**3. Results and discussion**

156 Recent Trends in Processing and Degradation of Aluminium Alloys

The investigation was performed on the two casting alumium alloys AlCu4Ni2Mg and AlCu6Ni. AlCu4Ni2Mg is the standard alloy used currently for highly stressed structural elements of engines and AlCu6Ni1 is an experimental alloy that was chosen to investigate the influence of the increased content of Cu on the phase composition, microstructure morphology and mechanical, technological and operational properties. The alloys were cast into metal moulds and subjected to X-ray inspection in order to exclude the presence of

The alloys were subjected to heat treatment T6 followed by annealing at 523 K and 573 K for 150 and 500 hours. After analysis of the results of preliminary tests it was found, that it is advisable to apply additional annealing times at particular temperature, i.e. 100, 300 and 750

Heat treatment conditions were established on the basis of the phase equilibrium diagrams Al-Si and Al-Cu and available heat treatment data for the alloys with similar chemical composition (both from literature and used in industry practice). The consideration was also given to requirements concerning mechanical properties of the alloys resulting from operation condition of the structural elements made of these alloys. The chemical composition of the investigated alloys and heat treatment parameters are

Element Element content, wt.%

Mn <0.10 0.90 Ni 2.10 1.10 Cu 4.30 6.36 Zr − 0.01 Fe 0.10 0.20 Si 0.10 0.10 Mg 1.50 0.05 Zn 0.30 −

Al balance balance

water cooling

air cooling

Table 1. Composition of AlCu4Ni2Mg and AlCu6Ni alloys and heat treatment parameters Examination of the alloys microstructure was carried out using light microscope (LM), as

solution treatment 793<sup>±</sup>5K/5h/

artificial ageing 523<sup>±</sup>5K/5h/

well as scanning (SEM) and transmission (TEM) electron microscopes.

AlCu4Ni2Mg AlCu6Ni

818<sup>±</sup>5K/10h/ water cooling

498<sup>±</sup>5K/8h/ air cooling

**2. Material and methodology** 

porosity or oxide films.

presented in table 1.

hours.

Figs. 1 to 4 show the results of microscopic observations of AlCu4Ni2Mg and AlCu6Ni alloys (in T6 condition). In both of investigated alloys large, irregular shaped precipitates of

Fig. 1. Microstructure of AlCu4Ni2Mg alloy in T6 condition (LM)

Fig. 2. Microstructure of AlCu4Ni2Mg alloy in T6 condition: precipitations of intermetallic phases in interdendritic areas (SEM)

Microstructural Changes of Al-Cu Alloys After Prolonged Annealing at Elevated Temperature 159

Based upon the EDS results the phases forming large size particles was identified as Al-Cu-Ni, Al-Cu-Ni-Fe and Al-Cu-Mn (fig. 5−6) (Mrówka-Nowotnik et al. 2007, Wierzbińska &

Fig. 5. AlCu4Ni2Mg alloy − EDS analysis of the areas 1−4

Sieniawski 2010).

intermetallic phases, located on the dendrite boundaries of solid solution α-Al and dispersive, spheroidal and strip shaped hardening phase precipitates homogenously distributed throughout the solid solution were observed.

Fig. 3. Microstructure of the AlCu6Ni alloy in T6 condition (LM)

Fig. 4. Microstructure of the AlCu6Ni1 alloy in T6 condition (SEM)

intermetallic phases, located on the dendrite boundaries of solid solution α-Al and dispersive, spheroidal and strip shaped hardening phase precipitates homogenously

distributed throughout the solid solution were observed.

Fig. 3. Microstructure of the AlCu6Ni alloy in T6 condition (LM)

Fig. 4. Microstructure of the AlCu6Ni1 alloy in T6 condition (SEM)

Based upon the EDS results the phases forming large size particles was identified as Al-Cu-Ni, Al-Cu-Ni-Fe and Al-Cu-Mn (fig. 5−6) (Mrówka-Nowotnik et al. 2007, Wierzbińska & Sieniawski 2010).

Fig. 5. AlCu4Ni2Mg alloy − EDS analysis of the areas 1−4

Microstructural Changes of Al-Cu Alloys After Prolonged Annealing at Elevated Temperature 161

Fig. 7. Microstructure of the AlCu4Ni2Mg alloy: a) precipitates of *S-Al2CuMg* phase, b) electron

diffraction pattern obtained from the precipitate, c) solution of the diffraction pattern

Fig. 8. Microstructure of the AlCu6Ni alloy: particle of α*-*Al2CuMg phase

Fig. 6. AlCu6Ni alloy − EDS analysis of the areas 1−5

Fig. 6. AlCu6Ni alloy − EDS analysis of the areas 1−5

Fig. 7. Microstructure of the AlCu4Ni2Mg alloy: a) precipitates of *S-Al2CuMg* phase, b) electron diffraction pattern obtained from the precipitate, c) solution of the diffraction pattern

Fig. 8. Microstructure of the AlCu6Ni alloy: particle of α*-*Al2CuMg phase

Microstructural Changes of Al-Cu Alloys After Prolonged Annealing at Elevated Temperature 163

The shape of Al2Cu particles was diversified from nearly regular polygons – "crystallites" to

Fig. 11. Microstructure of the AlCu6Ni alloy **–** precipitates of θ'-Al2Cu phase in the shape of

plates (TEM – thin foil)

strongly elongated – "rod-shaped" (fig. 11−12).

The intermetallic phases S-Al2CuMg (fig. 7) and α-Al2CuMg (fig. 8) as well as hardening phase θ'-Al2Cu (fig. 9-11) were identified in the alloy microstructure by electron diffraction analysis (Pearson, 1967).

Fig. 9. Microstructure of the AlCu4Ni2Mg alloy in T6 condition (TEM – thin foil). The precipitates of hardening phase θ'-Al2Cu

Fig. 10. Microstructure of the AlCu6Ni alloy in T6 condition (TEM – thin foil). The precipitates of hardening phase θ'-Al2Cu

The intermetallic phases S-Al2CuMg (fig. 7) and α-Al2CuMg (fig. 8) as well as hardening phase θ'-Al2Cu (fig. 9-11) were identified in the alloy microstructure by electron diffraction

Fig. 9. Microstructure of the AlCu4Ni2Mg alloy in T6 condition (TEM – thin foil). The

Fig. 10. Microstructure of the AlCu6Ni alloy in T6 condition (TEM – thin foil). The

analysis (Pearson, 1967).

precipitates of hardening phase θ'-Al2Cu

precipitates of hardening phase θ'-Al2Cu

The shape of Al2Cu particles was diversified from nearly regular polygons – "crystallites" to strongly elongated – "rod-shaped" (fig. 11−12).

Fig. 11. Microstructure of the AlCu6Ni alloy **–** precipitates of θ'-Al2Cu phase in the shape of plates (TEM – thin foil)

Microstructural Changes of Al-Cu Alloys After Prolonged Annealing at Elevated Temperature 165

Examination of the alloys microstructure after prolonged annealing revealed that the precipitates of Al6Fe and S-Al2CuMg phases and large precipitates of intermetallic phases at the dendrite boundaries practically did not change (fig. 13−14) even after very long time of annealing (750h). Whereas, significant increase in size of dispersive particles of θ'-Al2Cu

Fig. 13. Microstructure of the AlCu4Ni2Mg alloy after annealing: a) 523K/100h, b) 573K/750h

Fig. 14. Microstructure of the AlCu6Ni1 alloy after annealing: a) 523K/100h, b) 573K/750h

hardening phase was observed (fig. 15-18).

Fig. 12. Microstructure of the AlCu6Ni alloy **–** precipitates of θ'-Al2Cu phase in the shape of crystallites (TEM – thin foil)

Fig. 12. Microstructure of the AlCu6Ni alloy **–** precipitates of θ'-Al2Cu phase in the shape of

crystallites (TEM – thin foil)

Examination of the alloys microstructure after prolonged annealing revealed that the precipitates of Al6Fe and S-Al2CuMg phases and large precipitates of intermetallic phases at the dendrite boundaries practically did not change (fig. 13−14) even after very long time of annealing (750h). Whereas, significant increase in size of dispersive particles of θ'-Al2Cu hardening phase was observed (fig. 15-18).

Fig. 13. Microstructure of the AlCu4Ni2Mg alloy after annealing: a) 523K/100h, b) 573K/750h

Fig. 14. Microstructure of the AlCu6Ni1 alloy after annealing: a) 523K/100h, b) 573K/750h

Microstructural Changes of Al-Cu Alloys After Prolonged Annealing at Elevated Temperature 167

Fig. 17. Microstructure of the AlCu6Ni alloy **–** precipitates of the θ'-Al2Cu phase after

Fig. 18. Microstructure of the AlCu6Ni **–** precipitates of the θ'-Al2Cu phase after annealing at

annealing at 523 K for: a)100 h, b) 300 h, c) 500 h, d) 750 h

573K for: a) 100 h, b) 300 h, c) 500 h, d) 750 h

Fig. 15. Microstructure of the AlCu4Ni2Mg alloy **–** precipitates of the θ'-Al2Cu phase after annealing at 523 K for: a)100 h, b) 300 h, c) 500 h, d) 750 h

Fig. 16. Microstructure of the AlCu4Ni2Mg alloy – precipitates of the θ'-Al2Cu phase after annealing at 573K for: a) 100 h, b) 300 h, c) 500 h, d) 750 h

Fig. 15. Microstructure of the AlCu4Ni2Mg alloy **–** precipitates of the θ'-Al2Cu phase after

Fig. 16. Microstructure of the AlCu4Ni2Mg alloy – precipitates of the θ'-Al2Cu phase after

annealing at 573K for: a) 100 h, b) 300 h, c) 500 h, d) 750 h

annealing at 523 K for: a)100 h, b) 300 h, c) 500 h, d) 750 h

Fig. 17. Microstructure of the AlCu6Ni alloy **–** precipitates of the θ'-Al2Cu phase after annealing at 523 K for: a)100 h, b) 300 h, c) 500 h, d) 750 h

Fig. 18. Microstructure of the AlCu6Ni **–** precipitates of the θ'-Al2Cu phase after annealing at 573K for: a) 100 h, b) 300 h, c) 500 h, d) 750 h

Microstructural Changes of Al-Cu Alloys After Prolonged Annealing at Elevated Temperature 169

Results of the measurements showed that annealing of the alloys studied at 573K led to significant growth of hardening θ'-Al2Cu phase precipitates already after 150h. The biggest change both of size and shape factor of the particles (sevenfold increase) was observed in AlCu6Ni alloy. In the AlCu4Ni2Mg alloy precipitates growth was not so substantial – shape factor was only doubled. Increase in annealing time (750h) resulted in further growth of precipitates. However the process was not so dynamic as in the initial stages of annealing

Microstructure examination indicated that growth of the hardening phase precipitates is the main symptom of the microstructure degradation caused by long-term thermal loads. Coarsening and change of the shape of hardening phase particles lead to change of mechanism of their interactions with dislocations and as a consequence of that decrease of

Results of the static tensile test for the alloys studied in T6 condition and after additional annealing at 523 and 573K for 100, 150, 300, 500 and 750h are presented in table 3 and in

(a)

(b)

AlCu4Ni2Mg alloy as a function of annealing time at the temperature of a) 523K and b) 573K

Fig. 20. Ultimate tensile strength, 0.2% offset yield strength and elongation A5 for

(table 2, fig. 19) – only minor changes of shape factor were observed.

strength properties of the alloys (Hirth & Lothe, 1968).

figures 20 and 21.

Microstructure examination revealed that in both alloys i.e. AlCu4Ni2Mg and AlCu6Ni growth of the hardening phase precipitates occured as a result of long-term thermal loading, which was proportional to the temperature and time of annealing. However higher coarsening propensity was found for Al6CuNi alloy which arose from higher content of the element forming hardening phase (6% Cu). It was confirmed by analysis of the change of shape and size of the θ'-Al2Cu precipitates in both alloys after annealing at 573K for 150 and 750 h comparing to the standard T6 condition (table 2).


Table 2. Evolution of θ'-Al2Cu precipitates in AlCu4Ni2Mg i AlCu6Ni alloys during annealing at 573K for 150 and 750h

Fig. 19. Change of shape factor of the θ'-Al2Cu precipitates in AlCu4Ni2Mg and AlCu6Ni as a result of annealing at 573K for 150 and 750h

Microstructure examination revealed that in both alloys i.e. AlCu4Ni2Mg and AlCu6Ni growth of the hardening phase precipitates occured as a result of long-term thermal loading, which was proportional to the temperature and time of annealing. However higher coarsening propensity was found for Al6CuNi alloy which arose from higher content of the element forming hardening phase (6% Cu). It was confirmed by analysis of the change of shape and size of the θ'-Al2Cu precipitates in both alloys after annealing at 573K for 150 and

T6 + annealing at 573 K Alloy

width, w (nm) 25,20 131,15 158,19 AlCu4Ni2Mg

width, w (nm) 10,30 115,36 149,11 AlCu6Ni

Fig. 19. Change of shape factor of the θ'-Al2Cu precipitates in AlCu4Ni2Mg and AlCu6Ni as

Table 2. Evolution of θ'-Al2Cu precipitates in AlCu4Ni2Mg i AlCu6Ni alloys during

length, l (nm) 75,12 650,28 887,45

shape factor l/w 2,98 4,95 5,61 length, l (nm) 55,82 4465,60 6255,05

shape factor, l/w 5,42 38,71 41,95

Heat treatment conditions

150 h 750 h

750 h comparing to the standard T6 condition (table 2).

annealing at 573K for 150 and 750h

a result of annealing at 573K for 150 and 750h

Shape parameters of

<sup>θ</sup>'-Al2Cu precipitates T6

Results of the measurements showed that annealing of the alloys studied at 573K led to significant growth of hardening θ'-Al2Cu phase precipitates already after 150h. The biggest change both of size and shape factor of the particles (sevenfold increase) was observed in AlCu6Ni alloy. In the AlCu4Ni2Mg alloy precipitates growth was not so substantial – shape factor was only doubled. Increase in annealing time (750h) resulted in further growth of precipitates. However the process was not so dynamic as in the initial stages of annealing (table 2, fig. 19) – only minor changes of shape factor were observed.

Microstructure examination indicated that growth of the hardening phase precipitates is the main symptom of the microstructure degradation caused by long-term thermal loads. Coarsening and change of the shape of hardening phase particles lead to change of mechanism of their interactions with dislocations and as a consequence of that decrease of strength properties of the alloys (Hirth & Lothe, 1968).

Results of the static tensile test for the alloys studied in T6 condition and after additional annealing at 523 and 573K for 100, 150, 300, 500 and 750h are presented in table 3 and in figures 20 and 21.

Fig. 20. Ultimate tensile strength, 0.2% offset yield strength and elongation A5 for AlCu4Ni2Mg alloy as a function of annealing time at the temperature of a) 523K and b) 573K

Microstructural Changes of Al-Cu Alloys After Prolonged Annealing at Elevated Temperature 171

523 K 9 12 13 16 19 18 23 30 42 46 573 K 17 31 31 34 35 36 44 49 58 64

523 K 5 10 14 19 24 21 25 35 41 50 573 K 24 26 33 40 45 37 48 51 59 64

Table 4. Relative decrease of ultimate tensile strength and 0.2% offset yield strength of the

It was found that both alloys subjected to long-term annealing exhibit significant reduction of mechanical properties. This tendency was characterized by the coefficient calculated according to the formula [(R – R(T)) × R-1 × 100%] where: R – UTS or YS in T6 condition, R(T) – UTS or YS after annealing at 523/573K (table 4). The analysis of the dependence of that

(a)

(b) Fig. 22. Relative change of ultimate tensile strength (a) and 0.2% offset yield strength (b) of

the AlCu4Ni2Mg and AlCu6Ni alloys as a function of time of annealing at 523

Annealing [(UTS-UTS(T)) /UTS] × 100% [(YS-YS(T)) /YS] × 100% temperature 100h 150h 300h 500h 750h 100h 150h 300h 500h 750h

AlCu4Ni2Mg

AlCu6Ni

AlCu4Ni2Mg and AlCu6Ni1alloys after annealing at 523 and 573K


Table 3. Mechanical properties of the AlCu4Ni2Mg and AlCu6Ni alloys in standard T6 condition and after additional annealing at 523 and 573K

Fig. 21. Ultimate tensile strength, 0.2% offset yield strength and elongation A5 for AlCu6Ni alloy as a function of annealing time at the temperature of a) 523K and b) 573 K

properties T6 100h 150h 300h 500h 750h 100h 150h 300h 500h 750h AlCu4Ni2Mg alloy 0.2%YS, MPa 305 249 234 214 178 164 195 170 155 128 110 UTS, MPa 318 290 281 276 268 256 265 220 220 210 205 A5, % 0,8 1,3 1,7 1,5 2,3 2,1 3,4 5,2 6,1 6,8 7,8 AlCu6Ni alloy 0.2%YS, MPa 285 225 215 185 168 142 180 147 140 118 104 UTS, MPa 323 305 290 277 263 243 245 240 216 192 177 A5, % 0,7 1,3 1,9 2,6 3,1 3,8 2,4 4,2 5,6 6,8 7,0

Mechanical T6+523 K T6+573 K

Table 3. Mechanical properties of the AlCu4Ni2Mg and AlCu6Ni alloys in standard T6

(a)

(b) Fig. 21. Ultimate tensile strength, 0.2% offset yield strength and elongation A5 for AlCu6Ni

alloy as a function of annealing time at the temperature of a) 523K and b) 573 K

condition and after additional annealing at 523 and 573K

Heat treatment – temperature and time of annealing


Table 4. Relative decrease of ultimate tensile strength and 0.2% offset yield strength of the AlCu4Ni2Mg and AlCu6Ni1alloys after annealing at 523 and 573K

It was found that both alloys subjected to long-term annealing exhibit significant reduction of mechanical properties. This tendency was characterized by the coefficient calculated according to the formula [(R – R(T)) × R-1 × 100%] where: R – UTS or YS in T6 condition, R(T) – UTS or YS after annealing at 523/573K (table 4). The analysis of the dependence of that

Fig. 22. Relative change of ultimate tensile strength (a) and 0.2% offset yield strength (b) of the AlCu4Ni2Mg and AlCu6Ni alloys as a function of time of annealing at 523

Microstructural Changes of Al-Cu Alloys After Prolonged Annealing at Elevated Temperature 173

Values of the ultimate tensile strength and 0.2% offset yield strength of the alloy subjected to long-term thermal loads (573K/750h) characterize its ability to preserve strength properties

> *YS*(max), MPa in T6 condition

523 AlCu4Ni2Mg 305 164 18

573 AlCu4Ni2Mg 305 110 25

Table 5. Minimum values of the 0.2% offset yield strength of the AlCu4Ni2Mg and AlCu6Ni alloys after annealing at 523/573K for 750h and maximum values obtained for T6 condition Both alloys exhibit similar repeatability of tensile test results, however AlCu6Ni alloy shows slightly better stability of strength properties (table 5). However AlCu4Ni2Mg alloy is superior to AlCu6Ni alloy in terms of maximum and minimum yield strength after

In the AlCu4Ni2Mg and AlCu6Ni alloys degradation of the microstructure takes place as a result of long-term thermal loading. It consists largely in coarsening and the change of the shape of hardening phase particles (θ'-Al2Cu). The changes are proportional to the annealing time and temperature and lead to significant decrease of the mechanical properties of the alloys. The alloys studied are characterized by different content of Cu – primary element forming hardening phase. Increased Cu content in AlCu6Ni alloy caused only slight improvement of the stability of its strength properties. The AlCu4Ni2Mg alloy containing less Cu but with addition of Mg is characterized by better strength properties than AlCu6Ni alloy in T6 condition and preserves relatively high tensile strength and good ductility after long-term thermal loading. Taking into account criterion of mechanical properties and their stability both alloys studied can be successfully applied for highly stressed elements of

El-Magd, E. & Dünnwald, J. (1996). Influence of constitution on the high-temperature creep

Mrówka-Nowotnik, G., Wierzbińska, M., & Sieniawski J. Analysis of intermetallic particles

in AlSi1MgMn aluminium alloy. (2007). *Journal of Archieves in Materials and* 

Hirth, J.P. & Lothe, J. (1968). Theory of dislocations. McGraw-Hill, New York-London

AlCu6Ni 285 142 11

AlCu6Ni 285 104 18

*YS*(min), MPa after annealing for 750h Wz,%

Annealing 0.2% offset yield strength

particular heat treatment. It has also higher ultimate tensile strength.

aircraft structures operating in the temperature range of 523-573K.

behavior of AlCuMg alloy. *Metallkunde*, Vol.506, pp.411-414

Martin, J.W. Preciptation Hardening. (1968). Pergamon Press, Oxford

*Manufacturing Engineering*, Vol.1-2, No.20, pp.155-158

in operation condition of the castings (table 5).

Alloy

temperature, K

**4. Conclusions** 

**5. References** 

coefficient value on time of annealing enabled comparison of stability of mechanical properties of the investigated alloys (fig. 22 and 23).

Fig. 23. Relative change of ultimate tensile strength (a) and 0.2% offset yield strength (b) of the AlCu4Ni2Mg and AlCu6Ni alloys as a function of time of annealing at 573K

Repeatability of the mechanical properties of AlCu4Ni2Mg and AlCu6Ni alloys after longterm annealing was determined on the basis of variation of the static tensile test results (table 5). Five specimens were tested for each temperature and time of annealing. Coefficient of variation was calculated using formula:

$$\mathcal{W}\_z = \frac{s}{\overline{\overline{\chi}}} \times 100\tag{1}$$

where: *s* **–** standard deviation, *x* **–** average value

Values of the ultimate tensile strength and 0.2% offset yield strength of the alloy subjected to long-term thermal loads (573K/750h) characterize its ability to preserve strength properties in operation condition of the castings (table 5).


Table 5. Minimum values of the 0.2% offset yield strength of the AlCu4Ni2Mg and AlCu6Ni alloys after annealing at 523/573K for 750h and maximum values obtained for T6 condition

Both alloys exhibit similar repeatability of tensile test results, however AlCu6Ni alloy shows slightly better stability of strength properties (table 5). However AlCu4Ni2Mg alloy is superior to AlCu6Ni alloy in terms of maximum and minimum yield strength after particular heat treatment. It has also higher ultimate tensile strength.
