Dynamic observation of twin evolution during austenite grain growth in ...

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H. Hu et al.: Dynamic observation of twin evolution during austenite grain growth in an Fe–C–Mn–Si alloy

Haijiang Hua , Guang Xua , Feng Liua , Li Wangb , Lixin Zhouc , Zhengliang Xuea a Key

Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Hubei Collaborative Innovation Center for Advanced Steels,Wuhan University of Science and Technology, Wuhan, China b State Key Laboratory of Development and Application Technology of Automotive Steels (Baosteel Group), Shanghai, China c Daye Special Steel Company Limited, Hubei Xinyegang Steel Company Limited, Huangshi, China

Dynamic observation of twin evolution during austenite grain growth in an Fe–C–Mn–Si alloy For the first time, investigation of twin evolution during austenite grain growth in an Fe–C–Mn–Si alloy was conducted with in-situ observation using high temperature laser scanning confocal microscopy during austenization. It was found that twins nucleated in austenite grains, some of which might disappear during isothermal holding, even as new twins might nucleate at grain boundaries. In addition, the effects of twins and austenite grains on bainite morphology were also examined. The twins can restrain the growth of bainite sheaves and smaller austenite grains result in shorter bainite laths. The investigation of dynamic evolution of twins and the analysis of the effect of austenite twins on bainite morphology can only be realized by means of insitu observation. Keywords: Austenite twin; Dynamic observation; Austenite grain; Bainite morphology

1. Introduction Carpenter and Tamura [1] first observed annealing twins in a variety of deformed and annealed face-centered cubic (fcc) metals and alloys. Since then, numerous investigations have been conducted on deformation and annealing twins [2 – 13]. Cahoon et al. [2] studied the thermodynamic parameters involved in the formation of annealing twins of different metals after plastic straining. Song et al. [3] observed the annealing twins of cold rolled samples in a Pbbase alloy by using the electron back-scattered diffraction method (EBSD). Cheng et al. [4] investigated the formation of austenite annealing twins from the ferrite phase during aging in Fe–Mn–Al alloys by using optical microscopy (OM) and transmission electron microscopy (TEM). Baudin et al. [5] and Bystrzycki et al. [6] analyzed the annealing twins in copper wires and NiMn2 alloy, respectively. The above-mentioned studies focused on annealing twins, in addition, some research has been conducted on deformation twins. Mahajan [7] discussed the formation and growth of deformation and annealing twins in fcc materials. Zhang et al. [8] investigated the effect of the stacking-fault energy on deformation twin thickness in deformed Cu–Al alloys and Li et al. [9] discussed influence of grain size on the density of deformation twins in Cu-30 %Zn alloy by TEM. Field et al. [10] and Choi et al. [11] studied deformation twins by means of EBSD, Zhang et al. [12, 13] analyzed

structural change of a deformation twin in a Ti-45Al10Nb-2.5Mn (wt.%) alloy using TEM. Moreover, Mahajan et al. [14] proposed a microscopic model for the formation of annealing twins in fcc crystals. Pande et al. [15] developed a model of the mechanism of twin formation and they also presented experimental evidence for their model [16]. Over the years, different mechanisms on the formation of twins have been proposed by several researchers [17 – 24]. Summarizing existing studies, it can be said that almost all research focused on annealing and deformation twins during annealing and aging processes. Only Cheng et al. [4] observed the formation of austenite annealing twins from the ferrite phase during aging in Fe–Mn–Al alloys. So far, no study on twins during austenization has been reported. Furthermore, existing studies on annealing and deformation twins were conducted using conventional OM, TEM, and EBSD. The conventional method can only investigate the static twins at room temperature. As the morphology of bainite is affected by austenite twins, understanding the evolution of twins during austenization is important. In the present study, dynamic evolution of twins during austenite grain growth of an Fe–C–Mn–Si alloy was investigated for the first time with in-situ observation by using high temperature laser scanning confocal microscopy (LSCM). The effect of austenite twins on bainite morphology is also discussed. The dynamic evolution of twins means the formation and annihilation of twins.

2. Experimental procedure The steel in this study is an Fe–C–Mn–Si alloy with the chemical composition of 0.40 C, 2.81 Mn, 2.02 Si, and balance Fe (wt.%). The investigations were performed using an LSCM according to the test procedure shown in Fig. 1. The LSCM was a VL2000DX, a violet laser type microscope with the wavelength of 408 nm and a maximum scanning rate of 120 frames s–1. The material was refined in a vacuum induction furnace and cast into a small ingot followed by rolling to a 10 mm thick plate. Samples for LSCM were machined to a cylinder of 6-mm diameter and 4 mm height. The top and bottom surfaces of the samples were polished conventionally to maintain the measurement face level and minimize the effect of surface roughness [25]. The specimens were heated at a rate of 5 K s–1 to austenization temperatures of 1 000 and 1 100 8C and held for 30 min. Subsequently, the specimens were cooled to 330 8C and isothermally treated for 60 min for bainitic transformation fol1



Fig. 1. Experimental procedure.

lowed by final air cooling to room temperature. LSCM images were recorded continuously at 15 frames s–1 at 100 · magnification during both austenization at 1 000 and 1 100 8C and isothermal treatment at 330 8C. Moreover, after austenization at 1 100 8C and transformation at 330 8C, the crystallographic orientation of bainite sheaves and grain size were examined using the EBSD equipped in a Nova 400 Nano field emission scanning electron microscope (FESEM). The microscope was operated at an accelerating voltage of 30 kV. The step size for EBSD analysis was 0.75 lm, and was done using HKL Channel 5 software.

3. Experimental results and discussion 3.1. Dynamic observation of austenite twins The dynamic evolution of twins can be directly observed in-situ during austenization. Twins during austenization are termed austenite twins, and are presented in Fig. 2. It is shown that some twins appeared in austenite grains during austenization at 1 100 8C (as shown by A and B in Fig. 2a), some of which disappeared with holding time (as shown by A in Fig. 2b). It is known that annealing twins are stable and reluctant to disappear. However, the present study shows that austenite twins may disappear, confirming that formation and evolution of austenite twins is a dynamic process. Meanwhile, new twins appeared during austenization (as shown by arrow C in Fig. 2b). In addition, corner twins formed at the triangular grain boundaries indicated by arrow B in Fig. 2b and arrow D in Fig. 2c. Figures 2a and b are the same image field and Fig. 2c is another view field for the same sample. The twins observed during austenization in the present study are coherent twins whose morphology is a straight line under LSCM. It is very difficult to observe the growth process of twins because of their high formation rate. The appearance and disappearance of austenite twins are instantaneous, so that the twins are fully formed when they are observed (as shown by twin C in Fig. 2b). Therefore, the formation and annihilation speed of twins cannot be calculated from the frame rate of the LSCM. However, the formation and disappearance of austenite twins can be directly viewed. 2

Fig. 2. Dynamic evolution of twins during austenization at 1 100 8C. (a) Holding for 393 s; (b) holding for 740 s; (c) holding for 580 s.



Fig. 3. In-situ observed micrographs showing austenite grains of different austenization temperature and corresponding bainite morphology. (a) Austenite grain at 1 100 8C at the beginning; (b) austenite grain at 1 100 8C after austenization; (c) corresponding bainite morphology after transformation at 330 8C; (d) austenite grain at 1 000 8C at the beginning; (e) austenite grain at 1 000 8C after austenization; (f) corresponding bainite morphology after transformation at 330 8C.

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Several hypotheses have been proposed to explain the formation of twins. Some researchers assumed that twins form because of grain growth accidents [17 – 20]. Another hypothesis supposed that twins nucleate by stacking faults [21, 22]. Dash et al. [21] and Meyers et al. [22] presented an explanation for the formation of twins because of stacking-fault packets. In addition, Pande et al. [15] assumed that the formation of twins is based on the emergence of Shockley partial loops on consecutive {111} planes during grain migration. Mahajan et al. [14] tried to give a uniform explanation for the formation of annealing twins in fcc crystals and assumed that the formation of twins involves the nucleation of Shockley partial loops on consecutive {111} planes through growth accidents associated with a migrating grain boundary. The higher the velocity of the boundary, the higher is the probability of growth accidents, leading to more annealing twins. Austenite grain boundaries migrate during grain growth with isothermal holding. The twins form because of growth accidents or atomic arrangement faults on {111} planes during grain boundary migration. The twins discussed here are termed austenite twins because they nucleate during austenization. Annihilation of twins can be explained by the movement of stacking faults and dislocations. The twins are formed because of atomic stacking faults. Twins may annihilate because of slipping and interreaction of stacking faults and dislocation during austenization. As mentioned above, twins are a result of atomic arrangement faults; however, austenite twins are unstable compared with annealing twins because the driving force of atom movement is larger at high temperature, making it easy for austenite twins to disappear. 3.2. Effect of austenite grain size on bainite morphology The effect of austenization temperature on austenite grains can be directly analyzed through in-situ observation instead of conventional metallographic investigation through quenching after austenization. Figure 3 shows the change in austenite grains size with different austenization temperature and corresponding bainite morphology. Figure 3a and b indicates the remarkable growth of austenite grains in the samples austenized at 1 100 8C, although the austenite grains hardly grow during austenization at 1 000 8C, as shown in Fig. 3d and e. The coarsening of austenite grains occurs in the tested bainitic steel when the austenization temperature is above 1 100 8C. Figure 3c and f shows the morphologic feature of bainite sheaves after bainitic transformation for 60 min following austenization at 1 100 8C and 1 000 8C for 30 min respectively. It can be observed that the bainite sheaves in samples at 1 100 8C are significantly longer than that in samples at 1 000 8C. Larger austenite grains finally lead to longer bainite sheaves. Generally, the smaller austenite grains result in more grain boundaries, which impose more restrictions on the growth of bainite sheaves. Once impinging the grain boundary, the growing bainite sheaves are restrained because the bainite sheaves could not traverse grain boundary. To examine the final bainite microstructure more clearly after heat treatment, EBSD analysis was performed for the same sample austenized at 1 100 8C and transformed at 330 8C (Fig. 4). Figure 4 clearly shows the morphology and orientation of bainite sheaves. The areas with different 4

colors in parent austenite grains represent the bainite sheaves with different orientations. Areas in different colors result from the impingement of bainite sheaves with different orientations during growth. 3.3. Effect of austenite twins on bainite morphology Figure 5 presents the twin morphology after austenization at 1 100 8C and corresponding bainite morphology after bainitic transformation at 330 8C. It is apparent that the bainite morphology depends upon not only austenite grain size but also on austenite twins. The growth of bainite sheaves is restrained by austenite grain boundaries and austenite twins. Figure 5 illustrates the influence of twins on bainite morphology. The growth of bainite sheaves in Fig. 5b is stopped by austenite twins shown by arrows A and B in Fig. 5a during bainitic transformation. This means that austenite twins formed during austenization can refine bainite sheaves in the same manner as with austenite grain boundary. Therefore, it is important to investigate the austenite twins during austenization to analyze the morphology of transformed bainite and consequently the resulting mechanical properties.

4. Conclusions The dynamic evolution of austenite twins in an Fe–C–Mn– Si alloy was investigated with in-situ observation using LSCM and the effect of twins on bainite morphology was discussed. Following conclusions can be obtained: 1. It is found for the first time that austenite twins form during austenite grain growth. 2. The formation and evolution of austenite twins during austenization were directly observed. Austenite twins

Fig. 4. EBSD map of bainite morphology for the sample austenized at 1 100 8C for 30 min followed by transformation at 330 8C for 60 min, showing clearer bainite morphology and different orientations of bainite sheaves in parent austenite grains.



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(Received October 21, 2013; accepted January 15, 2014) Fig. 5. The effect of austenite twins on bainite morphology. (a) Austenite twins after austenizing for 30 min at 1 100 8C; (b) bainite morphology corresponding to (a).

form successively during isothermal holding, but some may disappear with isothermal time. 3. The morphology of bainite depends on both austenite grain size and austenite twins. The authors are grateful to the financial supports from the National Natural Science Foundation of China (NSFC) (No. 51274154), the National High Technology Research and Development Program of China (No. 2012AA03A504), the State Key Laboratory of Development and Application Technology of Automotive Steels (Baosteel Group), and the key project of Hubei Education Committee (No. D20121101).

Bibliography DOI 10.3139/146.111037 Int. J. Mater. Res. (formerly Z. Metallkd.) 105 (2014) E; page 1 – 5 # Carl Hanser Verlag GmbH & Co. KG ISSN 1862-5282 Correspondence address Dr. Prof. Guang Xu Faculty of Materials and Metallurgy Wuhan University of Science and Technology Wuhan 430081 China Tel.: +86-027-68862813 Fax: +86-027-68862807 E-mail: [email protected]

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