Compressive creep behaviour of AS41 alloy

Several works have been carried out on the creep of die cast magnesium alloys in the past [33, 38, 40, 41, 73, 74]. Most of those investigations were carried out on pure magnesium, AZ and AM series. Other works include those of references [39, 75-80]. Few activities have also been observed on AE and AS series [81-84, 43]. A different creep mechanism was suggested by Pekguleryuz [43] for AS41 below 150 °C. Since the creep investigations were carried out by the author of reference [43] only at 150 °C, and with limited samples, this claim could not be experimentally verified. To adequately address this, extensive creep investigation was carried out between temperatures of 100 °C to 200 °C and between stresses of 40 MPa and 90 MPa for the permanent mould cast AS41 alloy. Table 5 shows the creep conditions, minimum creep rate and corresponding creep strain after 200 hours.

From the calculation of stress exponent as seen in Figure 38, the stress exponent n as measured in this work were approximately 2 at low stress regions of 40 MPa and 55 MPa with temperatures of 100 °C to 150 °C. The activation energy for creep as shown in Figure 39 for constant stress condition of 55 MPa was 50 kJ/mol. This is well below the activation energy for self diffusion of magnesium which is 135 kJ/mol. Invariably, this means that diffusion will preferentially occur within the grain boundary and dendrite regions rather than the bulk of the matrix. This suggests creep by grain boundary diffusion and subsequent sliding at sufficient creep time within these stress and temperature range. At high stress regions of 70 MPa and 90 MPa for the permanent mould AS41, the stress exponent n was averagely between n~ 4-6. However, the activation energy for creep (Qc) within 70 MPa and 90 MPa constant stress condition were between 72 kJ/mol and 100 kJ/ mol. Although the activation energy are relatively high compared to 50 kJ/ mol as observed at low stress conditions, the activation energy for creep (Qc) was still below that of self diffusion of magnesium. Stress exponent values of between 4-6 suggest creep mechanism dominated by dislocation climb. Previous creep investigations of pure Mg and Mg-alloys show similar results for stress exponents but with

relatively varied activation energy for creep [85-89]. In those previous reports, n values close to 3 were believed to be creep controlled by viscous glide. For n values between 4 and 7, creep controlled by dislocation climb was proposed.

Stress exponents higher than 7 were believed to be particle strengthening controlled creep. However there is still need to correlate the microstructure with these creep mechanisms as the understanding of those are important to elevated temperature creep resistance Mg-alloy development.

200 nm 010

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200 nm 200 nm 010

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100 nm 100 nm

(a) (b)

Figure 74: (a) Dislocation lying in primary α-Mg matrix, zone axis [100], (b) TEM micrograph showing dislocation cross slip

Figure 74(a) represents TEM micrograph showing dislocation lying on the basal plan with zone axis [100]. From the result in this work, the dominant creep mechanisms for permanent mould AS41 Mg-alloy are grain boundary sliding and dislocation climb. Figure 74(b) shows the TEM micrograph of cross slip of dislocation. Al segregation within the eutectic and dendritic regions play significant role in the overall creep behaviour of AS41. Figure 75 shows SEM micrograph of pores development on the Al segregated regions of AS41 alloy deformed at 70 MPa/ 175 °C.

During solidification, the segregation of solute occurs near the grain boundaries and dendritic regions. It has been reported that Al segregation increases the local homologous temperature (TH) of Mg-Al alloys within these regions [72, 73].

The segregation of alloying elements cause these regions to yield first by pore development as shown in Figure 75 followed by cracks and fracture when the

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samples are tested in tensile mode. See also Figure 31 for comparison of as-cast and creep deformed AS41. The grain boundary regions are loosely bonded together in comparison to the α-Mg matrix: This leads to high atomic migration and consequently yields easily during creep deformation. The activation energy at the grain boundary regions are lower than that of self diffusion as also found in this work. Any act to reduce primary alloying element segregation or reinforcement of the grain boundary regions will definitely improve the creep properties of Mg-alloys.

α -Mg matrix α -Mg matrix

Figure 75: SEM micrograph of pores development on Al-segregated regions

Formation of stable intermetallics, which leads to reduction of grain boundary regions and increasing grain sizes, are some of the practical ways to delaying dislocation movement in Mg-alloys. Others include, reduction of the volume concentration of β-phase by alloying elements and again by solid solution strengthening [55, 89-94]. As reported by Huang et al. [73] and several other authors [93, 94], heat treatment is also a very important tool to improve creep properties of Mg-alloys. Application of T4 heat treatment brings about solute homogenization while T6 encourages precipitation that is beneficial to slowing down dislocation movement at elevated temperature. Another important way of Mg-alloys creep improvement is the addition of hard reinforcements to create Mg-alloy composites [95-97]. These include carbides and oxides such as SiC, B4C, and Al2O3. These processes have been proven to improve the creep performance of Mg-alloys by forming obstacles and consequently create resistance to dislocation motion. The role of borides such as ZrB2 and TiB2 in

creep of Mg-alloys is yet to be investigated. In the AS41 alloy studied in this work, the formation of “Chinese script (Mg2Si) helps to improve the creep response of this alloy. Although the coarse and brittle nature of Mg2Si is believed to be slightly detrimental to the creep behaviour of this alloy, addition of alloying elements like Ca has been proven to modify this “Chinese script”

morphology which improved its creep response [98].

There seems not to be any process suggesting a different creep mechanism below 150 °C for AS41 as suggested in reference [43]. In all the temperature and stress levels studied in this work, the dominant creep mechanisms are the grain boundary sliding at low stress regions of 40 – 55 MPa with temperature up to 150 °C. At high stress regions of 70 – 90 MPa and temperatures up to 200 °C, dislocation climb becomes the rate controlling creep mechanism.

Twinning is also an active mode of deformation during creep of AS41 alloy. The effect of solute segregation is however seen as having great influence on the creep property of AS41 Mg-alloy. The reduced volume fraction of β-Mg17Al12

phase and the presence of thermal stable Mg2Si within the grain and around the grain boundaries are instrumental to improved creep property as seen in AS41.

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