes two different heat treatments were carried out: the specimens were annealed either at 340 C for 5 h or at 450 C for 1 h under vacuum. The heat treatment was followed by cooling to RT and by quenching either into methanol at a temperature of )25 C or into liquid nitrogen. The peak temperature Tp and the onset tem￾perature Tx of the crystallization were measured by differential scanning calorimetry (DSC) at a heating rate of 40 C/min in flowing nitrogen gas using a Perkin– Elmer DSC 7. The specimens were used to prepare TEM foils by twin-jet polishing. The thinning was done in a Tenupol 3 with a solution of 75% CH3OH and 25% HNO3 ()22 C and 15 V). The different phases were analyzed by se￾lected area (SA) diffraction applying different beam di￾rections (BD) in a TEM (Philips CM 200 operating at 200 kV). Afterwards a HRTEM analysis was carried out using a Philips CM30 ST (operating at 250 and 300 kV) equipped with a Gatan slow scan CCD camera. 3. Experimental results 3.1. The nanostructured amorphous phase after HPT of different strains Fig. 1 shows a TEM study of a specimen immediately after the HPT deformation at a strain of S ¼ 6:7. As seen in Fig. 1(a) (bright field image) the specimen contains heterogeneously distributed nanocrystals (spots of dark contrast) that are embedded in an amorphous matrix and have a size between 5 and 30 nm. As analyzed by HRTEM and SA diffraction methods most of the larger nanocrystals contain B190 martensite whereas the smaller crystallites have the B2 structure. The contrast is reversed in the dark field image of Fig. 1(b) that was achieved by placing an objective aperture over part of the h110iB2, h
111iB190 and h020iB190 diffraction rings of the nano￾crystals. A band shaped area that contains only a very small volume fraction (<1%) of crystallites that have a diameter in the range of 5–15 nm is marked by L; an area containing a higher density of mainly larger crystals is marked by H. In the diffraction pattern of Fig. 1(c) the amorphous phase gives rise to diffuse rings superimposed by the rather sharp diffraction rings of the crystallites. As seen in Fig. 2 almost the entire volume of the specimens deformed at the higher strain S ¼ 7:3 is amorphous. A low density (less than about 1%) of nanocrystals is observed in bright and dark field images (cf. Figs. 2(a) and (b)). Their analysis shows that the nanocrystals are heterogeneously distributed; areas are observed containing a higher density of nanocrystals (e.g. near H) whereas other areas are almost free of them (e.g. near L). The crystallites have a size of less than about 15 nm and using HRTEM methods some of them with a diameter of only 3 nm were analyzed. In the diffraction pattern (see Fig. 2(c)) diffuse rings are ob￾served; the radius of the inner, bright ring corresponds to 4.7 nm
1 and therefore to the reflection g110 of the NiTi B2 lattice; additional rings being weak and broad correspond to g211 and g220 (as indicated in Fig. 2(c)). The analysis of several diffraction patterns shows that frequently the first diffuse ring is superimposed by weak h110iB2 reflection spots of the nanocrystals. In addition 138 T. Waitz et al. / Acta Materialia 52 (2004) 137–147

T. Waitz et al. Acta Materialia 52(2004)137-147 200nm 200nm L H ⊙ 5nm g Fig. 1. Ni-50.3at %Ti after HPT deformation; strain S=6.7.(a) TEM 0.3at %Ti after HPt deformation; strain S= 7.3.(a) TEM bright field image. The amorphous matrix contains retained nano- ous matrix contains a volume fraction als showing dark contrast. P marks a coarse particle of the Ti2Ni of retained nanocrystals of less than about I %.(b) Crystallites having a lattice.(b) TEM dark field image of the heterogeneously distributed diameter of less than about 15 nm she ystallites showing bright contrast(H and L mark areas containing a corresponding TEM dark field image. H marks a higher density of high and low density of them).(c) In the diffraction pattern broad nanocrystals whereas in the area marked L there are almost no fuse rings of the amorphous phase are superimposed with rings nanocrystals. (c)Sa diffraction pattern showing broad diffuse rings of containing B2 and B19 diffraction spots of the nanocrystallites. he amorphous phase having radii that correspond to the length of the diffraction vectors(1 10),(211>and(220) of the B2 lattice

Fig. 2. Ni–50.3at.%Ti after HPT deformation; strain S ¼ 7:3. (a) TEM bright field image. The amorphous matrix contains a volume fraction of retained nanocrystals of less than about 1%. (b) Crystallites having a diameter of less than about 15 nm show up as bright spots in the corresponding TEM dark field image. H marks a higher density of nanocrystals whereas in the area marked L there are almost no nanocrystals. (c) SA diffraction pattern showing broad diffuse rings of the amorphous phase having radii that correspond to the length of the diffraction vectors h110i, h211i and h220i of the B2 lattice. Fig. 1. Ni–50.3at.%Ti after HPT deformation; strain S ¼ 6:7. (a) TEM bright field image. The amorphous matrix contains retained nano￾crystals showing dark contrast. P marks a coarse particle of the Ti2Ni lattice. (b) TEM dark field image of the heterogeneously distributed crystallites showing bright contrast (H and L mark areas containing a high and low density of them). (c) In the diffraction pattern broad diffuse rings of the amorphous phase are superimposed with rings containing B2 and B190 diffraction spots of the nanocrystallites. T. Waitz et al. / Acta Materialia 52 (2004) 137–147 139

T. Waitz et al. Acta Materialia 52(2004)137-14 to the amorphous rings weak(200)B2 reflections were observed. It should be mentioned that a volume fraction of about 5% corresponds to coarse spherical particles of the Ti2 Ni lattice that have survived the hpt in both cases S=6.7 and 7.3(cf. P in Fig. 1(a) 3. 2. Nanocrystallization after different heat treatments The onset of the crystallization was measured by DSC (cf. Fig 3). The analysis of the dSc curve of the HPT specimen with the lower strain(S= 6.7) yields that both the crystallization temperature and the crystall zation enthalpy are lower than in the case of s=7.3. Tx (Tp)are 352C(374C)and 362C(379C)in the case of S=6.7 and 7.3, respectively. The crystallization en- harpies△H67and△H73 are about-1.4and-1.7kJ mol corresponding to S=6.7 and 7.3, respectively Fig. 4 shows TEM bright field images of annealed specimens. Figs. 4(a) and(b)correspond to the defo mations S=6.7 ands=7.3, respectively; in addition the R pecimens were isothermally annealed at a temperature T=352°c 0,0 S=6.7 -0.5 374°C 00 Temperature['CI 0.5 T=362°c 0,0 0.5 Fig 4 Nanocrystalline phase formed after isothermal annealing of HPT deformed amorphous Ni-503at %Ti. TEM bright field images. T=379°c (a)S=6.7 after annealing at 340C for 5 h. Most of the grains are -1.0 smaller than about 50 nm containing both B2 and R-phase.(b 7.3 after annealing at 340C for 5 h. Frequently areas are Temperature[C] observed that contain mainly smaller grains (e.g. near S) and are adjacent to areas where larger gra dominating (e. g. near M Fig. 3. DSC curves of the crystallization of HPT deformed Ni- B2 phase and R-phase are found in the smaller grains near S. R 503at%Ti (heating rate 40C/min). (a)S=6.7. An exothermic peak marks a grain of the R-phase. Martensite occurs in the grains near occurs at Tp=374C; the crystallization temperature T=352C is M having a diameter of about 120 nm.(c)S=7.3 after annealing at indicated.(b)S=7.3. The crystallization occurs at a higher tempera- 450.C for I h. Almost all grains larger than about 150 nm contain ture(p=379°C,Tx=362°C

to the amorphous rings weak h200iB2 reflections were observed. It should be mentioned that a volume fraction of about 5% corresponds to coarse spherical particles of the Ti2Ni lattice that have survived the HPT in both cases S ¼ 6:7 and 7.3 (cf. P in Fig. 1(a)). 3.2. Nanocrystallization after different heat treatments The onset of the crystallization was measured by DSC (cf. Fig. 3). The analysis of the DSC curve of the HPT specimen with the lower strain (S ¼ 6:7) yields that both the crystallization temperature and the crystalli￾zation enthalpy are lower than in the case of S ¼ 7:3. Tx (Tp) are 352 C (374 C) and 362 C (379 C) in the case of S ¼ 6:7 and 7.3, respectively. The crystallization en￾thalpies DH6:7 and DH7:3 are about )1.4 and )1.7 kJ/ mol corresponding to S ¼ 6:7 and 7.3, respectively. Fig. 4 shows TEM bright field images of annealed specimens. Figs. 4(a) and (b) correspond to the defor￾mations S ¼ 6:7 and S ¼ 7:3, respectively; in addition the specimens were isothermally annealed at a temperature Fig. 3. DSC curves of the crystallization of HPT deformed Ni– 50.3at.%Ti. (heating rate 40 C/min). (a) S ¼ 6:7. An exothermic peak occurs at Tp ¼ 374 C; the crystallization temperature Tx ¼ 352 C is indicated. (b) S ¼ 7:3. The crystallization occurs at a higher tempera￾ture (Tp ¼ 379 C, Tx ¼ 362 C). Fig. 4. Nanocrystalline phase formed after isothermal annealing of HPT deformed amorphous Ni–50.3at.%Ti. TEM bright field images. (a) S ¼ 6:7 after annealing at 340 C for 5 h. Most of the grains are smaller than about 50 nm containing both B2 and R-phase. (b) S ¼ 7:3 after annealing at 340 C for 5 h. Frequently areas are observed that contain mainly smaller grains (e.g. near S) and are adjacent to areas where larger grains are dominating (e.g. near M). B2 phase and R-phase are found in the smaller grains near S. R marks a grain of the R-phase. Martensite occurs in the grains near M having a diameter of about 120 nm. (c) S ¼ 7:3 after annealing at 450 C for 1 h. Almost all grains larger than about 150 nm contain martensite. 140 T. Waitz et al. / Acta Materialia 52 (2004) 137–147

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 and

of 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