*2.1. Materials and Processing*

The raw material Mg-9Li alloy ingots used in the experiment was purchased from Jiangsu Li Mg Aero Material Co., Ltd. The composition of the alloy was analyzed by GNR S3 spark direct reading spectrometer. The results are shown in Table 1. The raw material can be identified as Mg-9Li alloy according to the mass ratio of Mg to Li element. According to the binary phase diagram of Mg–Li alloy, the lithium content of the alloy in this paper falls in the range (between 5.7–10.3 wt. %) of the (α + β) duplex phase structure.

**Table 1.** Analysis of Mg-9Li alloy composition (wt. %).


ECAP was conducted at 200 ◦C with the samples extruded for 4, 8, and 16 passes. The rotary Die equal channel angular pressing (RD-ECAP) set-up as diagrammatically illustrated in Figure 1a with detailed information obtained from our previous investigation [22,27] consists of a plunger for forcing the sample through the die orifice with all four sides of the die sealed with punches with the upper punches protruding out of the die to be pressed by the plunger. This is used for continuously processing the samples. This method is in tandem with the principles of the conventional ECAP with the extruded inner angle being 90◦. The advantage over the conventional ECAP set-up is that unlike in the former, each pass needs to be rotated and then refilled with the mold and rotated clockwise at

the end of each pass as shown in Figure 1a. The rotary die can save a lot of experiment time, shorten the holding time of each pass and effectively reduce the dynamic recovery behavior after long time heating [27]. The inner parts of the die is cleaned with graphite emulsion to reduce friction during the extrusion process. After each pass, the sample squeezes from the upper channel into the left channel. The mold is then rotated clockwise with the sample reverting to its initial upper channel position, which is ready for the next pass. The as-cast sample is heated to 200 ◦C for half an hour in an oven and then processed for 4, 8, and 16 passes respectively. The preparation of the 16-pass ECAP sample requires another half an hour reinsertion of the die into the furnace after 8 passes is completed before performing the further 8 passes. This is to ensure that the sample does not crack during the extrusion process.

**Figure 1.** Schematic diagram of (**a**) rotary die equal channel angular pressing (RD-ECAP) process and (**b**) rolling technique [27].

The ECAPed specimens are further rolled at room temperature (RT) to obtain high strength and toughness. All the ECAP-processed specimens were further rolled along a 20 <sup>×</sup> 40 <sup>×</sup> 2.5 mm3 thin section perpendicular to the extruded surface using a rolling machine as diagrammatically illustrated in Figure 1b with detailed information obtained from references [27,31]. Multi-pass rolling at room temperature was used via the rolling speed of 0.1 mm·s−<sup>1</sup> in this process. Each pass reduction was 10% until the total reduction reached 50%. This method can effectively prevent the sample from cracking during the rolling process. The height between the two rollers were adjusted with the speed controlled. After each pass, the sample is reversed for the next roll to ensure the uniformity of the rolling. Each sample is designated with a specific name. The as-cast alloy, ECAPed alloys with 4, 8, 16 passes, cast-rolled alloy, 4-passes ECAP plus rolling alloy, 8-passes ECAP plus rolling alloy and 16-passes ECAP plus rolling alloy are given designated names of C, E4, E8, E16, CR, E4R, E8R, and E16R respectively.
