6.3.1 Brinell hardness of Al-Mg-Si and Al-Mg-Ge alloys
Precipitation hardening of A1 - D2 was confirmed by Brinell hardness measurements performed on specimens artificially aged for different times.
As an example, in Figure 6.3 a, HB for A1 - C2 is plotted against artificial aging time.
Already the first measured point after 30 min of artificial aging gives an increase of HB and it continues up to 180 min, where HB reaches its maximum. The peak hardness of A1 is nearly the same as it was measured for A356.0 aged for the same time. For the Mn-containing alloys A2 and A3, HB is somewhat higher indicating the sum effect of precipitation and Al(Mn, Fe)Si dispersoids. From the graph in Figure 6.3 b, it is clear that the precipitation process in A356.0, where the solid solution decomposes with the formation of and later phases, is similar to that of A2 where same precipitates are expected at different aging stages, but different from that in A201, which reaches a much higher microhardness.
Additional alloying with Li (B1 and B2) and Sc+Zr (C1 and C2) shows even lower HB values than that of A1 - A3.
a. b.
Fig. 6.3: HB of A1 - C2 as a function of artificial aging time (a) and for comparison with A2, C1, B1 and commercial A356.0, A201.0 alloys (b)
From Fig. 6.3 follows that HB of the commercial alloy A201.0 is the highest from all alloys tested. This result is not surprising, because A201.0 (Al-Cu system) belongs to the class of high strength alloys, as mentioned in Chapter 2. For sand casting (slow cooling rate), UTS reachea a level up to 440 MPa with an elongation up to 10% in T6 condition, [DIN96]. These are the highest values one can achieve for Al-based casting alloys subjected to conventional casting techniques. Such unique properties have two origins of, namely:
- high solute content in solid solution. EDX measurements show that in T6 condition the Al matrix contains 5.50 wt.% Cu, 3.50 wt.% Ag and 0.26 wt.% Mn which is much higher than in the alloys A1 - C2 and even in the commercial alloy A356.0;
- precipitation of strengthening phase during aging.
The main contributor to the high HB values are the precipitates of θ-CuAl2 phase as shown in Fig. 2.4. On the other hand however, poor castability, susceptiblity to solidification cracking and interdendritic shrinkage and the low corrosion resistance strongly restrict a widespread application of the A201.0 alloy in foundry industry. These are the reasons, why Al-Si and Al-Mg-Si are preferable for casting.
The lowest HB was measured for the Li-containing alloys B1 and B2 and for the Sc+Zr-containing alloys C1 and C2 HB was only somewhat higher. The decrease of HB indicates that alloying by precipitate-forming elements has on unfavorable effect on the mechanical properties of Al-based alloys. As it was discussed in Chapter 4, addition of Li makes the interlamellar spaces larger in the (Al)+(Mg2Si) eutectic. This result in a large distance between the Mg2Si particles after spheroidisation, therefore the absolute values of HB are lower than for A1 - A3.
HB for D1 and D2 in as-cast state is the lowest one among all alloys tested. After solution treatment, the HB values of the Al-Mg-Ge specimens fall down to the hardness of pure aluminum (Figure 5.3). During artificial aging at 175.0C, the maximum hardness was achieved after 120 - 360 min (Figure 6.4). No difference between HB of D1 and of Li-containing D2 was detected.
Prolonged ageing results in a slight decrease of HB for both D1 and D2.
The highest values of HB were measured for the A201.0 alloy artificially aged at 150.0C for 420 min, showing the leading position of Al-Cu alloys among casting alloys (Figure 6.3 b). Its strength originates from of needle shaped -precipitates (Table 2.4, Figure 2.4).
Thus for Al-Mg-Si, the
increase of HB can be mainly attributed to the precipitation process during aging, as known from the commercial A356.0 and A201.0 alloys.
6.3.2 Microhardness of Al-Mg-Si and Al-Mg-Ge alloys
It is clear that macrohardness such as HB is less sensitive to the structural changes taking place inside the solid solution.
Fig. 6.4: Hardness of D1 and D2 as a function of artificial aging time
a. b.
Fig. 6.5: Microhardness of A1 - C2 as a function of artificial aging time (a) and comparison of artificial aging kinetic for A2, B2, C1 and the commercial A356.0 and A201.0 alloys (b)
Therefore, HV measurements were applied to detect the changes of mechanical properties of 0.05 the -Al solid solution during aging. The results are summarized in Figure 6.5.
For the base alloys A1 - A3 HV significantly rises after 60 min of artificial aging and 0.05 reaches its maximum after 180 min. Further heating can not significantly improve microhardness, i.e., the HV values keep nearly the same level up to 24 hours artificial aging. 0.05 For A2 and A3 the microhardness is higher than in A1, similar as observed for HB in Figure 6.3 a. The reason is the formation of Mn-containing Al(Mn, Fe)Si dispersoids.
In the Li-containing alloys B1 and B2, HV was much lower than for the base alloys 0.05 A1 - A3. Similar as HB in Fig. 6.5 a, HV0.05 of, C1 and C2 reaches its maximum after 300 min artificial aging, but the values are still lower than for A1 - A3. It was shown in Chapter 5 (Section 5.4), which the Mg content in the solid solution of C1 and C2 is the lowest one.
After solution treatment at 575.0°C for 1 h, the Mg content in solid solution was about 1.20 wt.% in C1, which is dramatically lower to that measured in A1, where the solid solution contains 2.50 wt.% Mg.
It has to be noticed that the most unfavorable point for C1 and C2 is that the solid solution already decomposes during solution treatment and precipitates are formed (Figure 5.13).
It can be expected that no much precipitation will take place during aging, mostly growth of precipitates. Consequently, the volume fraction of precipitates should remain constant, only their size should increase. This has to be directly established by TEM examinations.
HV for the commercial casting alloys A201.0 and A356.0 is included in Figure 6.5 b 0.05
for comparison. The microhardness of A1 - A3 is lower than that of A201 alloy aged at 150.0C
Similar to HB, D1 and D2 alloys show the lowest microhardness in as-cast state. Measurements of
HV 0.05 after solution treatment, quenching and artificial aging show that it gradually increases and reaches a maximum after 360 min exposure. The obtained data indicate the ability of Al-Mg-Ge system to be age hardened. The
microhardness of the Li-containing D2 alloy increases at a earlier stage of aging than for D1 (Figure 6.6). However, the absolute values of HV are on the same level for both, D1 and D2. 0.05
In conclusion, the indirect methods DSC and hardness measurement gave indications for solid state transformations. The final prove of precipitation processes in A1 - D2 are examinations of thin foils using TEM, as presented in the following section.