Grain size refinement techniques

Magnesium alloys with refined grains could raise the strength and also improve the formability. In the study of AZ31 and ZK60, Bussiba et al. [26] suggested that magnesium alloys can possess better superplasticity at low temperatures or high strain rates with smaller grain size in the submicron scale. The grain refinement also affects the hardness value of magnesium alloys. Thus, the current research also focuses on producing high hardness magnesium material through effective grain refinement.

1.3.1 Grain size refinement techniques

In order to get good mechanical properties, different processes can be applied to refine the grain size, which would improve the mechanical properties such as strength, hardness, superplastic behavior, and so forth. The grain sizes of commercial alloys are generally tailored for specific applications by pre-determined thermechanical treatments in which the alloys are subjected in specified regimes of temperature and mechanical testing. However, the refining results of these usual produces are limited to the order of a few micrometers and can’t be used to produce materials with submicrometer grain sizes. Therefore, the other specific grain refining techniques that may be used to fabricate ultrafine-grained (UFG) materials with grain sizes in the submicrometer range are developed.

Two basic and complementary approaches have been developed for grain refinement and these are known as the “bottom-up” and “top-down” approaches [27]. The “top-down”

approach is dependent upon taking a bulk solid with a relatively coarse grain size and processing the solid to produce a UFG microstructure through heavy straining or shock loading. Unlike “bottom-up” approaches, these approaches can be readily applied to a wide range of pre-selected alloys and larger product sizes. Formally, the large strain is imposed on

the metal materials by severe plastic deformation (SPD) to gain fine grains via recrystallization or sub-grains by rearrangement of dislocations.

There are many different SPD processing techniques have been proposed, developed and evaluated. These techniques include rolling, equal-channel angular pressing (ECAP) [28-30], high-pressure torsion (HPT) [31], multi-directional forging [32,33], twist extrusion [34], cyclic-extrusion-compression [35,36], reciprocating extrusion [37,38], constrained groove pressing (CGP) [39], cylinder covered compression (CCC) [40], accumulative roll-bonding (ARB) [41,42]. All of these procedures are capable of introducing large plastic straining and significant microstructural refinement in bulk crystalline solids. Some of these techniques, such as ECAP, HPT, multi-directional forging and ARB are well-established methods for producing UFG materials.

Extrusion is the process by which a block of metal is reduced in cross section by forcing it to flow through a die orifice under a high pressure [43]. Semi-finished products of cylindrical bars or hollow tubes are most generally produced by extrusion. Lin and Huang [24,44] have extruded the AZ31 and AZ91 magnesium alloys, which are usually manufactured by thixomolding and casting, to refine the grain size from the initial average 75 μm to 1~5 μm via the occurrence of dynamic recrystallization during extrusion. These refined magnesium alloys can perform good superplasticity and higher room temperature tensile strength.

Rolling is the most widely used metalworking process because it lends itself to high production and close control of the final product. Rolling can be carried out at elevated temperatures (hot rolling), wherein the coarse-grained, brittle, and porous structure of the ingot or continuously cast metal is transformed into a wrought structure, with much finer

grain sizes. The rolling temperature plays an important role in rolling process. Rapid grain growth may be bought up by high rolling temperature that grains may not be effectively refined, while surface or edge cracking may take place at low rolling temperatures. Chang et al. [45] reported that grain sizes of AZ31 magnesium alloys could be refined from 13 μm to below 10 μm by rolling 2 mm thick plate to 0.5 mm for AZ31, and its thin sheet exhibited 300~325 MPa tensile strength. In addition, Kim et al. [46] also rolled AZ61 sheets with a thickness of 2.15 mm to 0.5 mm after nine passes at 648 K, and effectively refined grains from 16 μm to 8.7 μm.

In ARB process, the rolled material is cut, stacked and rolled again. In order to obtain the bulk material, the stacked sheets are bonded during rolling simultaneously (roll-bonding).

Therefore, the achieved strain is unlimited in this process because repetition times are endless in principle. Fig. 1-1 illustrates the principle of ARB process. Perez-Prado et al. [47] reported that grain sizes of AZ31 and AZ91 alloys can be refined from 38 μm to 3 μm and 23 μm to less than 1 μm, respectively. Therefore, ARB process is also an efficient way for grain refinement and the homogeneity of the microstructure can be improved by increasing the number of rolling process.

The ECAP process, performed by pressing samples into equal-diameter channel with a different exit direction, is a shear deformation maintaining the same input dimension of the pressed (or extruded) materials. The detailed illustration is shown in Fig. 1-2 [48]. This process can impose severe plastic deformation on materials after several passes to induce micro-scaled grains [48] or even to submicro or nano-scaled extrafine grains [49]. Therefore, this process appears to be a powerful technique. Mabuchi et al. [50] refined the AZ91 alloy to 1 μm by ECAP after eight passes, and the resulting alloys also showed a high elongation of 661% at a low temperature of 473 K. Recently, Matsubara et al. [51] and Lin et al. [52] have

developed a two-stage extrusion plus equal channel angular pressing (ECAP) to fabricate the UFG Mg alloys. The original coarse grain size can be reduced to less than 10 μm after extrusion at 300^oC and it is further reduced to around 0.7 μm after subsequent 8-pass ECAP at 200^oC.

As for the “bottom-up” approach, UFG materials are fabricated by assembling individual atoms or by consolidating nanoparticulate solids. Examples of these techniques include inert gas condensation [53], electrodeposition [54], spray forming [55,56], ball milling with subsequent consolidation [57] and cryomilling with hot isostatic pressing [58], where cryomilling essentially denotes mechanical milling in a liquid nitrogen environment. In practice, these techniques are often limited to the production of fairly small samples that may be useful for applications in fields such as electronic devices but are generally not appropriate for large-scale structural applications.

The spray forming process is an inert gas atomization of a liquid stream into variously sized droplets which then are then propelled away from the region of atomization by fast flowing atomizing gas, as shown in Fig. 1-3 [55]. Droplets are subsequently deposited and collected by a substrate on which solidification takes place. Finally a coherent and near fully dense perform is produced. This process can produce bulk scale nano-grained materials via rapid solidification at a cooling rate about 10² ~10³ K/s. Therefore, this process is very powerful tool, but it has an expensive issue related to patent. However, Chen and Tsao [56]

reported that spray-formed AZ91-3.34 wt% Si alloys can possess finer structures and better workability than the as-cast counterpart.

Besides the refinement technologies mentioned above, there are still other methods to refine the grain size. Andrade et al. [59] used shock-wave deformation to refine Cu alloys to