Dynamic Deformation Behaviour and Dislocation Substructure of AZ80 Magnesium Alloy over a Wide Range of Temperatures

The high strain rate deformation behaviour and dislocation substructure of AZ80 magnesium alloy are investigated at strain rates of 8x102 s-1, 1.5x103 s-1 and 2.2x103 s-1 and temperatures of -100 oC, 25 oC and 300 oC using a compressive split-Hopkinson pressure bar system. The flow stress, work hardening coefficient, strain rate sensitivity and temperature sensitivity all increase with increasing strain rate or decreasing temperature. Moreover, the dynamic deformation behaviour is well described by the Zerilli-Armstrong hcp constitutive equation. Transmission electron microscopy observations show that the dislocation density increases with a higher strain rate or a lower temperature. Finally, the flow stress varies linearly with the square root of the dislocation density in accordance with the Bailey-Hirsch model.


Introduction
Magnesium alloys have many favourable properties, including a low density, a high strength-to-weight ratio, excellent machinability, and good recyclability.As a result, they have received extensive attention in recent years as structural components for transportation and aerospace applications, and lightweight enclosures for 3C (computer, communication and consumer electronic) products [1][2].With a high strength and a low price, AZ80 magnesium alloy is one of the most widely-used commercial magnesium alloys [3][4].The mechanical properties and microstructure [5], or creep behaviour [6], or extrusion deformation [7] and weldability [8] of AZ80 are all well-understood.AZ80 magnesium alloy components often experience high strain rate loads during fabrication or in their subsequent service lives.
However, the literature contains only limited information regarding the high strain rate forging behaviour of AZ80.Thus, to ensure the physical integrity of AZ80 structural components, a more rigorous evaluation of the effects of strain rate and temperature on the flow behaviour and dislocation substructure of AZ80 magnesium alloy is required.
The present study investigates the high strain rate deformation behaviour of AZ80 magnesium alloy at strain rates of 8x10 2 s -1 , 1.5 x10 3 s -1 and 2.2x10 3 s -1 and temperatures of -100 ºC, 25 ºC and 300 ºC using a compressive split-Hopkinson pressure bar (SHPB) system.The correlation between the flow stress and the dislocation density is then investigated as a function of the strain rate and temperature.

Experimental procedure
The starting material for the impact tests was an extruded rod of AZ80 (Al 8, Zn 0.5 and a balance of Mg (wt.%) alloy with a 12 mm diameter.The rod was solution treated at 410 ºC for 12 h followed by water quenching, and then aged at 230 ºC for 8 h.Cylindrical specimens with a length of 8±0.1 mm and a diameter of 7.2 mm were machined from the heat-treated AZ80 alloy with the impact axis parallel to the extruded direction and finished to a final diameter of 7±0.1 mm via a centre-grinding process.To approximate a uniaxial deformation state during deformation (i.e., low friction conditions), the end faces of the specimens were lubricated with a glass paste consisting of 80% PbO and 20% B2O3 mixed with alcohol.
High strain rate forging tests were performed at strain rates of 8x10 2 s -1 , 1.5 x10 3 s -1 and 2.2x10 3 s -1 and temperatures of -100 ºC, 25 ºC and 300 ºC using the compressive SHPB system.The low testing temperature of -100 ºC was obtained by immersing the specimen in a refrigeration system filled with liquid nitrogen and alcohol.Meanwhile, the elevated deformation temperature of 300 ºC was achieved by enclosing the specimen in a clamshell radiant-heating furnace.
The microstructures and morphologies of the various specimens were observed by transmission electron microscopy (TEM).TEM foils were prepared using an FEI Nova 200 focused ion beam (FIB) milling system with a Ga ion beam and an operating voltage of 30 keV.The cross-sectional microstructures of the various specimens were observed using a Philips Tecnai F30 field emission gun transmission microscope with an operating voltage of 300 keV.The dislocation density ρ of each sample was calculated using Ham's interception method [9].Figures 1(a)~1(c) show the true stress-strain curves of the AZ80 specimens forged at strain rates of 8x10 2 s -1 , 1.5 x10 3 s -1 and 2.2x10 3 s -1 and temperatures of -100 ºC, 25 ºC and 300 ºC, respectively.It is seen that the flow stress curves have a parabolic shape under all of the considered loading conditions.However, the flow stress is sensitive to both the temperature and the strain rate.
For a given temperature, the flow stress increases with increasing strain rate.However, for a given strain rate, the flow stress decreases as the temperature increases.In other words, even though the flow curves have a similar shape, the specimens exhibit different degrees of work hardening and thermal softening under different strain rates and temperatures.This finding suggests that the dislocation density and multiplication rate both vary widely with the strain rate and temperature, and prompt a corresponding change in the plastic deformation resistance of the AZ80 specimens.

Effects of strain rate and temperature
Figure 2(a) shows the variation of the true stress with the strain rate as a function of the temperature and true strain (ɛ t = 0.015 and 0.07).For both values of the true strain, the true stress increases linearly as the strain rate is increased from 8x10 2 s -1 to 1.5x10 3 s -1 and 1.5x10 3 s -1 to 2.2x10 3 s -1 .However, the increase in the flow stress is more rapid in the higher strain rate range of 1.5x10 3 s -1 to 2.2x10 3 s -1 .Overall, the results presented in Fig. 2(a) suggest that the rate of dislocation multiplication and work hardening increases under an increasing strain rate or a decreasing temperature.Figure 2(b) shows the variation of the strain rate sensitivity with the strain as a function of the strain rate and temperature.It is seen that for a given strain and deformation temperature, the strain rate sensitivity increases rapidly with increasing strain rate; particularly in the higher strain rate range of 1.5x10 3 to 2.2x10 3 s -1 .Moreover, for a given strain rate range, the strain rate sensitivity increases with increasing strain, but decreases with increasing temperature.The higher strain rate sensitivity under higher strain rates and lower temperatures can be attributed to a more rapid multiplication of the dislocations during deformation.
The dislocations serve as effective obstacles in suppressing further dislocation motion, and therefore increase the flow resistance.
Figure 3(a) shows the variation of the flow stress with the deformation temperature as a function of the strain rate for true strains of 0.15 and 0.45, respectively.
An obvious thermal softening effect is observed under higher temperatures for all values of the strain and strain rate.Figure 3(b) shows the variation of the temperature sensitivity with the true strain as a function of the strain rate over the temperature ranges of -100 ºC to 25 ºC and 25 ºC to 300 ºC, respectively.It is observed that for a constant strain and strain rate, the temperature sensitivity decreases with increasing temperature.However, for both temperature ranges, the temperature sensitivity increases as the strain and strain rate increase.(1) where σ a , B , β 0 , β 1 , B 0 , α 0 and α 1 are constants; σ is the stress; ɛ is the equivalent plastic strain; ἐ is the strain rate; and T is the deformation temperature.

Dislocation configuration and distribution
Figures 5(a) and 5(b) show the dislocation substructures of the specimens forged at -100ºC under strain rates of 8x10 2 s -1 and 2.2x10 3 s -1 , respectively.For a strain rate of 8x10 2 s -1 , the substructure contains a large number of parallel dislocation lines (Fig. 5(a)).These lines indicate the occurrence of a significant pile-up and grouping of the dislocations during deformation, which prompts a strain hardening effect as a result.At a higher strain rate of 2.2x10 2 s -1 , dislocation cell structures and an increased dislocation density are observed (Fig. 5(b)).
The greater degree of dislocation tangling associated with these dislocation cells reduces the dislocation mobility, and hence also prompts a significant increase in the flow stress and strain hardening rate.Figures 5(c) and 5(d) show the dislocation substructures of the specimens forged at 25ºC under strain rates of 8x10 2 s -1 and 2.2x10 3 s -1 , respectively.As for the specimens deformed at a lower temperature of -100ºC, the substructures are characterised by dislocation pile-up and grouping under a strain rate of 8x10 2 s -1 (Fig. 5(c)) and dislocation cells at 2.2x10 3 s -1 (Fig. 5(d)).The dislocation density increases with an increasing strain rate.However, for each of the considered strain rates, the dislocation density is relatively lower, and the cell size relatively larger, than that in the specimen deformed at the same strain rate but a lower temperature of -100 ºC.
The lower dislocation density and greater cell size at higher temperatures leads to a loss in flow resistance (as shown in Fig. 1).For the specimens forged at 300 ºC, the dislocation density again increases with an increasing strain rate (Fig. 5(e)) and Fig. 5(f)).However, compared with the specimens forged at -100 ºC and 25 ºC, the https://doi.org/10.1051/epjconf/201818303010DYMAT 2018 elevated deformation temperature reduces the dislocation density and provides the dislocations with the thermal energy required to overcome obstacles to motion.Consequently, a significant reduction in the flow resistance occurs.
The TEM images presented in Fig. 5 show that the dislocation density depends strongly on the strain rate and temperature, and has a significant effect on the flow stress.According to the Bailey-Hirsch model, the flow stress varies linearly with the square root of the dislocation density as follows [11].
where σ 0 is the initial yield stress of the material, α 1 is a material constant, G is the shear modulus, b is the Burgers vector, and ρ is the dislocation density.Substituting values of G=17 GPa [12], b=3.2×10-10 m [13] and σ 0 =240 MPa (as measured using a Saginomiya 100 metal forming machine) into Eq.α 1 is found to have a value of 0.36 for the present AZ80 specimens.Figure 6 compares the results obtained from Eq. ( 4) for the variation of the flow stress with the square root of the dislocation density with the experimental measurements.
It is observed that a good agreement exists between the two sets of results.In other words, the Bailey-Hirsch model provides a good description of the dependence of the flow stress on the substructure dislocation density for the present AZ80 magnesium alloy specimens.

Conclusions
The impact deformation behaviour of AZ80 magnesium alloy have been investigated at strain rates ranging from 8x10 2 s -1 to 2.2x10 3 s -1 and temperatures of -100 ºC, 25 ºC and 300 ºC using a compressive split-Hopkinson pressure bar (SHPB) system.It has been shown that the flow stress increases significantly with increasing strain rate or decreasing temperature.The strain rate sensitivity is enhanced at higher strain rates (i.e., 1.5x10 3 s -1 to 2.2x10 3 s -1 ).The stress-strain values predicted by the Zerilli-Armstrong hcp constitutive equation for the AZ80 specimens are in good agreement with the experimental results for all of the considered strain rates and temperatures.TEM observations show that the dislocation density increases with increasing strain rate, but decreases with increasing temperature.The higher dislocation density impedes dislocation motion and therefore results in a strengthening effect.Finally, the flow stress varies linearly with the square root of the dislocation density, as predicted by the Bailey-Hirsch model.

Fig. 2 . 3 . 3
Fig. 2. (a) Variation of true stress with strain rate as function of temperature given true strains of 0.015 and 0.07.(b) Variation of strain rate sensitivity with true strain as function of temperature and strain rate.

Fig. 6 .
Fig. 6.Variation of flow stress with square root of dislocation density given constant true strain of 0.07.