Quantitative study of grain refinement in Al–Mg alloy processed by equal channel angular pressing at cryogenic temperature

doi:10.1016/j.matlet.2011.07.067

Materials Letters

Volume 65, Issues 23–24, December 2011, Pages 3472-3475

https://doi.org/10.1016/j.matlet.2011.07.067 Get rights and content

Abstract

Strain induced grain refinement of an Al–1 wt.% Mg alloy processed by equal channel angular pressing (ECAP) at cryogenic temperature is investigated quantitatively. The results show that both mean grain and subgrain sizes are reduced gradually with increasing ECAP pass. ECAP at cryogenic temperature increases the rate of grain refinement by promoting the fraction of high angle grain boundaries (HAGBs) and misorientation at each pass. The fraction of HAGBs and the misorientation of Al–1 wt.% Mg alloy during ECAP at cryogenic temperature increase continuously as a function of equivalent strain. Both {110} and {111} twins at ultrafine-grained size are observed firstly in Al–Mg alloy during ECAP. The analysis of grain boundaries and misorientation gradients demonstrates the grain refinement mechanism of continuous dynamic recrystallization.

Highlights

► Al–Mg alloy processed by ECAP at cryogenic temperature is investigated. ► ECAP at cryogenic temperature increases the rate of grain refinement. ► Twins at ultrafine-grained size are observed firstly in Al–Mg alloy during ECAP. ► Grain boundaries and misorientation gradients are discussed.

Introduction

Equal channel angular pressing (ECAP), one of the most promising severe plastic deformation (SPD) methods, has been widely applied for obtaining bulk ultrafine-grained (UFG) Al and its alloys. Much work has been done to minimize the grain size by optimizing the ECAP parameters [1], [2], [3], [4], [5]. It is well established that optimum processing by ECAP requires the lowest possible extrusion temperature and the highest accumulated strain (saturation strain). Increasing magnesium content in Al–Mg alloys, which decreases the stacking fault energy [2], normally results in smaller grain size [1], [3], [4]. However, crack and failure could occur when Al–Mg alloys containing more than 4 wt.% Mg are processed by ECAP at room temperature [5].

The aim of this study is thus to explore the possibility of Al–1 wt.% Mg alloy processed by ECAP at cryogenic temperature and analyze quantitatively the grain refinement during 1 to 4 passes of ECAP.

Section snippets

Experimental

Al–1 wt.% Mg alloy (Al–0.971 Mg, by wt.%) was chosen. Samples were received in the as-cast condition and were annealed to give an initial grain size of 960 μm. ECAP was performed with route Bc, using a 100 mm × 19.5 mm × 19.5 mm bar in a 90° die, which leads to an imposed strain of about 1.0 per pass. The die was kept at 243 K in a freezer for 8 h before ECAP and was surrounded by dry ice during ECAP to maintain the targeting temperature. All samples were kept in liquid nitrogen for 30 min before and between

Results and discussion

Fig. 1 shows the orientation maps and grain boundary maps obtained after 1, 2, 3 and 4 ECAP passes. In order to study the evolution of the grain boundaries in Fig. 1b,d and f, high angle grain boundaries (HAGBs), with misorientations in the range of 15–30° and above 30°, are marked with thick blue and black lines, respectively. The low angle grain boundaries (LAGBs), with misorientations in the range of 10–15° and 1.5–10°, are marked with thin blue and red lines, respectively. Fig. 1a and b

Conclusions

Microstructure evolution of Al–1 wt.% Mg alloy processed by ECAP at cryogenic temperature from 1 to 4 passes has been investigated quantitatively by EBSD. Both mean grain and subgrain sizes are reduced gradually with increasing ECAP pass. ECAP at cryogenic temperature increases the rate of grain refinement by promoting the fraction of HAGBs and misorientation at each pass. The fraction of HAGBs and the misorientation of Al–1 wt.% Mg alloy after ECAP increase continuously as a function of equivalent

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Quantitative study of grain refinement in Al–Mg alloy processed by equal channel angular pressing at cryogenic temperature

Abstract

Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Mater Sci Eng A

Acta Mater

J Alloys Compd

Mater Sci Eng A

Mater Sci Eng A

Scr Mater

Acta Mater

Mater Sci Eng A

Mater Sci Eng A

Mater Charact