Materials Letters 209 (2017) 94–96

Effect of pre-existing twinning on strain localization during deformation of a magnesium alloy

X. Hong1, A. Godfrey1, W. Liu1, A. Orozco-Caballero2, J. Quinta da Fonseca2

1. Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China.

2. The University of Manchester, School of Materials, MSS Tower, Manchester M13 9PL, United Kingdom.

Abstract: A fine dispersion of nanoscale gold surface markers was achieved on the surface of a Mg-alloy sample using a styrene-assisted remodeling process. The markers were used for a high resolution digital image correlation strain-field analysis, to achieve a better understanding of the effect of pre-existing twinning on strain localization during in-situ tensile deformation. A complex pattern of heterogeneous strain localization was found. The results show that twins do not deform uniformly, and that relative differences in Schmid factor may influence the early occurrence of strain localization. Additionally, strain localization was seen to occur preferentially at grain boundaries and twin boundaries due to the lack of efficient plastic mechanisms for strain transfer across such interfaces. The results demonstrate the ability to measure strain localization on a fine scale in Mg-alloys, and highlight the complex interaction between deformation mechanisms during plastic deformation.

Keywords: deformation and fracture; microstructure; metals and alloys; twinning; magnesium alloys; strain localization.

Cite this article as: “Effect of pre-existing twinning on strain localization during deformation of a magnesium alloy” X. Hong, A. Godfrey, W. Liu, A. Orozco-Caballero, J. Quinta da Fonseca, Materials Letters 209 (2017) 94–96

1. Introduction

Magnesium and its alloys deform plastically by activation of a variety of slip and twinning modes, where slip on basal planes and tensile {10-12}<-1011> twinning are the most easy deformation modes because of their relatively low critical resolved shear stress [1,2]. Twinning plays an important role in plastic deformation of HCP alloys and can both alter the orientation of original grains, and influence the activation of slip systems. Recently, the influence of twins introduced by pre-straining on reloading mechanical behavior and on the interaction between twinning and slip mechanisms during non-monotonic loading has been investigated by many researchers [2-7]. A remaining challenge is to explore the effect of twinning on strain localization, as due to small size of newly formed twins, it is difficult to experimentally determine the strain field at the required length scale.

Digital image correlation (DIC) is now a well-established and robust method for characterization of in-plane strain fields [8-10], although to obtain a spatial resolution high enough for the measurement of the strain field at the submicron scale, nanoscale patterns are required. Recently some methods to achieve suitable dispersions of speckle patterns by water or chemical vapor exposure have been reported [11,12]. Modifications to the reported procedures are, however, required for corrosion susceptible materials like magnesium to achieve a nanoscale dispersion of markers. In this study we show that with suitable markers high resolution DIC can be used to explore local strain partitioning in a tensile deformed magnesium alloy containing twins. As a similar process may also take place during deformation with strain path changes, this study is believed to be helpful in understanding the reloading behavior and fundamental deformation mechanisms of pre-strained material.

2. Experimental procedures

A hot-rolled AZ31 (Mg-3%Al-1%Zn) commercial sheet with a strong basal texture was used in the study. A dog-bone shaped tensile sample was cut from the sheet with gauge length (11x2x1 mm3) along the transverse direction (TD) of the rolled sheet. To induce some twinning in the microstructure, HV0.1 Vickers micro-hardness indents (100g load applied over 10s) were introduced on the sample surface. Gold nano-particles, produced by a styrene-assisted remodeling approach (based on a modification of the chemical vapor exposure method described in [12]) were used to decorate surface as markers for DIC analysis. In the modified procedure, a styrene-argon mixture was chosen as the remodeling reagent to avoid oxidation of sample surface (for further details see [13]).

A selected area containing several narrow twins was examined in the SEM before and after tensile deformation to nominal strains of 1.2% and 3.9% (determined from the change of gauge length). A series of images were recorded at a high magnification (x20000), using secondary electron imaging mode at 5KV and 8mm working distance. The DIC analysis was carried out using the VIC-2D software, employing a 41x41 interrogation window (corresponding to a sub-region size of 533x533nm2) and a step size of 5 (~88% overlap). After DIC analysis of each image, the results were stitched together to show the strain distribution for the entire observation area. After tensile deformation to 3.9% the speckled surface was lightly re-polished to allow orientation information to be collected using electron backscatter diffraction (EBSD) investigations.

3. Results and discussion

The surface morphology of an area containing several micro-hardness induced twins is shown in Fig. 1. The grain boundaries can be clearly discerned and some indenter-induced twins can be identified based on the image contrast. Fig. 1(b) shows the gold speckle pattern developed using the styrene-assisted remodeling approach. The sample surface was covered evenly and densely by nanoscale particles, with the majority of particles in the size range of 20-100 nm, and an average spacing of ~120nm between well discerned particles, allowing the strain field to be analyzed at submicron resolution.

Fig.1. (a) SEM micrograph showing the grain boundaries and the key hardness indent-induced twins examined in this study; (b) high magnification image showing the surface nanoparticles (area corresponding to the white box in (a)).

The xx component (along the tensile axis direction) of the strain field after deformation to a nominal strain of 1.2% is shown in Fig. 2(a). The average strain of this region as calculated from DIC is around 1.3%, in good agreement with the nominal strain.The strain distribution is, however, clearly inhomogeneous. In particular, strain is concentrated in some of the pre-existing twins, and clear strain localization occurs near several grain boundaries, and also in a region adjacent to twin boundary. Moreover, the high resolution DIC data reveals that the strain inside twins is also inhomogeneous, with the maximum local strain inside a twin almost 7 times the average value for the entire observation area. With increase of the deformation to 3.9%, the number of twins where strain is localized increases, and at the same time local strain accumulation continues near some grain boundaries (see Fig.2(b)). This evolution in strain distribution is illustrated in Fig. 3, which shows just the incremental value of the xx strain between the two deformation steps. Comparison of Fig. 3 with Fig. 2(a) also highlights the fact that in some cases the area within a twin where the strain is concentrated also changes with continued loading (e.g. twin T4).

Fig. 2. Maps showing the xx strain after deformation to a nominal strain of (a) 1.2%, and (b) 3.9%. Black lines show the grain boundaries; dotted lines highlight some clearly discerned twin boundaries.

Fig.3. Map showing the incremental xx strain component between 1.2% and 3.9%, superposed with some boundary information.

Table 1 lists the orientations of key individual grains and twins. Also shown are the Schmid factors for basal slip – the system expected to dominate given the initial orientation and loading orientation of the sample. To understand the pattern of strain localization the ratio of the maximum SF value for twin and parent grain are also calculated. The results show that although neither G4 nor T4 are well aligned for basal slip, significant strain localization took place in T4, at both strain steps. One possible reason for this may be the very high SFT/G ratio for this grain, such that strain localization takes place preferentially in regions much softer than their surrounding/neighboring volumes. This can also be demonstrated by T2, which despite its high SF, shows no strain localization in the first strain step, due to its significantly lower SFT/G ratio compared to T4.

Table 1. Orientations (Euler angles) and basal slip Schmid factors (SFs) for selected grains (G) and twins (T). Maximum SF for each is underlined.

Orientation

(0001)[2-1-10]

(0001)[-1-120]

(0001)[-12-10]

Ratio SFT/G

G1

116.2,169.8,55.9

0.001

0.135

0.137

3.00

T1

152.0,86.2,23.7

0.410

0.257

0.153

G2

21.2,10.4,43.1

0.054

0.005

0.059

7.06

T2

151.6,80.0,38.6

0.413

0.186

0.227

G3

95.6,165.6,49.4

0.056

0.230

0.174

2.05

T3

137.1,80.7,19.8

0.471

0.374

0.097

G4

177.9,174.7,36.0

0.003

0.001

0.002

47.41

T4

170.7,85.0,31.6

0.159

0.078

0.081

Strain localization also is found to take place in the region adjacent to some boundaries, for example between G3 and G4, and between T1 and G1. Here the strain localization can be analyzed in terms of the geometric strain compatibility factor, m’, defined as m’=coscos, where and are angles between the two slip directions and two slip plane normals for slip systems in two neighboring regions [14]. In the former case, although m’(G3,G4)max=0.88, and G3 is relatively soft, G4 has a very low SF on all basal systems, reducing the possibility for basal slip to be active. For G1 and T1, the twin T1 is rather soft (SF=0.41), but the corresponding basal system has poor geometric strain compatibility with all basal systems in G1 (the maximum m’ value is only 0.078). Strain localization at the G3/G4 and T1/G1 boundary may arise therefore as a result of the lack of efficient mechanisms for strain transfer across these interfaces.

4. Conclusions

A high resolution DIC study of the local strain distribution during slip-dominated tensile loading of an AZ31 magnesium alloy containing pre-existing twins was conducted utilizing nanoscale gold particles produced by a styrene-assisted remodeling method. The results reveal a complex pattern of strain localization at the twinning scale that can be rationalized by the crystal orientation and the compatibility of plastic mechanisms for strain transfer across internal interfaces. The ability to characterize strain heterogeneity at high spatial resolution in Mg-alloys is expected to be important for further development of crystal plasticity models that fully incorporate the interaction between different, and sometimes competing, deformation modes.

Acknowledgements

Financial support for this work from the National Key Basic Research Program of China (grant number 2013CB632204) is gratefully acknowledged.

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