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Supplementary MaterialEnhanced strength and ductility of A356 alloy due to composite effect of sub-rapid solidification and thermo-mechanical treatmentX. Zhanga,*, L.K. Huanga,*, B. Zhanga,*, Y.Z. Chen a,**, S.Y. Duana, G. Liub, C.L. Yanga,**, F. Liua,**a State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P.R. Chinab State Key Laboratory of Mechanical Behaviors Materials, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P.R. ChinaContent:1.Specific processing design of step I TMT (Fig. S1).2.Specific processing design of step II TMT (Table S1, Fig. S2).1.Specific processing design of step I TMT (Fig. S1)After T4 temper, non-uniform distributed eutectic Si particles in the matrix of sub-rapid solidified samples are shown in Fig. S1a. To obtain the microstructure with uniform distribution of eutectic Si particles, one ECAP pass of deformation could not achieve the purpose, and multipass ECAP deformation was adopted [1]. Fig. S1b shows that the billet broke after three continuous ECAP passes at room temperature, due to cracks which were formed in the eutectic Si particles and their surrounding areas arising from deformation (Fig. S1c). Initially, between each pass of ECAP, T4 temper was chose as the intermediate heat treatment to avoid cracks in the microstructure and reduce the stress concentration near eutectic Si particles. After 4 ECAP passes, the obvious cracks in the microstructure were effectively alleviated, but an evenly distributed of eutectic Si particles in the matrix was still not available, as shown in Fig. S1d. The areas of densely and sparsely distributed of Si particles appeared in the matrix, as indicated by the black arrows. Another question is raised, how to realize a uniform distribution of eutectic Si particles?Repeated experiments proved that adding ageing treatment to intermediate heat treatment can accelerate dispersion of eutectic Si particles during deformation, by increasing the matrix hardness. Note that the peak ageing treatment increased the matrix hardness from 85 in T4 temper to 115 (HV1) by serious precipitates strengthening, while the under ageing treatment was performed at 120 ℃ for 2 h, which increased the matrix hardness from 85 in T4 temper to 99 (HV1). Interestingly, 4 ECAP passes combined intermediate heat treatment with peak ageing, led to a large number of strip cracks in the microstructure (Fig. S1e), in contrast, the microstructure with uniformly distributed Si particles and without any internal cracks was achieved by 4 ECAP passes combined intermediate heat treatment with under ageing (Fig. S1f). So, 4 ECAP passes combined with the intermediate heat treatment (540 ℃, 1 h and 120℃, 2 h) was chosen as step I TMT.Fig. S1 Microstructure evolution during exploration and design for step I process. (a) The billet broke after three continuous passes of ECAP deformation, and (b) the cracks near the fracture of the billet, (c) morphology of eutectic Si particles in samples without any deformation in T4 temper as a reference; Morphology of eutectic Si particles in samples which were processed by 4 ECAP passes under different intermediate heat treatments: (d) 540 ℃ for 1 h (T4 temper), (e) T6 temper with peak ageing at 180 ℃ for 2 h, (f) T6 temper with under ageing at 120 ℃ for 2 h.2.Specific processing design of step II TMT (Table S1, Fig. S2)From previous studies about TMT process of heat-treatable aluminium alloys [S2, S3], ageing treatment can be performed before and after deformation, thus showing different effects on the final properties. Accordingly, four different processes were designed, where the step I processed samples in T4 temper, are further treated by process A (subjected to composite deformation without ageing treatment), B and C (subjected to composite deformation and then followed by low temperature aging treatment) and D (treated by peak ageing treatment and then subjected to composite deformation) respectively. The corresponding processes and the tensile properties are presented in Table S1. The process B and C slightly enhanced the strength but held the good ductility. However, the strength of A356 alloy were significantly improved by the process D, YS ~ 438 MPa and UTS ~ 483 MPa. Quantitative analysis of dislocation density of the above several representative processes of step II TMT was performed by X-ray diffraction (XRD) measurements. Following the Williamson-Hall method [S4], the effects of grain size (D) and lattice strain (ε) will contribute to the true peak broadening FW(S), which can be expressed as, FW(S)?cos(θ) = 0.9λ/D+2εsin(θ), where is the wavelength of Cu K radiation and is the Bragg angle. Considering that the minimum grain size in our work was 264 nm, the contribution of 0.9/D can be negligible. Plotting FW(S)?cos() vs sin(), and performing a linear regression analysis, the value of 2 was obtained from the slope of the fitting curve. Further, according to the method proposed by Williamson and Smallman [S5, S6], the dislocation density in fcc metals can be estimated as follows, ρ = 16.1ε2/b2, where b is the Burgers vector equal to 0.286 nm for FCC metals.The dislocation density of several representative processes of step II TMT is shown in Fig. S2. It is clear that the step I processed samples obtained the highest dislocation density under process D compared with other processes of step II, which is consistent with the highest UTS of process D shown in Table S1. Consequently, process D was selected as step II TMT.Table S1 Comparison of tensile properties obtained by different processes of step II.ProcessStep I+T4AgeingDeformationAgeingYS (MPa)UTS (MPa)EtF (%)A540℃+1h─ECAP+cryorolling─350±2425±211.9±0.5B540℃+1h─ECAP+cryorolling100℃+24h300±10380±418.2±0.5C540℃+1h─ECAP+cryorolling100℃+48h270±10364±419.0±0.6D540℃+1h180℃+2hECAP+cryorolling─438±5483±58.1±0.5A: single deformationB, C: deformation + low-temperature ageingD: peak ageing (T6) + deformationFig. S2 Dislocation density of the several representative processes of step II TMT. (a) XRD profiles of samples, and (b) Williamson-Hall plots for the diffraction patterns. Note that the labels of single deformation, deformation + low-temperature ageing and peak ageing + deformation correspond to process A, B, D of step II, and the step I processed samples were treated by T4 temper as a reference state.Reference:[S1] J.M. García-Infanta, S. Swaminathan, A.P. Zhilyaev, F. Carre?o, O.A. Ruano, T.R. McNelley, Microstructural development during equal channel angular pressing of hypo-eutectic Al–Si casting alloy by different processing routes, Mater. Sci. Eng. A 485(1-2) (2008) 160-175.[S2] J.K. Kim, H.G. Jeong, S.I. Hong, Y.S. Kim, W.J. Kim, Effect of aging treatment on heavily deformed microstructure of a 6061 aluminum alloy after equal channel angular pressing, Scr. Mater. 45 (2001) 901-907.[S3] S. Ferrasse, V.M. Segal, K.T. Hartwig, R.E. Goforth, Development of a submicrometer-grained microstructure in aluminum 6061 using equal channel angular extrusion, J. Mater. Res. 12(5) (1997) 1253-1261.[S4] G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminum and wolfram, Acta Metall. 1 (1953) 22-31.[S5] G.K. Williamson, R.E. Smallman, III. Dislocation densities in some annealed and cold-worked metals from measurements on the X-ray debye-scherrer spectrum, Philos. Mag. 1(1) (1956) 34-46.[S6] Y.Z. Chen, H.P. Barth, M. Deutges, C. Borchers, F. Liu, R. Kirchheim, Increase in dislocation density in cold-deformed Pd using H as a temporary alloying addition, Scr. Mater. 68(9) (2013) 743-746. ................
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