Elsevier

Acta Materialia

Volume 119, 15 October 2016, Pages 145-156
Acta Materialia

Full length article
In-situ X-ray diffraction studies of slip and twinning in the presence of precipitates in AZ91 alloy

https://doi.org/10.1016/j.actamat.2016.08.022Get rights and content

Abstract

The effect of basal plate precipitates on the hardening of basal slip and {101¯2} twinning modes was investigated for a non-aged and aged AZ91 alloy in the twin dominated strain paths. Exploiting in-situ synchrotron and laboratory based X-ray diffraction methodologies, we quantified the critical resolved shear stress (CRSS) for basal slip and twinning modes. The twin volume fraction changes were quantified from the intensity changes with applied load. We observed that the twin volume fraction changes with plastic strain is sensitive to the initial texture, while the relative hardening of different deformation modes are considered as a secondary effect. We also found that the twin interior stresses were significantly smaller and consistent with the high twin back stresses in the presence of precipitates. We propose, based on a simple analytical equation, that the leading edge of the propagating twin have a Burgers vector equivalent to 100 twinning dislocations and when the propagating twin is blocked by a precipitate, relatively high resolved stress is required for bowing the twin dislocation and hence the propagation of the twin occurs by the dissociation of the leading edge of the twinning dislocation.

Introduction

Optimized precipitate dispersions remain a potent means for achieving the next generation of high strength magnesium alloys [1], [2], [3], [4]. Although there has been some success in modelling the phenomena in this material [5], [6], the appropriate approach to adopt for predicting the impact of precipitation upon twinning is not yet clear.

Gharghouri et al. [7] showed that deformation twins in a Mg-7.7 wt% Al alloy engulf, by-pass or arrest at precipitates depending on their relative size compared to the twin. Basal slip was seen to accompany these phenomena. In aiming to ascertain how strengthening in this material should be understood, these authors [8] rejected Orowan looping as providing too little resistance and instead appealed to the strong backstresses set-up in the non-deforming precipitates. This effect was evidenced by significant differences in slopes of the solution treated and aged stress strain curves. Subsequent studies on material with similarly shaped particles have revealed that there is a strong impact of ageing upon the macroscopic yield stress, particularly in textured samples oriented for twinning [9], [10]. One study on the alloy AZ91 [9] reported hardening of the apparent critical resolved shear stress (CRSS) for basal slip by 5 MPa while the common {101¯2} twinning mode was hardened by ∼20 MPa by the same precipitate dispersion (produced by ageing at 200 °C). Remarkably, twins were seen to traverse ‘through’ multiple plate shaped precipitate barriers [10]. Orowan hardening was found to be insufficient to account for the observations. Back stresses from the deforming precipitates are clearly important but it is not clear how to understand these in the case of macroscopic yielding due to twinning (see also [11]) although recently Jain et al. [12] claim that the backstress can account for their observed twin hardening in a Mg-6wt%Zn alloy, which contains rod shaped particles.

Nie [13] and Robson et al. [14] have analysed the potential role of different precipitate morphologies on hardening against slip and twinning in magnesium alloys but these calculations assume a form of hardening consistent with Orowan looping between particles and as noted above it is not clear if this is the appropriate explanation. Experiments are required that allow the hardening of the different deformation modes to be deconvoluted. Towards this end, Agnew et al. [15] recently reported a neutron diffraction study of an aged alloy WE43 (Mg–Y–Nd–Zr alloy). Their ageing experiments showed a 50 MPa strengthening increment in the macroscopic flow stress but their modelling found that the twinning mode actually softened with ageing. In this case it is possible that solute depletion during ageing confounded the ability of the experiment to detect dispersion-twin interactions. Yttrium (Y) is known to have a potent solute strengthening effect on twinning [16], [17].

The present study combines the ability of diffraction techniques to determine the onset of plasticity due to different deformation modes with the study of alloy in which solute depletion during ageing is unlikely to be as significant as in previous work. Our aim is to quantify the impact of precipitate dispersions on deformation twinning in magnesium and to thereby establish a high-resolution X-ray based methodology for doing so with the highest level of accuracy.

In this paper, the effect of basal plate precipitates on the hardening of basal slip and twinning was studied using synchrotron and laboratory X-ray diffraction during in-situ compression and tension. Firstly, the calibration and profile fitting methodologies of the transmission XRD profiles are discussed for the accurate lattice strain measurement in different grain orientations. These are then used to calculate the critical stress required for basal slip and twinning modes in the ideal orientations. The stress in the parent and twin grains are then used to qualitatively explain the twin size differences in the non-aged and aged alloys based on the backstress experienced by the twin shear. Using an analytical equation we proposed that the leading edge of a propagating twin dislocation consists of ∼100 twinning dislocations.

Section snippets

Methodology

The alloy used in the current study was produced by wrought processing of a commercially available cast AZ91 alloy with a nominal composition of 9 wt% Al and 1 wt% Zn. Prior to wrought processing, the cast ingot was homogenised by heat treating the ingot at 420 °C for 24 h followed by quenching in water. The alloy was then hot rolled at 400 °C to a total strain of 0.8 in 21 passes. At the end of the final pass, the rolled slab was quenched in water. The final thickness of the rolled alloy was

Macroscopic stress – strain curves

The in-situ mechanical stress-strain curves of the non-aged and aged alloys obtained during in-situ C-RD and T-ND tests are shown in Fig. 4a and b respectively. Tests are performed on both the micro-deformation stage (Deben) and on an Instron universal testing machine. Since the in-situ measurements were done in extension control mode, the mechanical strain is determined by comparing the macroscopic extension data from the micro-deformation stage with the macroscopic strain determined by a

Discussion

The present material displayed an increase in macroscopic yield stress of 50 MPa in the peak aged condition. Micro-yielding in the grains favourably oriented for basal slip set up forward stresses (in the axial direction) in the precipitates ranging between 35 and 70 MPa, which, for a precipitate volume fraction of 6.6%, contributes ∼3.5 MPa to the macroscopic yield stress. In the magnesium matrix, it was found that ageing increased the CRSS for basal slip by 5 MPa and the apparent CRSS for {101

Conclusions

In-situ synchrotron and laboratory X-ray diffraction methodologies were utilised to investigate the effect of precipitates on the twin interior stresses and the twin volume fractions during C-RD and T-ND tests. The following conclusions can be drawn.

  • 1.

    The high intensity and highly parallel nature of the synchrotron beam together with a high resolution strip detector provided a high resolution diffraction setting. In addition, the optimized calibration and profile fitting methodology enabled the

Acknowledgements

The authors are grateful for discussions with Bevis Hutchinson and Sean Agnew, and for the technical support by John Vella. This research was supported under the Australian Research Council's Discovery (DP140102355) Projects funding scheme. This research was undertaken on the Powder Diffraction beamline at the Australian Synchrotron, Victoria, Australia.

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