Influence of Particulate Reinforcement and Equal-Channel Angular Pressing on Fatigue Crack Growth of an Aluminum Alloy

The fatigue crack growth behavior of unreinforced and particulate reinforced Al 2017 alloy, manufactured by powder metallurgy and additional equal-channel angular pressing (ECAP), is investigated. The reinforcement was done with 5 vol % Al2O3 particles with a size fraction of 0.2–2 μm. Our study presents the characterization of these materials by electron microscopy, tensile testing, and fatigue crack growth measurements. Whereas particulate reinforcement leads to a drastic decrease of the grain size, the influence of ECAP processing on the grain size is minor. Both reinforced conditions, with and without additional ECAP processing, exhibit reduced fatigue crack growth thresholds as compared to the matrix material. These results can be ascribed to the well-known effect of the grain size on the crack growth, since crack deflection and closure are directly affected. Despite their small grain size, the thresholds of both reinforced conditions depend strongly on the load ratio: tests at high load ratios reduce the fatigue threshold significantly. It is suggested that the strength of the particle-matrix-interface becomes the critical factor here and that the particle fracture at the interfaces dominates the failure behavior.


Introduction
Aluminum matrix composites (AMCs) are designed to meet the requirements of advanced engineering applications.Reinforcement with ceramic particles provides high strength and stiffness, and enhances wear resistance, thermal stability, and creep resistance [1,2].It has been shown that methods of severe plastic deformation (SPD) can help to homogenize the particle distribution [3], which improves the mechanical properties.Equal-channel angular pressing (ECAP) has emerged as one of the most frequently used SPD methods for processing particulate reinforced AMCs [4][5][6][7].For unreinforced materials, ECAP processing leads to strain-hardening of the material and decreases the grain size by the rearrangement of dislocations to subgrain boundaries and cell walls [8][9][10].
Focusing on potential future applications for AMCs produced by ECAP, fatigue crack growth is an important field of interest.To the best of our knowledge, previous studies only focused on crack propagation either in ultrafine-grained materials produced by ECAP or in AMCs.These investigations show the well-known effect of the grain or particle size on the fatigue crack growth.The grain refinement through ECAP leads to a minimized amount of crack deflection and roughness-induced crack closure, which results in higher crack propagation rates and, therefore, in minor thresholds [11][12][13][14][15][16][17][18].In reinforced materials, particle sizes larger than 2-5 µm have a contrary effect on crack propagation: by increasing crack deflection and roughness-induced crack closure, crack propagation rates are lowered [19][20][21][22][23][24].
The purpose of the present study is to investigate the influence of particulate reinforcement with a particle size less than 2 µm and additional ECAP on the near-threshold fatigue crack growth of an aluminum alloy.This small particle size was chosen in order to minimize local stress concentrations in the matrix material [25].It is well-known that small particles cause significantly higher strain hardening and that they are less susceptible to cracking as compared to coarser particles [26,27].As matrix material, the Al 2017 alloy has been chosen here and in preliminary studies [28], because it combines attractive properties, such as good fracture toughness in the underaged condition, high resistance against crack propagation, and excellent high-temperature strength, which enables future applications for lightweight structures, e.g., for the aerospace and aeronautical industries.

Material
A commercially available, gas-atomized, spherical Al 2017 powder alloy was used as matrix material.The particle size fraction of the powder, with a chemical composition such as that presented in Table 1, was below 100 µm.Al2O3 powder with a particle size fraction of 0.2-2 µm was used as reinforcing component.The composite powder, consisting of 95 vol % matrix material and 5 vol % Al2O3 particles, was processed in a high-energy ball mill and hot-degassed at 450 °C and 0.06 bar for 4 h.Afterwards, compaction was performed by hot isostatic pressing at 450 °C and 1100 bar for 3 h.The manufacturing process is described in more detail in [28].The non-ECAP-processed conditions, unreinforced, and reinforced, were solid-solution treated at 505 °C for 60 min, water-quenched, and subsequently naturally aged at room temperature for one month in order to adjust an underaged condition.For the ECAP-processed conditions, as described in [28], a pre-ECAP aging at 140 °C was carried out for 10 min to increase the workability.Immediately after the pre-ECAP aging, the material was ECAP-processed at 140 °C for one pass.The ECAP processing was performed in a device with an internal angle of 120 and a cross-section of 15 × 15 mm 2 , equipped with movable walls and a bottom slider.This processing resulted in an equivalent strain of 0.67 in each passage [29].After ECAP, the material was peak-aged at 140 °C for 360 min to optimize the ductility.With this combined treatment of ECAP and aging, both elevated strength and moderate ductility can be achieved [30,31].

Methods of Mechanical Testing and Electron Microscopy
Quasi-static tensile tests were performed in a Zwick-Roell servohydraulic testing machine (Zwick, Ulm, Germany) at a strain rate of 10 −3 • s −1 at room temperature.Cylindrical specimens were used with a cross section of 3.5 mm and a gauge length of 10.5 mm.
Fatigue crack propagation measurements were performed ΔK-controlled in a Rumul testronic resonant testing machine (Russenberger Prüfmaschinen AG, Neuhausen am Rheinfall, Switzerland) at room temperature and at three constant load ratios: R = 0.1, 0.4 and 0.7.Single-edge-notched bend (SEB) specimens with a geometry according to ASTM E399-12e3 [32] and a thickness of 5 mm were used.Figure 1 shows the position in the ECAP billet out of which the SEB specimens were extracted.In the choice of the extraction direction, it was considered that the crack is most likely sidetracked in the direction of the last ECAP shear plane [12].The crack length was measured continuously according to the principle of the indirect electric potential method with crack measurement foils.The ideal direction of crack growth is parallel to the ECAP shear plane.The plane from which samples for microstructural analyses were taken is marked in red.
After precracking at ΔK = 2-7 MPam 1/2 (depending on material condition and load ratio), ΔK was lowered in small steps until the fatigue threshold ΔKth was reached.Then, 0.5 mm crack growth were realized at this cyclic stress intensity to avoid any load history effect from the threshold region.In the following, ΔK was raised again in small steps up to a cyclic stress intensity of about 11 MPam 1/2 and a crack length of 5 mm.
Samples for microstructural analyses were extracted parallel to the extrusion direction, as shown in Figure 1, and electro-polished followed by a final ion milling polishing step.These samples were analyzed by scanning transmission electron microscopy (STEM) and quadrant back scatter diffraction (QBSD) at an accelerating voltage of 30 kV, as well as by electron back scatter diffraction (EBSD, EDAX TSL OIM 5.2, AMETEK GmbH, Wiesbaden, Germany) at 15 kV using a Zeiss Neon 40 field emission microscope (Carl Zeiss MicroImaging GmbH, Jena, Germany).

Tensile Tests
The tensile properties of the investigated material conditions are shown in Table 2.The unreinforced aluminum matrix (in an underaged heat treatment condition) exhibits the lowest yield strength with 252 MPa just as the highest ductility, which shows in a uniform elongation of 20%.ECAP processing (with subsequent aging as described in the experimental section) leads to a significant increase in yield strength (487 MPa, which is an increase of 93%) in combination with a moderate ductility (11% uniform elongation).The strategy of combining ECAP and aging, which is used here and which is referred to as "optimizing heat treatment" in [33], enables this favorable combination of strength and ductility: ECAP processing results in a severely strain hardened material with a high dislocation density.In the following process of thermal aging, numerous finely-dispersed precipitates form and strengthen the matrix.Simultaneously, thermal recovery takes place and increases the material's ductility.The dispersion strengthening, which is caused by the reinforcement with 5 vol % particles, leads to an improvement in strength, which is less compared to the strengthening, which is achieved by ECAP of the unreinforced material.ECAP processing of the particulate reinforced material increases the yield strength from 328 MPa to 509 MPa (which is an increase of 55%).This shows, that the effect of strain hardening during ECAP is less pronounced than in the unreinforced material.Both reinforced conditions exhibit minor ductility, which is ascribed to the particles which act as interior notches [28].

Microstructure
One ECAP pass of the unreinforced matrix material leads to a bimodal grain structure with inhomogeneously distributed refined grains.The initial grain size is significantly reduced in the shear bands, which is shown in Figure 2. In the outside parts, almost no grain refinement has taken place.
The reinforced material exhibits elongated areas with a width of 100-200 µm and a height of about 7 µm without reinforcement components.These areas are preferentially orientated parallel to the direction of extrusion.In Figure 3, micrographs of the reinforced areas are shown.The Al2O3 particles are finely dispersed and exhibit an intact interface to the aluminum matrix.In these micrographs, the presence of the coarse intermetallic phase Al2Cu can also be seen.Additional ECAP does not lead to significant grain refinement (see Figure 4).

Fatigue Crack Surface Analysis
For the unreinforced conditions the fatigue crack surfaces are shown in Figure 5.The non-ECAP-processed condition exhibits a strongly faceted surface with pronounced band-typed steps.This fissured crack surface can be explained by the large grain size of this condition and thus by a high amount of crack deflection.For the material after one ECAP pass, plateaus are present on the crack surface.Additionally, striations are clearly visible.Such signs of "micro-damage" have also been observed for other ECAP-processed aluminum alloys [16].They were interpreted as the severely strain-hardened material is not being able to compensate the stresses at the crack tip by deforming plastically.Compared to the unprocessed material, the crack path after ECAP is less tortuous and more straight-forward.Presumably, the crack propagates preferably along or within the shear bands.The surface is less rough and barely fissured as compared to the unreinforced conditions.Through additional ECAP, the particle failure is more likely to fracture than to break out.
In Figure 6, the fatigue crack surfaces of the reinforced conditions are shown.In contrast to the unreinforced conditions, the crack surfaces are characterized by a minor roughness.Hardly any faceting is noticeable.Due to the small grain and particle sizes of these conditions, crack deflection is reduced to a minimum.This outcome is in good agreement with the findings in [19,34].It is noticeable that, in the ECAP-processed condition, particle fracture is more pronounced, whereas the initial reinforced condition exhibits a higher rate of particle break outs.It cannot be concluded with certainty if the particle fracture proceeded due to fatigue loading or whether it was caused by ECAP.

Fatigue Crack Propagation
Fatigue crack growth curves near the threshold for the tested load ratios are shown in Figure 7.The matrix material exhibits the highest thresholds and the lowest fatigue crack growth rates for all tested conditions.Due to the major grain size of the matrix material, the amount of crack deflection and roughness-induced crack closure and, therefore, the strongly branched crack path lead to decreased fatigue crack growth rates, which results in major thresholds.One ECAP pass results in higher crack growth rates and lower thresholds.Presumably, due to the preferred crack propagation along the shear bands and the finer grains in this area, less crack deflection and closure occurs.Supposedly, also the high dislocation density induced by ECAP inhibits stress relief and, therefore, early damaging at the crack tip takes place [13].The thresholds for the reinforced conditions are lower than for the matrix material conditions.With the increase of the load ratio, this effect becomes more pronounced.The crack growth rates for the reinforced ECAP-processed conditions are generally the highest.
For the reinforced conditions, at low cyclic stress intensities, the crack propagation rates are higher as compared to the matrix material conditions.As the near threshold regime is highly sensitive to the microstructure, the main factor for this behavior is presumably the significantly smaller grain size and the resulting minimized effects of crack deflection and roughness-induced crack closure.For the ECAP-processed conditions, fatigue crack growth thresholds are further reduced as compared to their corresponding not-processed conditions.This is probably due to the high dislocation density in these materials, which leads to a diminished deformation ability at the crack tip.
Figure 8 presents the fatigue threshold depending on the load ratio for all tested conditions.The matrix material and both reinforced conditions exhibit a strong dependency on the load ratio.Although the curve progression is nearly parallel for these three conditions, the underlying mechanisms are different.The matrix material shows the highest thresholds and also a strong load ratio dependency.This can be explained by the well-known effect of the grain size on the crack growth at different load ratios [11][12][13]15,35].The amount of crack deflection is constant for all load ratios, whereas the effect of roughness induced crack closure is minimized by increasing the load ratio.The fatigue thresholds of the ECAP-processed matrix material show a minor dependency on the load ratio.Supposedly, the small grain size in the shear bands and therefore the generally low amount of crack deflection and closure lead here to minimized load ratio effects.
However, despite their small grain size, both reinforced conditions exhibit a strong dependency on the load ratio.In this case, assumedly, the thresholds depend critically on the interaction between the softer aluminum matrix where crack propagation takes place and the interface where decohesion or brittle particle facture can occur [36].At higher load ratios, the susceptibility to cracking of the particle-matrix-interface is strongly increased.On that account, fatigue thresholds further decrease for higher load ratios.This, in turn, results in a higher load-ratio-dependency.For the reinforced condition, additional ECAP does not influence the load ratio dependency.The ECAP-processed matrix material exhibits the least dependency on the load ratio.In contrast, the matrix material and the reinforced conditions depend strongly on the load ratio and display almost the same amount for the three conditions.

Conclusions
In this paper, the effect of particulate reinforcement and equal-channel angular pressing on the fatigue crack growth in an aluminum alloy produced by powder metallurgy is investigated.For the Al 2017 powder alloy, reinforced with 5 vol % Al2O3 with a size fraction of 0.2-2 µm, the fatigue properties are compared to the unreinforced material.Two different conditions for both materials are chosen: non-processed and after one pass of ECAP.
(1) One ECAP pass of the unreinforced material results in an inhomogeneous microstructure with shear bands.These shear bands are characterized by a considerably reduced grain size.In the surrounding areas, the grains are strongly elongated, but not refined.The grain size of the reinforced material is significantly lower, as compared to the unreinforced.Additional ECAP does not lead to a significant grain refinement, here.(2) The yield strength of both materials, unreinforced and reinforced, is increased by one pass of ECAP.
However, the strain hardening effect of ECAP is more pronounced in the unreinforced material.(3) Due to the major grain size of the unreinforced matrix material, the high amount of crack deflection and roughness-induced crack closure lead to a tortuous crack path and therefore to minor crack propagation rates.The grain refinement through one ECAP pass reduces these effects.Supposedly because crack propagation occurred in the finer-grained shear bands, the fatigue thresholds and their dependency on the load ratio are diminished for the unreinforced material after ECAP.(4) For the reinforced material, the fatigue thresholds are drastically reduced due to the significantly smaller grain size of these conditions.Despite their small grain size, the reinforced materials exhibit a strong dependency on the load ratio.Here, the interaction between the aluminum matrix and the interface is perceived as crucial: the susceptibility to cracking of the particle-matrix-interface is strongly increased at higher load ratios.This leads to a "particle-failure-induced" load ratio dependency.

Figure 1 .
Figure 1.Position of two single-edge-notched bend specimens within the ECAP billet.The ideal direction of crack growth is parallel to the ECAP shear plane.The plane from which samples for microstructural analyses were taken is marked in red.

Figure 2 .
Figure 2. Electron back scatter diffraction (EBSD) color-coded orientation maps of the unreinforced matrix material conditions: (a) non-ECAP-processed and (b) after one ECAP pass.In the shear bands, the grain size is considerably reduced after one ECAP pass.By contrast, the initial grain size outside the shear bands is almost maintained.

Figure 3 .Figure 4 .
Figure 3. Quadrant backscatter diffraction (QBSD, left) and scanning transmission electron microscopy (STEM, right) micrographs of Al 2017 with 5 vol % Al2O3 particles with a size of 0.2-2 µm.Al2O3 particles are marked with broken lines and the Al2Cu precipitates with arrows.The particles are finely dispersed and matrix and particles exhibit an intact interface.

Figure 5 .Figure 6 .
Figure 5. Fatigue crack surfaces of the unreinforced conditions: (a) non-ECAP-processed and (b) after one ECAP pass near the fatigue threshold.The crack propagation direction is marked with an arrow.The surface of the non-ECAP-processed condition exhibits a band-type faceted appearance, whereas ECAP leads to plateaus and clearly visible striations.

Figure 7 .
Figure 7. Fatigue crack growth curves of Al 2017 in unreinforced and reinforced conditions with additional ECAP at load ratios of: (a) R = 0.1, (b) R = 0.4, and (c) R = 0.7.The thresholds for the reinforced conditions are lower than for the matrix material conditions.With the increase of the load ratio, this effect becomes more pronounced.The crack growth rates for the reinforced ECAP-processed conditions are generally the highest.

Figure 8 .
Figure 8. Fatigue thresholds of Al 2017 in unreinforced and reinforced conditions with additional ECAP at different load ratios.The ECAP-processed matrix material exhibits the least dependency on the load ratio.In contrast, the matrix material and the reinforced conditions depend strongly on the load ratio and display almost the same amount for the three conditions.

Table 1 .
Chemical composition of the Al 2017 powder alloy.

Table 2 .
Tensile properties of the unreinforced and reinforced conditions.