Fatigue properties of Al2O3-particle-reinforced 6061 aluminium alloy in the high-cycle regime

Int. J. Fatigue Vol. 18, No. 7, pp. 475--481, 1996 Copyright © 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0142- ! 123/96/$ i 5.00

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0142-1123(96)00090-9

Fatigue properties of AI2Oz-particle-reinforced 6061 aluminium alloy in the high-cycle regime M. Papakyriacou*, H.R. Mayert, S.E. StanzI-Tscheggt and M. Gr6schl$ *University of Vienna, Institute of Solid State Physics, Vienna, Austria tUniversity of Agriculture, Institute of Meteorology and Physics, Vienna, Austria #:Technical University of Vienna, Institute of General Physics, Vienna, Austria (Received 30 May 1995; accepted 14 September 1995) The fatigue properties of A1203-particle-reinforced and unreinforced 6061-T6 aluminium alloys were investigated in the regime from 5 x 105 to 109 cycles to failure under constant-amplitude, fully reversed loading conditions (R =-1) using smooth specimens. Composites with different contents of reinforcing particles (12 vol.%, 15 vol.% and 21 vol.%) and different mean particle diameter showed about ten times fewer cycles to failure than unreinforced 6061-T6. The fatigue limits were 145 MPa for unreinforced 6061-T6 and 115 MPa for the reinforced alloys. Large, broken particles could be observed as preferential sites for fatigue crack initiation. Near crack initiation, A1203 particles are unsuitable obstacles for short crack growth, but brittle fracture of the reinforcing component was observed; arrangement of particles in clusters favours the initial damage process in addition. In order to optimize the fatigue properties of 6061 aluminium alloys, fine A1203 particles should be used; a high content of reinforcing component is beneficial for increasing the stiffness of the alloy. Copyright © 1996 Elsevier Science Limited (Keywords: metal-matrix composition; A1203p reinforced aluminium alloy 6061; high cycle fatigue)

Metal-matrix composites have advantages over the unreinforced material in many technical applications: addition of oxides, carbides or nitrides raises their wear resistance and static strength at elevated temperatures. Because of the high stiffness of the reinforcing components, the Young's modulus of the composite increases, and the temperature coefficient is reduced. An economical method for producing aluminium alloy composites is to add ground SiC or A1203 particles, l Particle-reinforced aluminium alloys are already used for car pistons and sleeves, and in the near future this type of material may attain increasing importance in the automobile industry, as well as for construction parts of aeroplanes. Cyclic loads are often imposed on automobile and aeroplane components, so that knowledge of the fatigue behaviour of this type of composite material is needed. Fatigue crack growth in SiC-particle-reinforced aluminium alloys has been studied in numerous investigations. 2-~° Reduced crack growth rates in comparison with the unreinforced matrix material due to crack trapping and crack deflection have been observed.5-7,9 Increasing content of SiC particles leads to superior fatigue crack growth properties. 6,9 Coarse SiC particles have been found to be more efficient than fine particles in improving fatigue crack growth properties for load ratio R = 0.1.3'8"9

For very low cyclic stress intensity factors, A1203particle reinforcement of 6061 aluminium alloy shows, as for SiC particles, a beneficial influence on fatigue crack growth properties. ~ The threshold stress intensity factor increases and the crack growth rates for very low cyclic loads decrease as a result of frequent deflection of the crack path by particles, which act as obstacles for crack extension. Nevertheless, the beneficial influence of AlaO3-particle reinforcement is, in contrast to SiC particles, restricted to near-threshold loading. For higher cyclic loads, unreinforced 6061-T6 showed lower fatigue crack growth rates than 6061/A1203 composites. ~1 The different fatigue crack growth behaviour was attributed to a lower bonding strength between the A1203 particles and the matrix in comparison with the SiC-particle-matrix interface. Fine A1203 particles were more efficient in improving fatigue crack growth properties than coarse particles, ~ which is in contrast to 6061/SiC-particle-reinforced composites for R = 0 . 1 . 3,8,9 When very high numbers of cycles are imposed on construction parts, such as 108 or above in the automobile industry, the lifetime is governed by the initiation and formation of a main crack; the crack extension period contributes less to the lifetime. The fatigue crack initiation process in particle-reinforced aluminium alloys, however, has received less attention

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than the crack propagation period in the past. Observations of crack initiation in SiC-reinforced 6061 aluminium alloy at a cyclic load slightly below the yield stress showed that cracks initiated from cracked, coarse particles. 4 In ref. 12, the low-cycle fatigue behaviour of three A1203-particle-reinforced 6061 aluminium alloys was compared with that of the unreinforced material. Although the static strength properties of unreinforced 6061-T6 are worse, poorer low-cyclic behaviour of A1203-particle-reinforced alloys was observed, especially in the low-cycle (i.e. high strain amplitude) regime. A comparison of the fatigue properties of SiC-particle- and A1203-particle-reinforced 6061 and unreinforced 6061-T6 was presented in ref. 13. The A1203-particle-reinforced material showed a lower number of cycles to failure than the unreinforced aluminium alloy in the entire regime investigated, from 103 tO about 107 cycles. The fatigue properties of SiCreinforced 6061 are approximately the same as those of unreinforced 6061-T6 at cycles to failure below 105 and better at lower cyclic loads. The difference in the fatigue behaviour of the two reinforced alloys was explained by the different particle size distribution (fine SiC particles, c o a r s e A1203 particles). Particles and particle clusters were assumed to be the origin of crack initiation. ~3 In the present paper, studies of the fatigue behaviour of AlzO3-particle-reinforced 6061 aluminium alloys are extended to the very high-cycle region (109 cycles). In addition, the use of three composite materials with different contents as well as different size distribution of the A1203 particles makes it possible to investigate the consequences of variation of the reinforcing component. MATERIAL Fatigue investigations have been performed using four different materials: pure 6061-T6 without particle reinforcement, 6061 reinforced with 11.7 vol.% A1203 particles (6061/AlzO3/12p), a composite with 15.3 vol.% particles (6061/AlzO3/15p), and one with 20.5 vol.% particles (6061/A1203/21p). The chemical compositions of the unreinforced and reinforced alloys are similar, as shown in Table 1. Preparation of the four different materials followed the same production and heat-treating procedure. Production of composites was according to the Duralcam TM process. 1 After extrusion in cylindrical rods with a diameter of 13 ram, ageing of the alloys was according to T6 (solution heat treatment at 525 °C for 0.5 h, followed by water quenching and ageing at 160 °C for 24 h). Heat treating led to strongly varying grain sizes with a mean diameter of about 50 ~m in the extrusion direction in all four alloys. The addition of A1203 Table 1 Chemical composition (in wt %) of A1203-particlereinforced and unreinforced 6061 Material 6061

Mg

Si

Cu

Fe

Mn

Cr

Zn

Ti

0.95 0.80 0.30 0.50 0.10 0.20 0.09 0.06 0.93 0.69 0.28 0.20 0.0090.10

EXPERIMENTAL PROCEDURE Fatigue experiments were performed using the highfrequency resonance testing method. In this procedure, specimens are excited to a resonance tension-compression vibration by use of a piezoelectric transducer. The loading frequency is about 20 kHz, which makes it possible to apply load cycles up to 109 in a timesaving manner. Specimen dimensions are chosen such that resonance criteria are satisfied; the specimen shape used is shown in Figure 3. The vibration amplitude varies along the specimen axis, with maxima at the specimen's ends. It becomes zero (vibration nodes) in the centre of the specimen, and the cyclic strain amplitude shows a maximum there. The cyclic strain is measured by means of strain gauges, which serve to calibrate and control loading. During the experiments a certain load amplitude is maintained with an accuracy of 99% using an electrical control unit. Using Hooke's law, the stress amplitude is calculated from the cyclic strain and Young's modulus (Table 2). No preload was applied in the experiments and the specimens were cycled under fully reversed loading conditions (R =-1). Details of the experimental procedure are described elsewhere. 14 Fatigue experiments were performed in a humid air environment (18-22 ° C, 40-60% relative humidity). Load was applied in a pulse-pause sequence with a pulse length of 1000 cycles. Periodic pauses between 50 and 500 ms served to carry off the heat produced by internal friction. To improve heat dissipation, a cooling fan was used. Table 2 Mechanical properties of A1203-particle-reinforced and unreinforced 6061 aluminium alloys Material

6061/A1203/12p 0.89 0.64 0.28 0.15 0.0040.11 0.007 0.06 6061/AI203/15p 0.8 0.68 0.27 0.20 0.0090.10 0.01 0.15 6061/A1203/21p

particles influences the static strength properties slightly, whereas a pronounced decrease of ductility and increase of Young's modulus with increasing particle content are obtained, as shown in Table 2. Besides the content, the particle size distribution also varies for the three reinforced alloys. Microsection surfaces normal to the extrusion direction were used (Figure 1) to analyse the particle size. A quantitative evaluation of particle size and distribution is summarized in Figure 2. The mean particle size is characterized by Ocircle and Oma x (Table 3). Ocircle is determined in the following way. The cross-section area of all particles visible on the microsection surfaces is measured, and a mean value is determined. Ocircle characterizes the diameter of a circle with the same cross-sectional area as this mean value. To determine Dma x, the maximum length of each particle visible on the microsection surfaces is measured. Dma× is calculated as the mean value of the maximum length of the particles.

0.02 0.10

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E (GPa)

Rpo.2%

Rm

(MPa)

(MPa)

A5 (%)

70.4

364 322 342 351

383 359 376 381

13.5 6.4 4.8 3.7

6061/A1203/12p 84.2 6061/A1203/15p 90.5 6061/AlzO3/21p 96.3

E, Young's modulus; Rpo.2%,yield strength (0.2% offset); Rm, ultimate tensile strength; As, tensile elongation

HCF properties of AI2-O3-particle-reinforced 6061 AI alloy

477

Figure 1 Microsection surfaces normal to the extrusion direction of A1203-particle-reinforced alloys (a) 6061/AlzO3/12p, (b) 6061/A1203/15p and (c) 6061/A12Of121p

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Number of Cycles to Failure Figure 4 Number of cycles to failure vs cyclic stress amplitude for the unreinforced and the three A1203-particle-reinforced 6061 alloys

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Particle Diameter, Dmax , (Iam) Figure 2 Distribution of A1203-particle diameters (Dmax) in the metal-matrix composites

Table 3

Mean particle sizes characterized by Dcircle (diameter of a circle with the same cross-sectional area as the mean crosssectional area of the particles) and Dmax (mean value of the maximum length of the particles) and mean density of the A1203 particles on microsection surfaces for all three reinforced 6061 aluminium alloys Material

Dcircle (/~m)

6061/A1203/12p 6.0 6061/A1203/15p 7.5 6061/A1203/21p 9.0

Figure 3

Dmax (/xm)

Numbers of particles per mm 2

9.5 11.0 13.5

5200 3500 2100

could be observed for any of the materials investigated. Nevertheless, materials handbooks often define a technical fatigue limit such that no fatigue failure may be expected within 5 x 108 cycles. This technical fatigue limit is 145 MPa for 6061-T6 and 115 MPa for the three investigated composites. As the Young's modulus increases with increasing particle content, equivalent stress amplitudes imply lower cyclic strain for the composite with the higher particle content. If strain instead of stress is plotted versus number of cycles to failure, the different fatigue behaviour of reinforced and unreinforced alloys becomes more evident (Figure 5). In this representation, A1203-particle reinforcement leads to a reduction of failure cycles by about two orders of magnitude. Comparison of the reinforced alloys shows a slight decrease of fatigue times with increasing particle content. For clearer representation, data only for 6061/A1203/12p and 6061/A1203/21p (without those for 6061/A1203/15p) are shown in Figure 5. The fracture surfaces of cyclically loaded specimens have been investigated with a scanning electron microscope (SEM). In Figure 6a the fracture surface around the crack initiation site of unreinforced 6061-T6 is shown after loading with a stress amplitude of 144 MPa. Crack initiation probably took place at the specimen surface. A transcrystalline and relatively fiat crack path can be observed. Figure 6b shows a typical fatigue fracture surface of the crack initiation area for A1203-particle-reinforced

Specimen shape (dimensions in mm) 4

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RESULTS The results of S-N fatigue tests using 6061-T6 as well as the three A1203-reinforced alloys are summarized in Figure 4. The arrows indicate specimens that did not fail within the applied number of load cycles. The resulting S-N curves can be well approximated by straight lines in double logarithmic plots in the measured range of 5×105 to 5x108 cycles. The three particle-reinforced alloys show similar fatigue behaviour. The S-N curves of the unreinforced and reinforced materials are approximately parallel; the fatigue lives of the reinforced alloys are about 10 times shorter than those of the unreinforced 6061-T6. No fatigue limit

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Number of Cycles to Failure Figure 5 Number of cycles to failure vs cyclic strain amplitude for the unreinforced and the three A1203-particle-reinforced 6061 alloys

HCF properties of AI2-O3-particle-reinforced 6061. AI alloy

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Figure 6 Crack initiation areas of A1203-particle-reinforced and unreinforced 6061 aluminum alloys: (a) 6061-T6 after loading with a cyclic stress amplitude of 144 MPa; (b) 6061/AlEO3/15p after loading with a cyclic stress amplitude of 180 MPa; (c) 6061/A12Oa/21p after loading with a cyclic stress amplitude of 125 MPa

480

M. Papakyriacou et al.

6061. The material is 6061/A1203/15p, and the cyclic load amplitude was 180 MPa. Particles are fractured and show a flat fracture surface. (Reinforcing A1203 particles appear darker in the photographs). The particle density there is relatively high. Most reinforcing particles fractured normal to the loading direction in the plane of fatigue crack growth. Therefore the resulting fracture surface is relatively flat and poorly structured. Figure 6c shows the crack initiation area of alloy 6061/AlzO3/21p loaded with 125 MPa at a higher magnification. A relatively large broken particle close to the specimen surface and some smaller, broken particles are visible. The crack extension area is normal to the loading direction. More detailed investigations showed that large broken particles frequently occur near the crack initiation area. For quantitative support, six fatigue fracture surfaces for each reinforced alloy have been investigated with an SEM. The site of crack initiation was difficult to identify exactly. However, the 'crack initiation area' could be identified quite accurately within a diameter of 100/xm. The size of each particle within this area of crack formation has been evaluated by determining its maximum length Dm,~. The mean value of Dma× of all particles in this area is 20 ~m for 6061/AlzO3/12p, 23/xm for 6061/A1203/15p and 28/~m for 6061/AlzO3/21p. Mean values of Dma x in the crack initiation area of all reinforced alloys are much larger than on the microsection surface (see Table 3). Large Dma × values such as those found in the fatigue crack initiation area are relatively rare on the microsection surface (Figure 2). This indicates that the crack initiated preferentially at large particles. Only a few holes in the matrix and decohered A1203 particles were observed in the crack initiation area, which means that fracture of the interface between the matrix and A1203 particles is very rare in this area of crack formation. The above characterization of fracture surfaces of reinforced and unreinforced alloys is restricted to the crack initiation area described. For places on the fatigue fracture surface outside this area, intercrystalline fracture and dimples become visible for all alloys investigated. In the reinforced alloys, holes and unbroken particles may be observed, which may be attributed to more frequent interface fracture between particles and 6061 matrix in this region of the fatigue fracture surface. DISCUSSION A1203-particle-reinforced 6061 aluminium alloys show a significantly reduced fatigue strength in comparison with unreinforced 6061-T6. As smooth specimens with a relatively small diameter of 5 mm were used for the fatigue tests, crack initiation and short crack growth are the main time-governing processes, whereas the number of cycles necessary to advance a crack by about 1 mm to final fracture is negligible; the lower lifetimes of reinforced materials may therefore be attributed to a relatively short crack initiation period in comparison with unreinforced 6061-T6. Deteriorated crack initiation properties contrast the beneficial influence of AlzO3-particle reinforcement on fatigue crack growth in the near-threshold regime. Crack trapping in the wake of particles, crack deflection and branching when the crack bypasses the obstacles

are the main mechanisms to effectively reduce crack growth rates. ~ Obviously, these processes are of minor importance for crack initiation and the growth of a short crack. SEM investigations show significant differences between fracture surfaces in the crack initiation area and the remaining fatigue fracture area. Fracture of the interfaces between A1203 particles and the 6061 aluminium alloy matrix is frequently observed outside the crack initiation area whereas, in the region of initial crack growth, interface debonding is rare. In the literature 9'~3 a lower bonding strength of the interface between 6061 matrix and A1203 particles is reported than for SiC particles. This means that the weak interface between the matrix and the A1203 particles influences the fatigue crack propagation behaviour, !1 but is of minor influence on crack initiation. Large, brittle-fractured A1203 particles with flat fracture surfaces normal to the loading direction can frequently be found within the crack initiation area (Figure 6b and c). In the virgin composite material, reinforcement particles are subjected to high compression stresses, which occur during cooling of the melt owing to the lower thermal expansion coefficient of aluminium oxide (8.3 x 10 -6 K -l) in comparison with the 6061 matrix (2.4 x 10-5 K-~). In the matrix material around particles, tension stresses arise that are sufficiently high to cause plastic deformation 5'~5. Nevertheless, AlzO3-particle-reinforced 6061 showed cyclic softening for strain amplitudes below 8 x 10-3.12 It may be assumed, therefore, that cyclic loading causes a reduction of internal stresses, as unreinforced 6061-T6 showed cyclic stability. ~2 As soon as the compression stresses on the particles decrease, fatigue crack initiation at edges and flaws becomes possible. An additional decrease of particle strength is caused by cracks and microcracks, which originate from milling during the production procedure. ~ Investigations of SiC particles showed that damage of larger particles is more probable than that of small ones. j6 Similarly, more frequent failure of large A1203 particles may be expected, which coincides with frequently found large, fractured particles in the crack initiation area. Broken particles increase local stresses, which raise the probability of failure of the matrix and particles in their vicinity. Further fracture of particles increases local damage and serves as a starting point for the formation of a long crack; the successive crack extension period is then relatively short. Brittle fracture prevents effective retardation of short-crack propagation by A1203 particles. In some specimens, fracture surfaces with particle clusters in the crack initiation area were found, which means that the initial crack grows preferentially in regions of high particle concentration. Particle failure in the crack initiation area contrasts with the observation that longer fatigue cracks bypass A1203 particles. It may be speculated that larger plastic deformation in the plastic zone of a long crack allows variations in the crack path whereas, for initial crack growth with a small plastically deformed volume, the crack extension direction is more restricted. The size distribution of reinforcing particles is different in the three composites investigated (Figure 2). Based on the above considerations, the number of cycles to failure for 6061/A1203/2 lp containing the

HCF properties of Ale-O3-particle-reinforced 6061 AI alloy largest particles is expected to be lower than for 6061/AlzO3/15p, and for 6061/A1203/12p with the finest particles the highest number of cycles to failure is expected. Presentation of cycles to failure vs cyclic stress amplitude based on the experimental results does not show the expected difference of the three composites: identical curves could be found within the range of scatter. This can be explained by the fact that the local stress field around particles, which determines the fatigue behaviour of these reinforcements, is insufficiently described by the applied cyclic stress amplitude. A comparison of the fatigue properties of the three composites on the basis of cyclic strain amplitudes gives a better approximation of actual loading conditions of particles, and is shown in Figure 5. Comparing 6061/A1203/2 lp and 6061/A1203/12p (Figure 5), the alloy containing finer particles shows about five times higher number of cycles to failure. This difference in fatigue lives is pronounced and well exceeds statistical scatter. It should be noted that the largest A1203 particles, expressed in terms of Dma x values are about 30 ~m in alloy 6061/AlzO3/12p and about 38 ~m for 6061 A1203/21p, which shows that the crack initiation properties are sensitive to small variations in mean particle size. In the present experiments, the deteriorated crack initiation properties of 6061/A1203/21p due to relatively coarse particles are compensated by the increased stiffness due to the high particle content. Nevertheless, better fatigue crack initiation properties and higher number of cycles to failure may in general be predicted for alloys containing finer particles if A1203-particle-reinforced composites with equivalent volume contents of particles are compared. Finally, it should be pointed out that the present fatigue tests give information about the fatigue crack initiation process. For larger construction parts, however, the crack propagation period may give a significant contribution to lifetime. It has been shown in addition in ref. 11 that fatigue crack growth properties in the threshold regime are better for A1203-particlereinforced 6061 than for unreinforced 6061-T6. Therefore, longer lifetimes may be expected, especially for large construction components, if A1203-particlereinforced alloys instead of unreinforced alloys are used.

initiation are large, cracked particles. In an area of about 100 ~m around the crack initiation site, particles do not really act as obstacles for short crack growth. Brittle fracture of A1203 particles, especially when arranged in particle clusters, enhances the initial damage process and the formation of a long crack. Failure of the particle-matrix interfaces in the area of crack initiation is rare and therefore not a significant reason for crack initiation. 3. For optimized fatigue properties of AlzO3-particlereinforced 6061 aluminium alloys, fine particles should be used as reinforcement. A homogeneous distribution to avoid cluster formation is beneficial. To increase stiffness of the composite a high content of reinforcing particles is favourable. ACKNOWLEDGEMENT The authors thank Dr P. Degischer, Dr W. Ktihlein and Dr G. Henkel, AMAG Austria, for providing the testing material and information on composition, grain size and static properties (Figures 1 and 2, and Tables 1-3). They are also indebted to the Forschungsf6rderungsfonds der gewerblichen Wirtschaft (Austrian Industrial Research Promotion Fund) for financial support. REFERENCES 1

2 3 4 5 6

7 8

9 10

CONCLUSIONS 1. In the regime of 5x105 to 5x108 load cycles, composites with a 6061 aluminium alloy matrix and 12-21 vol.% of A1203 particles show approximately 10 times lower cycles to failure than unreinforced 6061-T6. The technical fatigue limit decreases from 145 MPa for 6061-T6 to 115 MPa for the A1203particle-reinforced materials. 2. Low fatigue lives may be attributed to early formation of an initial crack. Preferential sites for crack

481

11 12 13 14

15 16

Lloyd, D.J. in 'The 3rd International Conference on Aluminium Alloys (ICAA3)' (Eds L. Arnberg, O. Lohne, E. Nes and N. Ruym), Norwegian Institute of Technology, Trondheim, Norway, 22-26 June 1992, Vol. III, pp. 145-168 Logsdon, W.A. and Liaw, P.K. Eng. Fract. Mech. 1986, 24 (5), 737 Shang, J.-K. and Ritchie, R.O. Acta Met. 1989, 37 (8), 2267 Kumai, S., King, J.E. and Knott, J.F. Fract. Eng. Mater. Struct. 1990, 13 (5), 511 Levin, M. and Karlsson, B. Mater. Sci. Technol. 1991, 7, 596 Kobayashi, T., Ninomi, M., Iwanari, H. and Toda, H. in 'Proc. RASELM 9th Conf. Recent Advances in Sci. and Eng. of Light Metals', Sendai, Japan, 1991, p. 543 Ishii, H., Tohgo, K. and Araki, H. Eng. Fract. Mech. 1991, 40 (4/5), 821 Downes, T.J., Knowles, D.M. and King, J.E. in 'Fatigue of Advanced Materials' (Eds R.O. Ritchie, R.H. Dauskardt and B.N. Cox), Materials and Component Engineering Publications Ltd, Birmingham, UK, 1991, pp. 395-407 Kumai, S., Yoshima, K., Higo, Y. and Nunomura, S. Int. J. Fatigue 1992, 14 (2), 105 Kumai, S., King, J.E. and Knott, J.F. Fatigue Fract. Eng. Mater. Struct. 1992, 15 (1), 1 Papakyriacou, M., Mayer, H.R., Tschegg-Stanzl, S.E. and Gr6schl, M. Fatigue Fract. Eng. Mater. Struct. 1995, 18(4), 477 Perng, C.-C., Hwang, J.-R. and Doong, J.-L. Compos. Sci. Technol. 1993, 49, 225 Hochreiter, E., Panzenb6ck, M. and Jeglitsch, F. Int. J. Fatigue 1993, 15 (6), 493 Stanzl, S.E. and Ebenberger, H.M. in 'Fatigue Crack Growth Threshold Concepts' (Eds D. Davison and S. Suresh), Met. Soc. of AIME, 1984, pp. 399-416 Nakamura, T. and Suresh, S. Acta Metall. Mater. 1993, 41 (6), 1665 Brechet, Y., Embury, J.D., Tao, S. and Luo, L. Acta Metall. Mater. 1991, 39 (8), 1781