Effect of Heat Treatment on Microstructure and Mechanical Properties of ZM6 Alloy Prepared by Solid Recycling Process

The extruded specimens from the consolidated chips were solution treated at 803 K for 2.5 h, and then aged at temperature of 473 K for various periods of time. The variation in tensile strength with aging time plotted as the aging curve is given in Fig. 2. It takes about 16 h to reach peak aging. The peak-aged specimen presents an ultimate strength of 294.5 MPa with an elongation to failure of 13.5%. After 16 h aging, the alloy suffers an obvious overaging with the decrease in tensile strength from 294.5 to 252.1 MPa.

Figure 3 shows the evolution of the aging microstructure on the plane perpendicular to the extrusion direction. As the aging process reaches the time of 16 h, a large amount of the second phase particles precipitate homogeneously from the α-Mg matrix. The average grain size was 25.3 μm. The solution and aging treatment results in a more homogeneous distribution of the microprecipitates throughout the entire volume of the ZM6 alloy. The particles of the specimen aged for 96 h are much bigger than that of the specimens aged for 16 h. Figure 4 presents transmission electron micrographs recorded from an alloy sample aged at 473 K for 8 h, with the incident electron beam approximately parallel to [0001]_α. Many shaft-like precipitates were formed and distributed in the matrix, as shown in Fig. 4(a). In the corresponding selected area electron diffraction (SAED) pattern (Fig. 4b), there was a hexagonal D0₁₉ structure in the alloys, namely β′′ (Ref 8). At the peak- and over-aged stages, plate-shaped precipitates were observed. Figure 5(a) and (b) shows bright field images recorded from the alloy samples aged at 473 K for 16 and 96 h, respectively. From the aging time of 16-96 h, the plate-shaped precipitates grew from typical ~200 nm in length, with a thickness of <10 to ~400 nm in length, with a thickness of ~10 nm. The SAED pattern corresponding to Fig. 5(a) was face-centered cubic structure, namely β′. The plate-shaped β′ precipitates make the strength to be the maximum (Ref 9).

Figure 6(a)-(c) shows the microstructures of the alloy in the as-extruded, extruded-T4, and extruded-T5, respectively. The extruded specimens exhibited partial dynamic recrystallization and the grains refined greatly. It can be seen from Fig. 6(a) that some coarse brittle intermetallic phases were broken into small particles and moved from grain boundaries to grain interiors during the hot extrusion process (Ref 10). The solution heat treatment of the alloy at 803 K for 2.5 h reduced the Nd-rich second phases at grain boundaries and left only some discrete, rounded precipitates, as shown in Fig. 6(b). This kind of discontinuous particle morphology can improve the ductility of the alloy by reducing the nucleation, growth, and coalescence of cracks and cavities at grain boundaries (Ref 11). In the extruded-T5 condition, the average grain size was 12.3 μm. The grain refinement and less grain growth tendency may be attributed to the Mg₁₂Nd particles which pin the grain boundaries and impede grain growth during extruding and subsequent heat treatment (Ref 12).

Figure 7 presents the mechanical properties at room temperature of the alloy in different conditions. The as-extruded rods have the UTS of 232.2 MPa, the TYS of 118.8 MPa, and the elongation of 23%. After solution treatment (extruded-T4), the UTS and the TYS have a little decline, but the elongation increases from 23 to 30%. Compared with those of the as-extruded rods, both the UTS, and the TYS of the extruded-T5 rods increase while the elongation decreases due to the precipitation hardening. Large improvements of the UTS and the TYS are obtained from extruded-T4 condition to extruded-T6 condition, but the elongation is greatly reduced. The extruded-T6 rods have the highest UTS of 294.5 MPa and the highest TYS of 142.9 MPa.

During the hot-extrusion process, Mg₁₂Nd intermetallics were destroyed and broken into small particles, responsible for a substantial strengthening. Furthermore, hot extrusion may promote the formation of finer microstructure. According to the well-known Hall-Petch relation, the increase in yield stress depends on the grain size as follows (Ref 13):where \( \Updelta \upsigma_{0.2} \) is the increase in yield stress due to grain refinement, K is a constant, and d is the grain size. In the extruded-T4 condition, many of the particles were resolved into the matrix, which contribute to the tensile strength decreasing and the elongation increasing. In the extruded-T5 condition, the other potential sources of strengthening are the Mg₁₂Nd particles, which are observed to exist as relatively small particles. After solution heat treatment and aging treatment, the second phase particles precipitate from the matrix and retard the dislocation movement and grain boundaries migration, producing strengthening effects (Ref 14). In addition, due to the uncoordinated deformation with the matrix, the precipitates may act as crack sources and in turn decrease the elongation to fracture of tensile samples.

Figure 8(a) and (b) shows SEM images of the tensile fracture surfaces of the alloys in the as-extruded and extruded-T6 conditions. It can be seen from Fig. 8(a) that the failure surfaces are composed of some deep and small dimples. The diameter of dimples is about 8-15 μm. Moreover, apparently tearing ridges are observed in the fracture surface of the as-extruded alloys. Cleavage planes, some dimples and tear ridges are observed in the failure surfaces of the extruded-T6. The cleavage planes means that the direction of the cracks frequently changes during propagation and the crack extension resistance increases. Many small particles are seen in the dimples. The particles are Nd-containing intermetallics by EDS analysis as shown in Fig. 9. Due to the fragile characteristic of Nd-containing compounds, the particles were broken during the tensile test (or during extrusion) and become a cracking source.