Pure Al matrix composites produced by vacuum hot pressing: tensile properties and strengthening mechanisms
Introduction
It is attractive to use particulate reinforced aluminum (PRA) matrix composites in structural applications because of their excellent stiffness-to-weight and strength-to-weight ratios [1], [2]. Such PRA materials also exhibit generally good wear resistance, thermal conductivity, and low thermal expansion, all of which makes them good multifunctional light-weight materials [3], [4]. These superior properties suggest many possible uses in weight-sensitive components for aerospace or land transportation. A wide variety of fabrication techniques have been explored for metal matrix composites. These include powder metallurgy, molten metal methods, semi-solid casting, pressure infiltration, and spray deposition [1], [4], [5], [6].
The powder metallurgy processing technique is attractive for several reasons [6], [7], [8]. This approach offers microstructural control of the phases that is absent from the various routes that involve a liquid phase. Powder metallurgy processing employs lower temperatures and, therefore, reduced diffusion rates with better control of interface reaction kinetics. Because of their basis as a powder, PRA composites often have been deformation processed after powder consolidation to develop the best properties. In this manner, the composites behave like high strength aluminum alloys made by the powder metallurgy technique: i.e., the prior particle oxide skins must be broken up by metal working before the true properties of the matrix metal and, hence, the composite can be achieved. The most common primary breakdown process has been extrusion. Other metal working processes such as rolling, forging, shear spinning and swaging have also been demonstrated. Also, the typical ceramic reinforcements (e.g., SiC) for PRA composites give rise to dulling of many common machine tools, decreasing the machinability of these composites. Thus, it would be quite beneficial if a powder metallurgy process for high strength PRA composites with good ductility could be developed that avoided extensive mechanical deformation and permitted net shape die forming without machining.
Strengthening mechanisms of composites have been studied extensively for many years. The continuum shear lag model was first developed to predict the direct strengthening of continuous fiber reinforced composites originally by Cox [9]. The predominant direct strengthening factor is the volume fraction of reinforcement. Powder processed material tends to give somewhat higher strengths than melt processed composites, probably because of additional strengthening from residual oxide dispersoids from prior particle surfaces, and from the somewhat finer grain size. Unfortunately, for the low aspect ratio whisker or particulate reinforcement particles typically used in current metal matrix composites, the shear lag model underestimates the strength [10], [11]. Nardone and Prewo suggested that better agreement could be obtained if the shear lag model was modified to allow for whisker or fiber end loading effects [12]. The theoretical prediction by means of this modified direct strengthening model was closer to the experimental results when the reinforcement aspect ratio is small. The difficulty with the continuum approach of Cox [9] is that it ignores the secondary strengthening influence of reinforcement particles on the micromechanics of composite deformation, such as the very high matrix work hardening at low strains. The improved shear lag model [12] still ignores detailed modifications in composite microstructures, such as dislocation density increases and residual stresses from processing effects [13].
A model to predict the yield strength of a particle-reinforced metal matrix composites by considering the dislocation density due to mismatch between thermal expansion coefficients (ΔCTE) of particle and matrix was developed by Arsenault and Shi [14], [15]. It was proposed that the high matrix dislocation density caused by ΔCTE should account more closely for the observed secondary strengthening. While this enhanced dislocation density strengthening model is in agreement with the general trends of the strengthening results, it is not sufficient to account for the complex combined strengthening effects in many composite materials [16], [17]. For example, Orowan strengthening is not a major factor with the 5 μm and larger reinforcement particles usually used, but particles of this size can result in quench hardening and enhanced work hardening because of elastic misfit back stress hardening [14], [18]. Reinforcement particle shape, in terms of aspect ratio, also can influence composite strength, but for the typical SiC particulate aspect ratio range of up to 2:1, it is not expected to be a major factor.
In order to study predominantly the direct strengthening contributions to the total strength of Al alloy based composites, Krajewski and Chawla et al. [19], [20] elected to use a series of complex thermomechanical treatments to balance the complex secondary strengthening contributions in both the unreinforced Al alloy and in the composite material. These treatments were needed to ensure that the starting density and distribution of the dislocations and precipitates were similar in the unreinforced Al alloy and in the composites in order to compare the direct and secondary strengthening more unambiguously. In this paper, elemental Al powder was chosen as the matrix material to avoid the complications of matrix heat-treatment designs to isolate the strengthening mechanisms of composite materials and to permit the use of a high temperature consolidation temperature, without promoting partial melting.
In recent research [16], results on elemental Al matrix composites, reinforced by 30 vol.% of spherical Al–Cu–Fe alloy particles and consolidated by quasi-isostatic forging, were introduced. Because of the fine (diameter <10 μm) matrix and reinforcement particle sizes that match closely and a homogenous spatial distribution, the yield strength (YS) of this model composite material was improved over the matrix properties by 220% for the commercial purity composite sample. Remarkably, for an equivalent composite material produced from higher purity metal powders, i.e., with much thinner oxide surfaces, the yield strength of the composite was improved over the matrix properties by 328%. Because these strengthening results were so far above previous observations, typically <100% [16], the time seems right to revisit the predictive capability of the previous strengthening models for particulate reinforced metal matrix composites. The elastic modulus values [16] of these composites, in both versions, were slightly below the calculated upper bound value from the rule of mixtures (ROM) for 30 vol.% loading, using ultrasonically measured values of the specific Al matrix phases and the (assumed) quasicrystalline reinforcement phase. Such high elastic modulus measurements are also very unusual and add further motivation to re-examine composite strengthening for such materials.
These results suggested also that the selection of potential reinforcement phases for PRA materials should be broadened from refractory ceramics to include, e.g., intermetallic compounds that also can form a strong bond with the Al matrix by high temperature solid-state sintering. Moreover, the results clearly indicated that PRA composites made from powders with a thinner oxide surface can achieve significant improvements in tensile properties over the same composites made from commercial powders with residual impurities and thicker oxide surfaces [21].
In light of the interesting mechanical property measurements of the previous work [16], a broader study of this type of metal matrix composites was undertaken for this report. In this paper, we will report the tensile properties of similar Al/Al–Cu–Fe composites made by vacuum hot pressing (VHP) with a wide range of reinforcement loadings. Compared to quasi-isostatic forging, VHP has a further decreased amount of interparticle shear, strain hardening rate and almost no grain anisotropy compared to the forging method [7]. This vacuum hot pressing approach can be viewed as one step closer to the goal of powder consolidation process simplification for direct net shape forming of PRA composite parts. New results also have demonstrated that the Al–Cu–Fe, quasicrystalline reinforcement phase in the composite actually transforms during consolidation by both quasi-isostatic forging and vacuum hot pressing to a crystalline intermetallic phase of similar properties. This more complete set of tensile property results will be compared to the predictions of the direct and the (relevant) secondary strengthening models for the YS increase of the composite samples, as suggested above.
Section snippets
Materials
Powders used for the two kinds of composite samples are shown in Table 1. For the baseline experiments, commercial inert gas atomized (CIGA) Al and Al63Cu25Fe12 (quasicrystal) powders were obtained. The CIGA Al powder (99.7% purity) had been evaluated thoroughly in earlier work [21], [22], [23] to characterize its surface oxide properties. A patented gas atomization reaction synthesis (GARS) technique was also used to produce 99.99% pure Al and Al–Cu–Fe quasicrystal powders [24] in our
General microstructures
As shown in the low magnification micrographs of Fig. 1, Fig. 2, the reinforcement particles are spherical in shape and distributed quite uniformly in all samples. Some local clustering still can be found [30] in all the samples, especially in 30 vol.% loading samples. The density measurements of the composite samples are given in Table 3, which shows that all of the samples are essentially fully dense.
X-ray diffraction of composites
X-ray diffraction measurements of the consolidated composites revealed elemental Al matrix
Testing of composite strengthening models
During the past two decades a large number of investigations have been carried out to reveal the strengthening mechanisms of metal matrix composites, where both continuum and micromechanical models have been developed. As a result of these investigations the major mechanisms that may contribute to the direct and secondary strengthening of a composite have been deduced:
- (a)
Direct strengthening results from load transfer from the matrix to the reinforcement via shear stresses at the interface between
Conclusions
- 1.
Microstructural analysis of elemental Al matrix composites reinforced by Al–Cu–Fe alloy particles demonstrated that the quasicrystalline phase in the as-solidified Al–Cu–Fe particles transformed during high temperature consolidation processing to a crystalline ω phase, which has similar elastic modulus, coefficient of thermal expansion, and hardness properties, but a reduced density. Tensile test results also were collected from an extensive set of composite samples with reinforcement loadings
Acknowledgements
The authors greatly appreciate the assistance of the Materials Preparation Center of the Ames Laboratory for the CIP and VHP consolidation processing, performed by Mr. Paul Wheelock, and the SEM studies performed by Mr. Fran Laabs. The ultrasonic measurements were performed by Mr. Dan Barnard. Mr. Arne Swanson helped with tensile testing. Dr. Dan Sordelet provided HIP consolidated bulk Al–Cu–Fe quasicrystal and Al7Cu2Fe (ω phase) alloy samples for CTE and microhardness measurements. The
References (45)
- et al.
Acta Metall. Mater.
(1991) - et al.
Scripta Metall.
(1986) - et al.
Mater. Sci. Eng. A
(1986) - et al.
Mater. Sci. Eng. A
(2003) - et al.
Acta Metall. Mater.
(1991) - et al.
Mater. Sci. Eng. A
(1998) - et al.
J. Light Met.
(2002) - et al.
Mater. Sci. Eng. A
(1991) - et al.
J. Non-Cryst. Solids
(1993) - et al.
J. Non-Cryst. Solids
(1993)
Scripta Mater.
Mater. Sci. Eng. A
Acta Metall. Mater.
Composites
JOM
JOM
JOM
Chemtech
J. Inst. Met.
Int. Mater. Rev.
Cited by (155)
Achieving novel copper–steel joints with a combination of high strength and ductility reinforced by in-situ Fe-rich particles
2024, Journal of Materials Science and TechnologyConstitutive equation and microstructural evolution of one distinctive Al–based hybrid composite reinforced by nano–AlN and micro–TiC particles during hot compression
2023, Materials Science and Engineering: AMicrostructure features and mechanical properties of non-heat treated HPDC Al9Si0.6Mn–TiB<inf>2</inf> alloys
2023, Journal of Materials Research and TechnologyEmployment of intragranular reaction to enhance dispersion strengthening through dispersoid proliferation in Al matrix composite
2023, Journal of Alloys and CompoundsEffect of Sc microalloying on fabrication, microstructure and mechanical properties of SiC<inf>p</inf>/Al–Cu–Mg-Sc composites via powder metallurgy
2023, Materials Science and Engineering: A