Mechanical properties and microstructures of Al–1Fe–(0-1)Zr bulk nano-crystalline alloy processed by mechanical alloying and spark plasma sintering

doi:10.1016/j.msea.2012.02.017

Materials Science and Engineering: A

Volume 541, 15 April 2012, Pages 152-158

https://doi.org/10.1016/j.msea.2012.02.017 Get rights and content

Abstract

A high strength bulk nano-crystalline Al–1Fe–(0-1)Zr alloys were processed by mechanical alloying and spark plasma sintering. The high yield strength of ∼850 MPa was achieved in an Al–1Fe–0.5Zr alloy with 19% strain in compression. The high strength was attributed to the nanosized Al–Fe grains and the dispersion of fine Al₆Fe particles. High temperature compressive yield strength of 455 MPa and elongation of 16.7% was achieved at 250 °C.

Highlights

► A yield strength of ∼850 MPa was achieved with 19% strain in compression. ► The high strength due to the nanosized Al–Fe grains and fine Al₆Fe particles. ► A compressive yield strength of 455 MPa and elongation of 16.7% at 250 °C.

Introduction

There is a continued drive for the development of high strength light alloys due to the strong demand to improve the energy efficiency in the transportation sector. Precipitation hardened aluminium alloy is one of the commercially available light alloys for structural applications. The maximum tensile yield strength is ∼800 MPa for the Al–Cu–Li based Weldalite alloy [1], which is attributed to the uniform dispersion of T1 (Al₂CuLi) and δ′ (Al₃Li) precipitates. It is intriguing to explore the possibility of strength up to 1 GPa in Al-based alloys. To open up the possibility, one promising technique is the refinement of grain size down to a nanometer scale.

Al–TM systems (TM: transition metals) have been studied for a long time as potential heat resistant light alloys. However, the limited solubility of TM in Al hinders the precipitation of sufficient amount of secondary phases by conventional processing routes. The effort to extend the solubility of TM in an α-Al matrix was made by non-equilibrium processing such as rapid solidification, mechanical alloying and electron beam deposition. These non-equilibrium processing routes also resulted in the successful fabrication of “nanocrystalline” supersaturated alloys or nanocomposites with ultra-high strength [2], [3], [4], [5], [6]. Recently, Sasaki et al. [7] reported the successful fabrication of bulk nanocrystalline Al–5 at%Fe alloy by the combination of mechanical alloying (MA) and spark plasma sintering (SPS). This alloy was strengthened by the nanocrystalline grains and high volume fraction of intermetallic particles, and exhibited the ultra-high strength over 1 GPa at room temperature and 500 MPa at 350 °C with large plastic strain in compression [7]. However, none of these alloys showed tensile ductility due to the high volume fraction of the intermetallic particles.

One way to enhance the ductility retaining the high strength is to reduce the high volume fraction of the brittle Al₆Fe and Al₁₃Fe₄ phases while keeping the nanocrystalline grain structure. The previous investigations suggested that Zr worked as an effective alloying element as a grain refiner in conventional Al alloys and Al–Fe–Zr alloy processed by powder metallurgy process [1], [2], [8], [9]. Therefore, the substitution of Fe with a small amount of Zr could be a promising way to reduce the volume fraction of the brittle intermetallics and retain nanocrystalline structure to achieve high strength with a small amount of the alloying element (<1.5 at%). In this work, we have investigated the effect of Zr additions on the microstructures and mechanical properties of mechanically milled and spark plasma sintered Al–Fe alloys.

Section snippets

Experimental procedure

Pure Al (99.9% purity and 53–106 μm diameter), Fe (99.9% purity and 53–106 μm diameter) elemental powders and Al₃Zr alloy powders were used in the present investigation. The Al₃Zr alloy powders were used to avoid the formation of ZrH₂ phase during mechanical alloying [5], [10], [11]. The stoichiometric Al₃Zr alloy made by arc melting was crushed into a powder. Mechanical alloying (MA) was carried out at ambient temperature using a Fretsch Pulverisette P-6 planetary ball mill in a hardened

Results

Fig. 1(a) shows XRD profiles for the Al–1Fe–0.3Zr alloy mechanically alloyed for different times. Only Al peaks could be found in these profiles indicating the dissolution of Fe and Zr as solid solution. Fig. 1(b) shows the variation in grain size as a function of the MA times. The grain size varies from ∼52 nm after 60 h of MA to a minimum value of ∼45 nm after 90 h, and increases with further MA time of 100 h to ∼50 nm. The XRD spectra recorded for mechanically alloyed powders with increased Zr

Discussion

In this work, we have fabricated bulk nanocrystalline Al–Fe–Zr alloys processed by MA and SPS, and investigated their processing–structure–property relationship. The Al–1Fe–0.5Zr alloy fabricated by the optimized processing condition exhibited relatively high compressive yield strength of ∼854 MPa with 19% plastic strain. Although the strength is lower than that of the Al–5 at%Fe alloy exhibiting 1 GPa class strength [7], it is interesting to note the high strength up to 850 MPa has been achieved

Conclusions

Bulk nano-crystalline Al–1Fe–0.5Zr alloys fabricated by mechanical alloying and spark plasma sintering exhibited high compressive yield strength of 854 MPa with a plastic strain of 19% at room temperature and 455 MPa and 16.7% at 250 °C. The high strength and high plastic strains were attributed to the presence of high density of nano-crystalline grains with grain size of 65 nm and a small volume faction of coarse grains distributed through the microstructure and a fine distribution of Al₆Fe and Al₃

Acknowledgments

This work was in part supported by World Premier International Research Center for Materials Nanoarchitectonics (MANA) and the Center for Nanostructured Materials Technology (CNMT) under the 21st Century Frontier R&D Programs of the Ministry of Science and Technology, Korea through the Korea Institute of Science and Technology (KIST).

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