Resistance to Fracture of AlZnMgCu ( AA7000) High Strength Aluminum Alloys

Material of complex specified properties, such as strength, ductility, fracture resistance and stress corrosion cracking resistance are required for the application in structures operating under heavy service loadings (like in aircraft and rocket industry) with prescribed high level of safety margin.

Aluminum alloys, due to convenient strength-to-density ratio, have found extensive application in heavy-duty structures. This is the case with AlZnMgCu alloys exhibiting the highest strength level, but in the same time high level of susceptibility to fracture and stress corrosion.

Mechanical properties of AlZnMgCu alloys depend on:

• Chemical composition, primarily on total Zn+Mg content and Zn/Mg ratio
• The presence of Mn, Cr, Zr and impurities Fe and Si,
• The microstructure as a result of fabrication thermo mechanical and heat treatment conditions, e.g. on morphology, distribution and volume portion of intermetallic (IM) phases and texture.

In the microstructure of AlZnMgCu alloy, the presence of three types of particles could be expected:

• Large inclusions, 0.1 to 10 mm in diameter, produced during crystallization (solidification), mainly unsoluble IM phases rich in Fe and Si; these phases are weak points regarding the fracture resistance properties;
• Intermediate particles, rich in Cr, Mn, or Zr, 0.05 to 0.5 mm in size;
• Small precipitates, with dimensions 0.01 to 0.5 mm, as a result of solid solution decomposition - thermal precipitation (ageing).

Based on requirement to develop Al alloy with ultimate tensile strength above 650 MPa and satisfactory level of ductility and fracture toughness, several alloys had been designed with Zn content between 6 and 8.8% and Mg content of 2.15 to 3.5%, saving the same amount of other alloying elements (Cu, Mn, Cr, Zr).

In order to get a closer insight in metallurgical effects of hot and cold work, thermo mechanical and heat treatments on micro structural variations and mechanical properties, many tests had been performed with the designed alloys. The morphology and distribution of unsoluble phases sized above 0.6 µm had been investigated by quantitative metallographic analysis and their influence on fracture resistance was evaluated.

Tensile properties, ultimate tensile strength Rm and yield strength R_P0.2 are tested on small size proportional specimens, 8 mm in diameter. Graphic presentation in the Figure 1 is a result of (here produced and tested) tests on 200 heats of AlZnMgCu alloys, heat treated on 460^oC for 2 hours, water quenched and artificial by aged in one step (120^oC/24h).

Table 1 - Chemical composition of experimental alloys

Alloy	Chemical composition (wt %)						Impurities
Design.	Zn	Mg	Cu	Mn	Cr	Zr	Fe	Si
A02	6.00	2.20	2.58	0.28	0.25	0.15	0.28	0.11
B01	7.15	3.15	1.53	0.23	0.22	0.15	0.28	0.11
C02	7.38	2.21	1.44	0.29	0.24	0.15	0.28	0.11
C03	7.20	2.21	1.44	0.29	0.24	0.15	0.21	0.06
C04	7.20	2.15	1.46	0.28	0.16	0.12	0.12	0.05
D02	8.30	3.30	1.63	0.24	0.23	0.15	0.28	0.15
D03	8.20	2.35	1.45	0.29	0.19	0.15	0.15	0.07
D04	8.80	3.50	1.45	0.27	0.21	0.15	0.14	0.07

From the statistic evaluation following relations could be derived for Zn and Mg in wt%:

Rm = 461.74 + 20.16 Zn + 33.82 Mg
R_P0.2 = 350.81 + 28.40 Zn + 35.10 Mg

Plain strain fracture toughness, K_IC, had been tested according to ASTM E 399 on SE (B) specimens of 20x40 mm cross-section. Fatigue pre-crack was produced on high-frequency pulsating machine.

a)                                                          b)

Figure 1. The effect of Mg and Zn content on yield strength R_P0.2 (a) and ultimate tensile strength Rm (b) of AlZnMgCu alloys

Figure 2 presents results obtained for several alloys of different Zn and Mg contents produced according above given regime. Relation of fracture toughness with Zn and Mg wt% can be defined in following form:

K_IC = 66.78 - 2.6 Zn - 7.22 Mg

Figure 2. The effect of Mg and Zn content on fracture toughness K_IC of AlZnMgCu alloys

In order to get a closer insight in mechanical properties, in Figure 3 K_IC is presented versus yield strength for tested alloys. It is clear from Figure 3 that C0 type alloy series could be accepted (for in detail analysis) as most promising, regarding strength and fracture resistance, offering K_IC ranged between 28.9 and 41.4 MPa m^1/2 for yield strength between 630 and 665 Mpa.

Figure 3. Fracture toughness K_IC related to yield strength R_P0.2 for the set of alloys

However, the selection of best combination of strength and fracture toughness of AlZnMgCu alloy is also influenced by Fe content. The effect of Fe content on fracture toughness for selected AlZnMgCu alloys for the same Zn and Mg content is presented in Figure 4.

Figure 4. The effect of Fe content on K_IC of AlZnMgCu alloys with same Zn and Mg content for ageing regimes.

The presence of secondary intermetallic phases in number, size, volume portion (especially coarse particles, rich in Fe and Si) has been analyzed on structural analyzer TAS plus. The analysis of fractured surface after fracture mechanics test had been performed on a scanning electron microscope SEM 511 with EDAX 9900.

Performed investigation revealed that the increase of Zn and Mg content generally increase strength properties and decrease fracture toughness. Although required strength (ultimate tensile strength over 650 MPa) had been achieved by several alloys (B01 type, C02, C03, C04, D02, D03, D04), only C0 type alloys (~ 7.3% Zn, ~ 2.2% Mg) had offered satisfactory fracture toughness.

It has also been proved that decreasing Fe content in alloys (C02 to C04), and corresponding decrease in coarse intermetallic particles of second phase content result in increasing fracture resistance. Coarse particles, rich in Fe and Si, are deformed and fractured during tests, where as fine regularly distributed intermetallic phases contribute to ductile behavior. In the last case, the particles are fractured only seldom.

By comparing obtained results, one can conclude that best combination of strength and fracture toughness properties has been achieved by C04 alloy composition, with low Fe content and corresponding regular distribution of fine second phase particles.

Two steps precipitation ageing regime (100^oC/5h + 160^oC/5h) for C04 alloy can be recommended when highest fracture toughness is required, resulted in slightly reduced strength parameters compared to one step (129^oC/24h) or alternative investigated two steps (100^oC/5h + 130^oC/10h) precipitation ageing.