No abstract

Microstructure development and mechanical properties of Al-xMg alloys (x = 0, 1, 5-10 wt%), processed by ECAP at room temperature, have been investigated. The results show that the microstructures of Al-xMg alloys are refined by the interaction of shear bands and their increase in number during ECAP. The addition of magnesium to aluminum promotes the grain refinement. Misorientation increase induced by particles along grain boundaries is observed by using high resolution EBSD. As ECAP strain increases up to 4, the strength of Al-6 wt% Mg alloy increases progressively while the elongation decreases from 31.7% to 5.5%. A good combination of both strength and ductility has been obtained by annealed ECAP. The change in softening mechanism of the Al-6 wt% Mg alloy, processed by 6 passes of annealed ECAP, occurs in the range of 523-573 K.(1) Introduction At present, the development of equal channel angular pressing (ECAP) has attracted much attention from the material scientists because of its ability to easily refine the grains of metals with accumulative strain [1]. The metals processed by ECAP normally present a very high strength due to their small grains and high dislocation density [2]. Many papers have reported the promising results of Al and its alloys processed by ECAP. In particular, fine grained Al-3 wt% Mg alloy with a mean grain size of 200 nm was obtained by ECAP at room temperature followed by cold rolling [3]. The grain size of Al-1.5 wt% Mg reached 280 nm and 230 nm after ECAP strain up to 8 and 13, respectively [4]. In general, the mean grain sizes of commercial aluminum-based alloys can be reduced into a submicrometer range by ECAP at room temperature [5].However, processing of Al-xMg alloys containing more than 4 wt% Mg by ECAP is not so easy. It normally requires elevated temperature to reduce the possibility of cracking. Cracks have been found in a 5083 Al-Mg alloy processed by ECAP at room temperature [6]. The grain size may grow during ECAP and between ECAP passes when the materials are exposed at high temperature. Thus the grain size is normally larger than the ECAP at low * Corresponding author at: Alfred Getz vei 2B, No-7491 Trondheim, Norway.Tel.: +47 73594921; fax: +47 73590203.E-mail address: happywinner01@gmail.com (Y.J. Chen).temperature [7]. In addition, electron backscatter diffraction (EBSD) investigation of Al-xMg alloys after ECAP is very difficult due to the experienced low intensity of the Kikuchi diffraction patterns caused by strain and magnesium addition, especially for high magnesium content (≥5 wt%). As a result, there are few publications reporting the EBSD investigations of Al-xMg alloys (≥5 wt%) after ECAP. The aim of the present paper is to process the Al-xMg alloys by ECAP at room temperature and to investigate the microstructure development and the improvement of mechanical properties.(2) Experimental procedureInthis study the Al-xMg alloys (x = 0, 1, 5-10 wt%) were received in the as-cast condition. The chemical composition of all alloys used in this study is shown in...

Keywords:Al-Mg alloy Electron backscattering diffraction (EBSD) Severe plastic deformation (SPD) Grain refinement Microstructure Grain boundaries a b s t r a c tStrain 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.

SummaryUltra-fast pattern acquisition of electron backscatter diffraction and offline indexing could become a dominant technique over online electron backscatter diffraction to investigate the microstructures of a wide range of materials, especially for in situ experiments or very large scans. However, less attention has been paid to optimize the parameters related to ultra-fast electron backscatter diffraction. The present results show that contamination on a clean and unmounted specimen is not a problem even at step sizes as small as 1 nm at a vacuum degree of 6.1 × 10 −5 Pa. There exists an optimum step size at about 50 data acquisition board units. A new and easy method to calculate the effective spatial resolution is proposed. Effective spatial resolution tends to increase slightly as the probe current increases from 10 to 100 nA. The fraction of indexed points decreases slightly as the frame rate increases from 128 patterns per second (pps) to 835 pps by compensating the probe current at the same ratio. The value 96 × 96 is found to be the optimum pattern resolution to obtain optimum speed and image quality. For a fixed position of electron backscatter diffraction detector, the fraction of indexed points as a function of working distance has a maximum value and drops sharply by shortening the working distance and it decreases slowly with increasing the working distance.