Martensite and its reverse transformation in nanocrystalline bulk Co

Materials Science and Engineering A 438–440 (2006) 420–426

Martensite and its reverse transformation in nanocrystalline bulk Co Chunsheng Wen, Baoxu Huang, Zi Chen, Yonghua Rong ∗ School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China Received 4 July 2005; received in revised form 19 January 2006; accepted 14 February 2006

Abstract A nanostructured surface layer of Co with the thickness of about 20 ␮m, considered as a bulk sample, was prepared by means of surface mechanical attrition treatment (SMAT). The average grain sizes of the samples prepared by 30 and 60 min SMAT are determined as 26 and 23 nm, respectively, by X-ray diffraction, and confirmed by transmission electron microscopy. Differential scanning calorimetry analysis for the above samples and a coarse-grained sample reveals that start temperature As of the ␣ (hcp) → ␤ (fcc) reverse martensitic transformation can be described as: TAS = 456–293/d (in ◦ C, 15 nm ≤ d ≤ 100 nm, d is grain size). The nanocrystalline high-temperature ␤ (fcc) phase with grain size smaller than about 35 nm obtained by heating SMAT samples for proper duration exhibits thermal stability during cooling from 500 ◦ C to ambient temperature even at −196 ◦ C. However, these thermally stable nanocrystalline ␤ (fcc) phase samples can undergo the ␤ (fcc) → ␣ (hcp) martensitic transformation when treated by SMAT again. Thermal stability of the nanocrystalline low-temperature phase ␣ (hcp) was observed in SMAT Co, that is, when the grain sizes are smaller than 15 nm, the reverse transformation will not occur until to 815 ◦ C. © 2006 Elsevier B.V. All rights reserved. Keywords: Surface mechanical attrition treatment; Differential scanning calorimetry; Reversal transformation; Thermal stability; Nanocrystalline Co bulk

1. Introduction Phase transformations in nanocrystalline materials have attracted considerable scientific interest in the past decade because phase transformation behaviors in nanocrystalline materials differ from coarse-grain bulk materials [1–8]. However, the investigated materials are almost limited in nanosized particles (grains), powders or films due to the difficulties in synthesizing 3-dimensional bulk nanocrystalline samples without porosity, contamination. For example, the inert gas condensation and in situ warm consolidation technique cannot satisfy the above requirement of nanocrystalline bulk in addition to the complexity of technology [9]. Surface mechanical attrition treatment (SMAT), accomplished by surface shot peening treatment, creates localized plastic deformation which leads to grain refinement progressively down to the nanometer region in the surface layer of metallic materials [10]. It has been successfully applied in many material systems [11,12] to achieve nanocrystallization surface layer with 15–50 ␮m thickness. While 15–50 ␮m thickness surface implies that there exist 150–500 grains with 100 nm diame-

∗

Corresponding author. Tel.: +86 21 62932558; fax: +86 21 62933910. E-mail address: [email protected] (Y. Rong).

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.191

ter in the thickness direction, equivalent to 1.5–5 mm thickness sample with 15–50 ␮m diameter grains (traditional coarse grain size range). In other words, the nanocrystallization surface with 15–50 ␮m thickness can be considered as a nanocrystalline bulk sample. As a consequence, SMAT will supply a new method for the preparation of nanocrystalline bulk samples without porosity, contamination and change of composition to investigate phase transformations. The comparison of phase transformation in nanocrystalline bulk materials prepared by SMAT with its traditional coarse-grain bulk ones will be simpler and more direct since their difference is only in grain size. Pure Co is of typical ␤ (fcc) → ␣ (hcp) phase transformation (also called martensite transformation), in the present work we will study the martensite transformation and its reversal transformation in nanocrystalline Co bulk by combination of SMAT and heat treatment. 2. Experimental Pure Co plate was first heated at 500 ◦ C for 60 min following water quenched to obtain the uniform grain size. Samples of 70 mm × 70 mm × 5 mm were prepared for SMAT at 50 Hz frequency with spherical stainless steel balls of 8 mm diameter. In this work, all samples were treated at ambient temperature, and the treatment durations are 30 and 60 min, respectively.

C. Wen et al. / Materials Science and Engineering A 438–440 (2006) 420–426

The start temperature As of ␣ (hcp) → ␤ (fcc) reversal martensite transformation in SMAT and quenched samples were measured by differential scanning calorimetry (DSC) with scanning rate of 20.0 K/min on Perkin-Elmer DSC7 (20–500 ◦ C) and Netzsch DSC404 (20–1000 ◦ C) instruments, respectively. The 60 min SMAT samples with about 0.2 mm thickness from surface layer were sealed in a vacuum quartz glass tube filled with argon gas to avoid the oxidation, and then were heated up to 500 ◦ C for different holding time following water quenched to investigate the effect of grain size on ␤ (fcc) → ␣ (hcp) martensite transformation. Lattice structure of the surface layer was studied by X-ray diffraction (XRD) on a D8 Discover with GADDS X-ray diffractometer with Cr K␣ radiation and the scanning extent of diffraction angle (2θ) was between 40◦ and 140◦ . Microstructures of the SMAT samples were examined using optical microscopy (OM) and transmission electron microscopy (TEM) on a JEM-100CX microscope. The thin foils for DSC and TEM were prepared first by cutting the corresponding surface layer and then mechanically polishing the sample from the untreated side down to about 50 ␮m thickness. 3. Results and discussion 3.1. Microstructure of the surface layer obtained after SMAT Fig. 1 shows XRD patterns of one quenched and two SMAT Co samples. The obvious Bragg-diffraction peak broadening and shifting in spectrum “B” and “C” compared with “A” may be attributed to grain refinement and/or an increase of microstrain. Quantitative XRD results (using the Scherrer formula, 1 0 1␣ selected) indicate that the average grain sizes in top surface layer with thickness of about 20 ␮m are, respectively, 26 nm for 30 min SMAT, and 23 nm for 60 min SMAT. The average grain size in the water-quenched sample is too large to be accurately measured by XRD.

Fig. 1. XRD patterns of the surface layer of Co metal before and after SMAT.

421

Fig. 2. Morphology of hcp (␣) in Co metal water quenched after 500 ◦ C heated for 60 min.

Dark field image in Fig. 2 shows the typical morphology of hcp (␣) in coarse-grain Co metal based on the diffraction pattern inserted (belong to [2 1¯ 1¯ 0]α zone), a lot of parallel strips can be seen clearly. The cross sectional optical morphology of the Co sample after SMAT 60 min is exhibited in Fig. 3, the plastic deformation degree decreases along the depth, and the severe plastic deformation just occurs in the region from top surface to about 20 ␮m depth (pointed by arrow in Fig. 3) though deformation trace can be found at about 50 ␮m depth from top surface. The grain sizes in undeformed region are about 30–50 ␮m (average grain size of the water-quenched sample). To study the effect of the inhomogeneous plastic deformation on the microstructure, TEM observation was carried out on the surface layers of the 30 and 60 min SMAT samples, respectively, as shown in Fig. 4. It is found that the top surface layer consists of roughly equiaxed nanocrystalline grains, selected area electron diffraction patterns indicate that these nanocrystalline grains possess random crystallographic orientations based on their ring diffraction patterns inserted. Compared the size distribution of nanocrystalline grains in Fig. 4(a and b), the fraction of small size grains in the 30 min SMAT sample are less than that of 60 min SMAT sample. The average grain size of the nanocrystalline

Fig. 3. Cross sectional optical micrograph of sample by SMAT 60 min.

422

C. Wen et al. / Materials Science and Engineering A 438–440 (2006) 420–426

Fig. 4. Nanosized grains in the surface layer of SMAT sample for different time: (a) 30 min, (b) 60 min.

grains is about 23 nm (30 min) and 16 nm (60 min), respectively, based on the statistics results of Fig. 4(a and b). They are slightly smaller than the measurement results of XRD in Fig. 1, which is a reasonable result since measurement of XRD refers to the average grain size of the top surface layer with several ␮m depth. 3.2. Effect of grain size on As Fig. 5 shows the DSC curves of three samples with different grain sizes using Perkin-Elmer DSC7, i.e., a water-quenched and two SMAT samples. An endothermal peak in each heating curve of Fig. 5(a) indicates the occurrence of ␣ (hcp) → ␤ (fcc) transformation. Comparing the curves, As lowers from 456.50 → 448.83 → 441.50 ◦ C with the decrease of grain size from 50 ␮m → 26 nm → 23 nm. The effect of grain size on As can be described as TAS = 456 −

k ◦ ( C) d

(1)

where d is grain size (measured in nm scale), k is a constant mainly depended on grain boundary energy and the latent heat of phase transformation, and determined as 293 by fitting. To examine the possible influence of dislocations and/or defects produced by SMAT on As , tensile test was carried out

Fig. 5. DSC curve (20–500 )of Co metal before and after SMAT (a) heating course (b) cooling course.

for coarse-grain Co sample, and the severe deformation fragment cut from tensile test specimen was used to measure the DSC curve, as shown in “D” of Fig. 5(a). Comparing “D” with “A” in Fig. 5(a), the As of severe deformation fragment is a little higher than that of water-quenched sample (which might be that the strength increase of matrix ␣ (hcp) under the unidirectional tensile condition and suspends its reverse transformation), as a consequence, we can infer that the variation tendency of As indeed results from the change of grain size in nanometer scale rather than crystal defects. Comparing their cooling curves of Fig. 5(b), however, it can be found that there exist an exothermal peak only in the water quenched sample and no exothermal peaks in SMAT samples. It implies that in the coarse-grain sample the ␤ (fcc) → ␣ (hcp) martensite transformation occurs completely during cooling, but nanocrystalline ␤ (fcc) grains in SMAT samples show excellent thermal stability at room temperature. The conclusion from some experiments reported [16–19] is that the grain size almost cannot affect As of the reverse martensite transformation. However, our experiments demonstrate the effect of grain size on As . The difference between experimen-

C. Wen et al. / Materials Science and Engineering A 438–440 (2006) 420–426

Fig. 6. XRD patterns of the surface layer of Co metal SMAT 60 min heated at 500 ◦ C for different holding time and water quenched afterwards.

tal results may result from experimental conditions required. In order to reveal the effect of grain size on the reverse transformation, the following experimental conditions in nanocrystalline samples should be considered: good resistance of oxidation and no porosity, contamination. In addition, the nanocrystalline grains in samples do not easily grow during heating up to As . The SMAT nanocrystalline Co samples keep the features of its coarse-grain samples, including no porosity, contamination and good resistance of oxidation, meanwhile, the As of cobalt is lower than 500 ◦ C, and such a low-temperature makes nanocrystalline grains not easily grow during heating. This is why we choose SMAT Co to investigate the characteristics of the reverse martensite transformation. 3.3. Effect of grain size on martensite transformation In order to study the effect of grain size on martensite transformation, the experiments were designed to obtain hightemperature phase ␤ (fcc) grains with different nanometer scale sizes through the reversal martensite transformation of ␣ (hcp) nanocrystalline grains in a SMAT Co. Fig. 6 demonstrates a series of XRD patterns of 60 min SMAT Co samples and then heated at 500 ◦ C for different holding time following water quenched. It means that ␣ (hcp) nanocrystalline grains in SMAT samples undergo the reverse martensite transformation during heating. Based on the above XRD patterns, ␤ (fcc) phase in SMAT samples with nanocrystalline grains keeps stable during cooling down to room temperature, the volume fraction and average grain size of ␤ (fcc) phase were calculated, as shown in Fig. 7, and the average grain size of ␣ (hcp) phase is also shown in Fig. 7. It is clear that the volume fraction of ␤ (fcc) increases with the holding time up to a maximum of about 17% obtained at about 30 min holding time, then decrease to a certain value of about 11%, after that the volume fraction of ␤ (fcc) almost keeps unchanged with the increase of holding time. At the same time, the average grain sizes of ␤ (fcc) and ␣ (hcp) show little changes within holding time of 30 min, about 35 and 15 nm, respectively,

423

Fig. 7. Volume fraction and grain size of phases in Co metal SMAT 60 min with different holding time at 500 ◦ C afterwards.

and then they will both increase with the increase of holding time. Fig. 8 exhibits a series of XRD patterns of coarse-grain Co samples heated at 500 ◦ C for different time following water quenched. It is clear that ␣ (hcp) with coarse grains transforms to ␤ (fcc) during heating, then transforms to ␣ (hcp) completely from ␤ (fcc) again during cooling. While ␤ (fcc) in SMAT samples with nanosized grains keep stable during cooling down to room temperature (see Figs. 6 and 7). These results are well consistent with those in DSC, namely, during cooling ␤ (fcc) completely transforms to ␣ (hcp) martensite only in coarse grains, in other words, the martensitic transformation is suppressed in SMAT samples with nanocrystalline grains. It is worthy to point out that only 17% or less of ␤ (fcc) phase for SMAT samples is obtained during 500 ◦ C heated for different time. The results are out of our prediction, even so, it is believed that the majority of martensites are not the product of martensite transformation during cooling, which is reflected by almost no exothermal peaks in DSC cooling curves of SMAT samples, and the relative research will be described in Section 3.4.

Fig. 8. XRD patterns of the surface layer of coarse grains Co metal heated at 500 ◦ C for different holding time and water quenched afterwards.

424

C. Wen et al. / Materials Science and Engineering A 438–440 (2006) 420–426

Fig. 9. XRD patterns of Co surface layer treated by A (cooled into liquid nitrogen) and B (continue SMAT 30 min) after holding 30 min at 500 ◦ C following SMAT 60 min.

Many researches [2,20–22] reveal that martensite transformation in some materials, such as Fe, Co, Fe–Ni alloy and ZrO2 , will be suppressed when their grain size is less than a certain critical grain size. For example, the ␤ (fcc) → ␣ (hcp) martensite transformation in Co will be suppressed and high-temperature ␤ (fcc) keeps stable at room temperature when its grain size is less than the critical grain size of 35 nm predicted by our theory. Our experiments confirm the above conclusion. For example, the volume fraction of ␤ (fcc) phase decreases from about 17 to 11% with the increase of grain size from about 35 to over 45 nm (Fig. 7). While for the samples holding time over 120 min, ␤ (fcc) still keeps stable at ambient temperature although the grain sizes of these samples are larger than the critical grain size, which may be attributed to the excellent thermal stability of nanocrystalline grains, rather than oxidation effect, and this deduction is made by comparing with coarse samples (Fig. 8). When these samples were cooled to −196 ◦ C in liquid nitrogen, martensite transformation still does not occur, but does when these samples suffered SMAT again (see XRD results in Fig. 9). It indicates that ␤ (fcc) high-temperature phase with nanocrystalline grains has a good thermal stability, but is metastable under stress. 3.4. Thermal stability of nanocrystalline α (hcp) phase at high temperature When SMAT samples were heated at 500 ◦ C for different time, only 17% or less amount of ␤ (fcc) phase was obtained based on the result of Fig. 7. It can be deduced that nanocrystalline ␣ (hcp) martensite in SMAT samples might not completely transform into ␤ (fcc) phase during heating, which is quite different from coarse-grain samples. Considering the ␣ (hcp) grain sizes of different holding time in Fig. 7, they are much smaller than those of ␤ (fcc) in each corresponding sample, we can infer that there might exist a critical grain size, below which reversal transformation does not occur. According to the results in Fig. 7, the critical grain size of the reverse transfor-

Fig. 10. Nano grains (␤ (fcc) and ␣ (hcp)) in the surface layer of Co metal held for 2 min at 500 ◦ C after SMAT 60 min, (a) g = (0 0 2)␣ , (1 1 1)␤ , (b) selected area diffraction pattern.

mation for ␣ (hcp) can be determined as 15 nm. In other words, most of the grain sizes of ␣ (hcp) in SMAT sample surface layer are smaller than 15 nm, they will show excellent thermal stability, and if the SMAT sample is heated to higher temperature, the ␣ (hcp) → ␤ (fcc) reversal transformation will occur. The prediction will be verified by following experiment. While the grain sizes are larger than 15 nm, reversal transformation begin to occur above As . Considering the thermal stability of ␣ (hcp) with grain size less than 15 nm, the effect of grain size in nanometer scale on the As temperature in Eq. (1) should be approximately revised as: TAS = 456 −

293 d

(15 nm ≤ d ≤ 100 nm, d is grain size.) (2)

During cooling, martensite transformation will occur in the ␤ (fcc) phases whose sizes are larger than 35 nm. Since the amount of these grains is so small (about 6%), an exothermal peak produced by the ␤ (fcc) → ␣ (hcp) transformation can hardly be observed in the cooling curves of DSC in Fig. 5(b). In order to confirm the existence of plenty of ␣ (hcp) without undergoing the reverse transformation, TEM investigations were carried out. Fig. 10(a) exhibits the nanocrystalline grains

C. Wen et al. / Materials Science and Engineering A 438–440 (2006) 420–426

Fig. 11. Nano grains (␣ (hcp)) in the surface layer of Co metal held for 2 min at 500 ◦ C after SMAT 60 min, (a) g = (1 0 0)␣ , (b) selected area diffraction pattern.

in the surface layer of Co metal held for 2 min at 500 ◦ C after SMAT 60 min, they were identified as a mixture of ␣ (hcp) and ␤ (fcc) by the selected area diffraction pattern (Fig. 10(b)). Some areas with majority of ␣ (hcp) were selected to be observed. Fig. 11 shows that most of ␣ (hcp) grains are less than 15 nm, which is well consistent with XRD results. Similar to the stability of high-temperature phase at low temperature, for stability of low-temperature phase at high temperature there also exists a critical grain size. This phenomenon needs to be confirmed by more experiments and be explained by theory. In order to reveal the stability of ␣ (hcp) with grain size less than 15 nm, high temperature DSC (20–1000 ◦ C)measurement was performed on the NETZSCH DSC404 instrument with scanning rate of 20.0 ◦ C/min, as shown in Fig. 12. It is obvious that there is another endothermal peak at about 815 ◦ C in SMAT samples (B and C) in addition to at about 436.9 ◦ C.In DSC heating curve of SMAT sample B with a thicker surface layer (larger grains) than SMAT sample C, in addition, an endothermal peak at about 436.9 ◦ C results from reversal transformation of ␣ (hcp) grains larger than 15 nm, while an endothermal peak at about 815 ◦ C results from that of plenty of ␣ (hcp) grains less than 15 nm. In DSC heating curve of sample A with coarse-grains there exists only one endothermal peak at about 459.5 ◦ C. The

425

Fig. 12. DSC curve (20–1000 ◦ C) of Co metal before and after SMAT(a) heating course, (b) cooling course.

experiment strongly verified our prediction on the stability of ␣ (hcp) grains less than 15 nm. While in DSC cooling curves, there exist only one exothermal peak, moreover, the exothermal amount per gram increases with the increase of grain size, which implies that some of ␤ (fcc) in SMAT samples keep stable during the cooling from 1000 to 20 ◦ C. 4. Conclusions (i) Co nanocrystalline surface layer without porosity and contamination can be prepared through SMAT, moreover, different average sizes of nanograins in the 20 ␮m thickness surface layer were obtained by changing SMAT time, accordingly, SMAT can supply a good nanocrystalline bulk sample to investigate the effects of grain size on martensitic transformation and its reversal transformation. (ii) DSC experiment reveals that the start temperature As of ␣ (hcp) → ␤ (fcc) reversal martensite transformation in SMAT Co metal decreases with the decrease of grain size. The effect of grain size in nanometer scale on As for SMAT samples can be determined as: TAS = 456–293/d (in ◦ C, 15 ≤ d ≤ 100 nm, d is grain size).

426

C. Wen et al. / Materials Science and Engineering A 438–440 (2006) 420–426

(iii) When grain size of high-temperature ␤ (fcc) phase is less than the critical grain size (about 35 nm), it keeps stable during cooling from 500 ◦ C to room temperature, even at −196 ◦ C, however, the ␤ (fcc) → ␣ (hcp) martensite transformation will occur when this sample suffered SMAT, which indicates that ␤ (fcc) high-temperature phase with nanocrystalline grains has a good thermal stability, but is metastable under stress. (iv) Thermal stability of nanocrystalline low-temperature phase ␣ (hcp) at high temperature has been found in SMAT Co sample, that is, when the grain sizes of ␣ (hcp) are smaller than 15 nm, it will not occur for reversable transformation until to 815 ◦ C and ␣ (hcp) shows excellent thermal stability.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [16] [17]

Acknowledgements The present work was supported financially by the Science and Technology Foundation of Shanghai under Grant no. 0210nm017, the author also wants to show his sincere thanks to Ph.D student Kaiyun Zheng for the kind help during the experiment.

[18] [19] [20] [21] [22]

R.E. Cech, D. Turnbull, Trans. AIME 206 (1956) 124. Y. Bando, Trans. JIM 5 (1964) 135. S. Kajiwara, S. Ohno, K. Honma, Phil. Mag. A63 (1991) 625. Y.H. Zhou, M. Harmelin, J. Bigot, Mater. Sci. Eng. A133 (1991) 775. T. Tadaki, Y. Murai, A. Koreeda, et al., Mater. Sci. Eng. A217/218 (1996) 235. K. Asaka, Y. Hirotsu, T. Tadaki, Mater. Sci. Eng. A273/275 (1999) 262. Y.H. Rong, Q.P. Meng, T.Y. Hsu, PRICM-4, December11–15, Hawaii, USA, 2001. C. Kuhrt, L. Schultz, J. Appl. Phys. 73 (1993) 1975. H.B. Wang, J.H. Zhang, T.Y. Hsu, Mater. Sci. Eng. A 380 (2004) 408. K. Lu, J. L¨u, J. Mater. Sci. Technol. 15 (1999) 193. K. Lu, J.L. L¨u, Mater. Sci. Eng. A375–377 (2004) 38. G. Liu, S.C. Wang, X.F. Lou, et al., Scripta Mater. 44 (2001) 1791. Y.H. Zhou, M. Harmelin, J. Bigot, Mater. Sci. Eng. A133 (1991) 775. Y.H. Rong, Q.P. Meng, T.Y. Hsu, Proceedings of the Fourth Pacific Rim Conference On Advanced Materials and Processing, Jpn. Inst. Metals, vol. 1, 2001, p. 147. O. Kitakami, H. Sato, Y. Shimada, Phys. Rev. B 56 (1997) 13849. Q.P. Meng, Y.H. Rong, T.Y. Hsu, Acta Mater. 30 (2002) 467. R.C. Garvie, J. Phys. Chem. 69 (1965) 1238. Q.P. Meng, Y.H. Rong, T.Y. Hsu, Sci. China 45 (5) (2002) 485. Y.H.Rong, Q.P.Meng, Y.L.Zhang, T.Y.Hsu(Xu Zuyao). Phase stability and its intrinsic conditions in nanocrystalline materials, (Keynote in ICOMAT’05).