The fatigue and final fracture behavior of SiC particle reinforced 7034 aluminum matrix composites

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Abstract

In this research paper, the cyclic stress-amplitude-controlled fatigue response and fracture behavior of aluminum alloy 7034 discontinuously reinforced with silicon carbide particulates (SiCp) is presented. In view of the limited ambient temperature ductility, test specimens of the 7034/SiCp composite, in both the under-aged and peak-aged conditions, were cyclically deformed under stress-amplitude-control at an elevated temperature corresponding to the aging temperature of the alloy. The cyclic fatigue tests were conducted at two different load ratios with the objective of documenting the conjoint influences of intrinsic composite microstructural effects, nature of loading, and magnitude of cyclic stress amplitude on cyclic fatigue life and fracture characteristics. The final fracture behavior of the composite is discussed in light of the concurrent and mutually interactive influences of composite microstructural effects, deformation characteristics of the composite constituents, nature of loading, and resultant fatigue life.

Introduction

The promise of offering exceptionally high specific modulus (E/ρ), strength-to-weight (σ/ρ) ratios, fatigue strength, wear resistance, and tailorable properties such as thermal expansion are few of the important attributes that have made the discontinuously reinforced aluminum alloy based (DRA) metal-matrix composites (MMCs) an attractive and viable candidate for use in weight-sensitive and stiffness-critical components [1], [2], [3], [4], [5], [6], [7], [8], [9]. The presence of the discontinuous reinforcement phase in a continuous aluminum alloy metal-matrix results in properties not attainable by other means, thereby enhancing the potential range of possible applications [7]. The attainable improvements in properties are dependent on mutually interactive influences of: (a) the intrinsic properties of the composite constituents, and (b) the size, shape, orientation, volume fraction and distribution of the reinforcing phase in the metal-matrix [1], [2], [3], [4], [5], [6], [10]. Driven initially by the demand for high performance military and space applications, the discontinuously reinforced MMCs, based on aluminum alloy metal matrices, have progressively evolved to engender considerable scientific and technological interest, particularly in the time period spanning the last two decades. Recent years have seen expanded uses to specifically include applications spanning the areas of automotive [8], aerospace products [7], [11] and even recreational goods [12]. However, the inadequate fracture toughness, limited damage tolerance and poor tensile ductility compared to the unreinforced counterpart coupled with inferior fracture-related properties, size limitations on available product forms, and intrinsic material variability, are often severe limitations to the wide spread application and use of these composites in performance-critical applications [13], [14], [15], [16], [17].

Selection of reinforcement type, geometry (shape) and volume fraction is critical to obtaining the best optimum combination of properties, in the aluminum alloy composite, at a substantially low cost [18]. Silicon carbide particulates (SiCp) are the most preferred reinforcements because enhanced properties are achievable with little or no density penalty [8], [11]. For example, incorporation of discontinuous particulate reinforcements in a ductile aluminum alloy metal-matrix has resulted in a 15–40% increase in strength and a 30–70% increase in stiffness, compared to the unreinforced counterpart [19], [20]. The increase in strength is often more pronounced at elevated temperatures [2], [3], [5]. Improvements in elastic modulus of up to 100% have been reported for an aluminum alloy discontinuously reinforced with 40 vol% of silicon carbide [21]. The presence of the reinforcement has been found to result in decreased grain size, accelerated aging and even in precipitate size, morphology and distribution in the metal-matrix [22], [23], [24]. The high density of dislocations both at and near the reinforcement/metal-matrix interfaces arises as a result of the mismatch in the coefficient of thermal expansion between the SiC particle and the aluminum alloy metal-matrix. The enhanced expansion of the matrix induces plastic deformation during cooling with a concomitant increase in the density of dislocations [24], [25], [26]. More importantly, the DRA MMCs maintain their amenability to conventional metallurgical processing, fabrication and characterization methods currently used for the unreinforced aluminum alloy counterparts [27].

Use of DRA MMCs in performance-critical components often involves repeated loading, and therefore the fatigue response and resultant failure and fracture characteristics are of need and interest. Earlier studies [28], [29], [30], [31], [32], [33], [34], [35], [36], [37] have found that reinforcing an aluminum alloy metal-matrix has a beneficial or detrimental influence on cyclic fatigue resistance depending upon diverse and yet related factors such as: (i) method of synthesis (primary processing method), (ii) reinforcement type, (iii) geometrical constitution (size, shape, volume fraction, and distribution of the reinforcing phase) in the metal-matrix, (iv) nature of secondary processing (aging treatments), and (v) strengths of the matrix and the reinforcement/matrix interfaces.

Several related studies have focused on understanding the influence of reinforcement particle on matrix microstructure and the resultant influence of composite microstructure on stress-controlled and strain-controlled fatigue behavior of MMCs [38], [39], [40], [41], [42], [43], [44], [45], [46]. Complex relationships do exist between the fatigue properties and fracture characteristics of a discontinuous particulate-reinforced aluminum alloy-based MMC. These include: (a) the intrinsic properties of the matrix (composition, aging condition and microstructure), (b) intrinsic properties of the particulate reinforcement phase (composition, morphology, size and volume fraction), (c) the influence of secondary processing on microstructure, and (d) the influence of test parameters such as nature, type and magnitude of loading, and extent of cyclic plasticity.

With this underlying background the motivation for this research study was to examine the influence of microstructure (aging condition), load ratio and magnitude of stress amplitude on cyclic fatigue life and fracture behavior of a 7034 aluminum alloy metal-matrix composite. The cyclic fatigue life and fracture behavior of the composite is discussed in light of concurrent and mutually interactive influences of composite microstructural effects, deformation characteristics of the composite constituents, nature of loading, magnitude of stress amplitude and test temperature.

Section snippets

Material

The aluminum alloy MMC used in this research investigation was made using the spray processing technique and provided as an extruded billet by the Air Force Materials Laboratory (Dayton, OH, USA). The 7034 aluminum alloy and SiC particles were processed by powder metallurgy technique and extruding the consolidated sample through a standard shear face die to get a billet. The as-received billet was subject to the following heat treatment: (a) solution heat treated at 490 °C for 4 h, (b)

Experimental techniques

Samples were cut from both the 7034/SiC/15p-UA and 7034/SiC/15p-PA composites and prepared by standard metallographic procedures for observation in an optical microscope. The morphology of the reinforcing silicon carbide particulate (SiCp), their distribution in the 7034 aluminum alloy metal-matrix and other observable microstructural features were examined in an optical microscope and photographed using a bright-field technique.

Smooth cylindrical test specimens (6.25 mm gage diameter and 25 mm

Undeformed microstructure

The optical micrographs illustrating the microstructure of the 7034/SiC/15p-UA and 7034/SiC/15p-PA composites are shown in Fig. 1. The SiCp, in the 7034 aluminum alloy metal-matrix, were non-uniform in size, irregularly shaped and randomly dispersed. At regular intervals, a clustering or agglomeration of the SiCp, of varying sizes, was observed resulting in SiCp-rich and SiCp-depleted regions. An agglomerated site consisted of the smaller SiCp intermingled with few larger SiCp. No attempt was

Conclusions

A study of the high cycle fatigue and final fracture behavior of 7034/SiC/15p-UA and 7034/SiC/15p-PA metal-matrix composites provides the following key observations.

The initial microstructure of the 7034/SiCp MMCs revealed that the reinforcing SiC particulates were non-uniform in size, irregularly shaped and randomly dispersed. At regular intervals, a clustering or agglomeration of the SiCp, of varying sizes, was observed resulting in SiCp-rich and SiCp-depleted regions. An agglomerated site

Acknowledgements

Thanks and appreciation are extended to the Air Force Materials Laboratory of Wright Patterson Air Force Base (Dayton, OH, USA) for providing the material used in this study (Program Manager: Dr D.B. Miracle).

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