Temperature-dependent formation of gradient structures with anomalous hardening in an Al–Si alloy

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Abstract

The temperature effect on forming gradient structure in an Al–Si alloy during surface severe deformation is studied in this report. The intermediate temperature (473 K) produces the steepest gradient structure by an anomalous hardening on the top layer compared to lower (300 K) and higher temperature (673 K) counterparts. Our analysis shows profuse aluminum oxide particles in the top layer of the gradient structure under 473 K lead to anomalous hardening by oxide-dispersion strengthening. The counterintuitive enhancement of strengthening at the intermediate temperature is explained by the dynamic interplay between thermal-driven, mechanical-aided oxidation and the wear-induced loss of surface materials as a function of temperature, which yields a critical processing temperature to achieve the steepest gradient structure.

Introduction

Heterogeneous microstructure design through gradual changes in grain size, defect density, texture, etc., has garnered extensive interest in the community during the past decades [[1], [2], [3]]. There is a growing research interest in gradient structured (GS) pure metals and alloys ranging from conventional metals such as Cu and steels to emerging multi-principal element alloys [[4], [5], [6]]. Compared to the conventionally homogeneous counterparts, the GS metals were reported to exhibit extraordinary mechanical properties, including outstanding strength-ductility synergy [2,7], enhanced fatigue resistance [8], and remarkable wear resistance [9]. Fundamentally, those superior mechanical properties are intrinsically associated with the microstructure characteristics of GS, such as the gradient steepness and the volume fraction of GS in the whole material [10,11]. For example, to boost the strength in GS metals while maintaining decent ductility, a sufficiently steep gradient across the spatial distribution of its microstructure is often required to induce substantial mechanical incompatibility upon plastic deformation [12,13]. It is this mechanical incompatibility that promotes the emergence of the strain/stress gradient and the storage of geometrically necessary dislocations on the fly during deformation, which is commonly believed to play an essential role in achieving the extra work hardening and the exceptional strength-ductility synergy [3,14,15].

Naturally, the microstructure characteristics of GS are determined by its fabrication route and conditions. To date, the primary fabrication method to produce bulk GS metals is surface mechanical treatment (attrition, grinding, rolling, peening) to generate gradients in the materials via severe plastic deformation [3]. Most of these fabrications were carried out at ambient temperature rather than elevated temperatures, with the main objective of suppressing the growth of most refined grains at the outermost layer to achieve a steep gradient in grain size. It is noteworthy that increasing the processing temperature may also induce microstructural evolutions benefiting the formation of a steep gradient structure, for example, partial oxidation during deformation. Forming a surface oxide layer, especially when the oxides are uniformly dispersed in the metal matrix, can provide effective strengthening [16], potentially inducing a significant mechanical incompatibility from the exterior to the interior, i.e., a steep gradient. However, the competition between the loss of grain boundary strengthening (due to grain growth) and the gain of oxide dispersion strengthening to mediate the formation of GS as a function of temperature is not well understood. In addition, the dynamical nature of the interplay between deformation and in-situ oxidation adds another layer of complication to comprehending the formation process of GS at different temperatures.

In this study, we investigate the formation of a GS in an Al–Si alloy by high strain cyclic shear deformation at three different temperatures. The focus is to unveil how processing temperature influences the obtained GS steepness through the dynamic interplay between the surface severe deformation and the attendant oxidation. The Al–Si alloy system, selected for this study is a popular cast binary alloy system, representing about 85–90% of the total cast Al products, and thus holds crucial significance for applications [17]. Our proof-of-concept study is applicable to a range of surface deformation-based solid-phase processing techniques that result in gradient microstructures.

Section snippets

Materials and methods

The high-purity Al-4at.%Si alloy was melted in a vacuum furnace protected with an argon atmosphere and then cast. A thin plate was cut from the cast ingot, metallographically polished using 0.02 μm colloidal silica solution. Fig. 1a presents the microstructure of this starting material, consisting of coarse dendritic grains and networks of the eutectic Al–Si phase (bright). Dry sliding and reciprocating wear tests were performed using a RTEC Universal Reciprocating Tribometer with temperature

Results and discussion

The backscatter electron micrographs of the transverse cross-section of the wear tracks at three temperatures (300, 473, and 673 K) are shown in Fig. 1c. The cyclic tribological deformation on the surface creates the shear-zone trenches. The depth of the trenches increased with increasing temperature. Along with the increasing depth, the coefficient of friction and wear rate both demonstrate a similarly increasing trend as a function of temperature (Fig. 1d). Given the identical load and strain

Summary

In closing, we studied the GS formation in Al-4at.% Si alloy by surface severe deformation under various temperatures. A steeper gradient is surprisingly obtained at the intermediate temperature owing to the in situ formation and dispersion of oxide particles in its top layer. A detailed microstructural characterization reveals that the oxide-dispersion influences the steepness of the GS under different temperatures. The high-density (6.4 × 1011 mm−3) and narrow spacing of fine-scale oxide

Originality Statement

I write on behalf of myself and all co-authors to confirm that the results reported in the manuscript are original and neither the entire work, nor any of its parts have been previously published. The authors confirm that the article has not been submitted to peer review, nor has been accepted for publishing in another journal. The author(s) confirms that the research in their work is original, and that all the data given in the article are real and authentic. If necessary, the article can be

CRediT authorship contribution statement

Xiaolong Ma: Conceptualization, Methodology, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. Matthew Olszta: Methodology, Formal analysis, Investigation, Writing – review & editing. Jia Liu: Methodology, Formal analysis, Investigation, Writing – review & editing. Miao Song: Methodology, Investigation, Writing – review & editing. Mayur Pole: Methodology, Formal analysis, Investigation, Writing – review & editing. Madhusudhan R. Pallaka: Methodology, Formal

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.

Acknowledgments

This research was supported by the SPPS (Solid Phase Processing Science) Initiative at the Pacific Northwest National Laboratory, USA. The authors are grateful to Anthony Guzman for the preparation of specimens for microstructural characterization, and Professor Mathaudhu Suveen for advice. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy under contract DE-AC06-76101830.

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    Current address: Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC, 27695-7907, USA.

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