Elsevier

Wear

Volumes 462–463, 15 December 2020, 203507
Wear

Functionally graded aluminum reinforced with quasicrystal approximant phases – Improving the wear resistance at high temperatures

https://doi.org/10.1016/j.wear.2020.203507Get rights and content

Highlights

  • Functionally graded aluminum (FGA) showed a high temperature wear resistance.

  • The wear resistance of Al was improved by quasicrystalline approximant phases.

  • Friction behavior of the FGA was similar to fully quasicrystalline coatings.

  • Protective oxide layer containing refined quasicrystal approximant phase was found.

  • The wear mechanisms acting at different temperatures were established.

Abstract

Functionally graded aluminum was produced and displayed two well-defined metallurgically bonded layers (A and B). Layer A exhibited a ductile Al-matrix (70 HV0.01) reinforced by 53.4 vol% of hard quasicrystal approximant α-Al12(Fe,Mn,Cr)3Si phase (909 HV0.01) distributed homogeneously. Layer B presented the typical structure of a hypoeutectic Al–Si alloy. The tribological behavior of both layers was evaluated in sphere-on-plate configuration at different temperatures: room-temperature, 100, 200, and 300 °C. At room temperature, layer A ensured low coefficient of friction values around 0.2. These values were comparable to those of fully quasicrystalline coatings under dry-sliding testing conditions. The specific wear rate at room temperature of the layer A was about one order of magnitude lower than that of the layer B (1.2 × 10-4 versus 2.7 × 10-3 mm3/Nm, respectively), and three orders of magnitude lower at 300 °C (2.3 × 10-5 versus 2.8 × 10-2 mm3/Nm, respectively). Improved wear resistance and low friction of layer A were achieved through the formation of a protective and compacted Al-rich oxide layer, where the quasicrystal approximant phase played a critical role. The present results and the discussed wear mechanisms provide significant insights into the development of novel alloys for service in critical components.

Introduction

Friction and wear are the main causes of degradation of aluminum alloys used in engineering components in relative motion. The optimization of the tribological properties of aluminum alloys therefore results in economic, environmental and energy benefits, mainly in transportation industry [1]. Al–Si alloys are used in many automotive parts due to their high strength to weight ratio and excellent thermal conductivity. However, the poor wear and scuffing resistances of these alloys limit their use in applications where high tribological performance is critical, such as cylinder liners in engine blocks [2]. Cast iron cylinder liners remain the best cost-effective solution for engine blocks in modern passenger cars [3,4], but different strategies have been proposed in recent decades to improve the wear resistance of Al-based alloys aiming automotive applications.

Functionally graded materials (FGM) exhibit a gradual change in composition and microstructure, which can perform specific functions and meet stringent performance requirements [5]. Different methods have been used to fabricate in-situ functionally graded aluminum (FGA) aiming to increase their wear resistance. These include powder metallurgy, vapor deposition, centrifugal method, additive manufacturing technologies, and others [[6], [7], [8], [9]]. These advanced engineering materials are rapidly finding applications in aggressive environments with steep temperature gradients, such as, for instance, automotive parts [10]. Thus, the design of novel Al-based alloys having functional properties and the development of low cost process to produce these lightweight materials are particularly important nowadays.

The wear behavior of Al–Si casting alloys depends on their design, microstructure and the amounts of impurities [[11], [12], [13]]. Iron is the most common and unwanted impurity in Al-alloys. Since Fe have a low solid solubility in aluminum, various brittle intermetallic phases may form. These intermetallic phases may be deleterious regarding wear resistance, with the extension depending on their morphology, fraction, size, and distribution [14]. The brittle β-Al5FeSi intermetallic is the most common and harmful to the mechanical properties because of its characteristic plate-like morphology [[15], [16], [17]]. The effect of Fe and the role of β-Al5FeSi phase on the sliding wear of hypoeutectic Al–Si alloys has been investigated by Taghiabadi et al. [18,19]. It was found that above a critical Fe content (~0.7 wt%Fe), the formation of large β-Al5FeSi plates reduced the wear resistance and facilitated the subsurface microcracking and delamination. Investigations have shown that small transition metals (TM) additions such as Mn and Cr is a strategy to slightly decrease the wear rate of Al–Si casting alloys, reaching typical values about 10-3 mm3/m [11,[20], [21], [22], [23], [24]]. Such TM additions promote the formation of α-Alx(Fe,TM)ySiz phase with polygonal morphology, which is less harmful than the β-Al5FeSi phase. However, it is not yet understood how to control the microstructure of alloys containing the α-Alx(Fe,TM)ySiz phase to achieve optimized mechanical and tribological properties [[25], [26], [27], [28], [29], [30]].

The α-Alx(Fe,TM)ySiz phase is a 1/1 cubic approximant with lattice parameter ranging from 1.256 to 1.268 nm [31]. Because of the non-conventional structures, quasicrystals and their approximants provide properties such as low coefficient of friction, high hardness, and good corrosion resistance [32,33]. However, the origin of the low friction properties of quasiperiodic structures is still an open discussion in the literature [34], and several explanations have been presented, such as the inherent lack of translational periodicity [[35], [36], [37]], the high hardness and Young's modulus [38,39], and the reduced surface adhesion energy [40]. However, depending on the experimental conditions, many of these aspects may be contributing to the low coefficient of friction values, especially under ambient air conditions where the surface of the quasicrystal and/or approximant phase will be oxidized [41].

The inherent brittleness of quasicrystals is another aspect that has limited their use as surface coating applications [42,43]. Thus, the development of composites consisting of Al-alloys reinforced with quasicrystals and their approximants can be an interesting alternative to explore the potential properties of these phases [[44], [45], [46], [47]]. Kang et al. [48] investigated the wear behavior of an Al-based composite reinforced by a large amount of Al91Cr5Fe4 quasicrystal phase prepared by selective laser melting process from powder mixture. Despite a non-uniform dispersion of quasicrystals in the microstructure of the consolidated composite material, high wear resistance has been reported 10-4 mm3/Nm. Designing these alloys is a real challenge due to the chemical complexity and the use of expensive processing routes. Recent studies have shown the possibility to manufacture these advanced materials using conventional low cost processes and precursors [31,49,50], but further studies are required to correlate the resulting structure with functional properties such as wear.

This work reports the wear resistance at elevated temperatures of a functionally graded aluminum (FGA) reinforced with quasicrystal approximant α-Alx(Fe,TM)ySiz phase. The structure composed of an Al-FCC matrix and more than 50 vol% of homogeneously distributed hard primary α-phase ensured low friction and specific wear rate values far better than most of Al-alloys, and comparable with fully quasicrystalline coatings. This improved wear behavior of the FGA was associated with the formation of a stable and compact Al-rich oxide layer upon wear.

Section snippets

FGA surface characterization

A FGA was produced in a cylindrical-like geometry (127 mm diameter, 400 mm height, and ~40 mm thick) by a rotational outward solidification casting process, outlined in Fig. 1a. Details of this novel casting process and a deep structural characterization of this innovative FGA can be found in a previous work [31]. The cylindrical casting produced was cut into slices of 50 mm height, as illustrated in Fig. 1b. The macrostructure revealed a radial growth of two well-defined layers (named A and B)

Characterizations of the FGA surfaces

Fig. 2(b–c) shows the microstructures of FGA polished surfaces (layers A and B). The structure of layer A is formed mainly of large primary α-Al12(Fe,Mn,Cr)3Si particles and an Al-FCC matrix (See Fig. 2b). Some phases resulting from an eutectic reaction are also present, being: pure Si from the Al–Si eutectic, θ-Al2Cu, and Al7Cu4Ni. The layer B, in contrast, is formed mainly of Al-FCC grains with phases from eutectic reactions in the interdendritic regions, such as β-Al5FeSi, Si, Al7Cu4Ni, Al2

Wear testing at room temperature and at 100 °C

The COF values of layer A at room temperature were similar to those reported for fully quasicrystalline and approximant coatings during dry-air sliding against hard steel ball [57,58]. This is an interesting result, since layer A is composed only of 53.4 vol% of quasicrystal approximant α-phase.

Fig. 8(a–c) show a close examination of the worn surface morphology of layer A tested at room temperature. A smooth surface can be observed in the center of the worn track, while craters are located on

Conclusions

The wear resistance of a functionally graded aluminum alloy (FGA) produced by rotational outward solidification casting was investigated at room and high temperatures. Microstructural evolution upon wear was used to discuss the resistance of FGA and the related mechanisms. The following conclusions can be drawn:

  • The FGA was composed by two distinct metallurgically bonded layers: layer A composed of 53.4 %vol. of primary α-Al12(Fe,Mn,Cr)3Si quasicrystal approximant phase embedded within an Al-FCC

CRediT authorship contribution statement

Tales Ferreira: Conceptualization, Methodology, Formal analysis, Investigation, Visualization, Writing - original draft. Guilherme Yuuki Koga: Conceptualization, Methodology, Formal analysis, Writing - review & editing. Ivanir Luiz de Oliveira: Conceptualization, Formal analysis, Writing - review & editing. Claudio Shyinti Kiminami: Conceptualization, Formal analysis, Writing - review & editing. Walter José Botta: Conceptualization, Formal analysis, Writing - review & editing. Claudemiro

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was financed in part by FAPESP through a Thematic Project (grant number 2013/05987–8) and by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) - Finance Code 001. The authors are also grateful to the Laboratory of Structure Characterization (LCE-DEMa-UFSCar) for the microscopy facilities.

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