Ultrafine-grained Al–5 wt.% Fe alloy processed by ECAP with backpressure

doi:10.1016/S0921-5093(03)00215-6

Materials Science and Engineering: A

Volume 357, Issues 1–2, 25 September 2003, Pages 159-167

https://doi.org/10.1016/S0921-5093(03)00215-6 Get rights and content

Abstract

A process for increasing both strength and ductility in a brittle Al–5 wt.% Fe alloy by Equal Channel Angular Pressing (ECAP) was investigated. The increase in solid solubility of iron in the Al matrix produced by intense deformation under ECAP permitted age hardening in this alloy although it is not hardenable by conventional processing. An ultrafine-grained microstructure of 325–450 nm was obtained in a brittle, cast Al–5 wt.% Fe alloy by ECAP with various numbers of passes and backpressure levels from 40 to 275 MPa. A supersaturated solid solution with a maximum solubility of 0.6 wt.% of iron in an aluminium matrix was obtained during ECAP, which allowed ageing of the conventionally non-hardenable alloy. Strength, ductility and microhardness of the cast alloy processed by the ECAP technique were significantly enhanced (e.g. strength and ductility from 102 MPa and 3.4% to 261 MPa and 5.8%, respectively). The subsequent artificial ageing resulted in a further increase in strength to 272 MPa. It was shown by scanning electron microscopy (SEM) that the type of fracture of tensile specimens taken from material subjected to ECAP was predominantly ductile. An increase in backpressure retards cracking of intermetallic particles and enhances the workability and ductility of such alloys processed by ECAP.

Introduction

Al–Fe-alloys are very attractive as a base for development of cheap, high-strength constructional materials for application in the automotive and aircraft industries. Alloying of aluminium by iron can enhance resistance to heat because of the presence of dispersed second phase particles in the structure [1]. However, the solubility of iron in the aluminium lattice is less than 0.03 at.% even at high temperatures, and these alloys are not age-hardenable. Apromising approach to overcoming this and strengthening such alloys is to use severe plastic deformation (SPD) as a processing method for nano- or ultrafine-grained metals. This method is now well known among researchers in materials science [2], [3]. It allows microstructure to be refined and various solid state properties, especially strength and ductility, to be increased greatly. Moreover, application of one of the techniques of SPD, for instance high pressure torsion (HPT) [4], [5], enhances solubility of iron in aluminium and allows the strength of Al–Fe during following artificial ageing to be greatly increased.

Equal channel angular pressing (ECAP) is the most attractive method among the SPD techniques because it can be used to produce not only laboratory samples but bulk nanostructured billets for subsequent metallurgical operations [6]. In recent years, many original and review papers have been devoted to this technique, connected with the features of ultrafine-grained structure (UFG) and the development of properties during ECAP. The influence of the number of passes, processing routes, strain rate, temperature and the design of ECAP tooling has been considered in detail [3]. However, the role of backpressure as one of the important processing parameters, has been reported in few papers [7], [8], [9], [10]. This parameter could be especially important for ECAP of alloys with low ductility, that otherwise fail after even one pass. When backpressure is applied, the occurrence of damage in deformed samples decreases because the shear strain takes place under a compressive hydrostatic stress [7], [8], [9], [10]. The influence of the level of backpressure on the structure and, particularly, on mechanical properties of ECAP material has not been investigated to the present.

As described in the present paper, an attempt has been made for the first time to produce a high strength nanostructured state by the ECAP technique and to study the features of the structure, ageing effects, thermal stability and mechanical behaviour as a function of the level of backpressure in bulk non-age-hardenable Al–5 wt.% Fe alloy.

Section snippets

Experimental material and procedures

The Al–5 wt.% Fe alloy was supplied by IPM (Ekaterinburg, Russia). It was prepared by melting and mixing commercially pure aluminum and iron carbonyl in a steel crucible at 1100 °C for 2 h. The metal was cast into a brass mould. Ingot dimensions were approximately 30 mm diameter and 85 mm long. Before SPD by ECAP the ingots were machined to 20×20×85 mm. Detailed information on the ECAP apparatus used in this study can be found in [7], [8]. ECAP was performed in a 90° die, (Fig. 1a), at room

Characterisation of optical microstructure

Fig. 2a, b shows the typical optical microstructures of the Al–5 wt.% Fe alloy in the transverse direction before and after ECAP. The structure of the cast alloy consists of an Al matrix, with large primary and fine eutectic and secondary Al₁₃Fe₄ aluminides. The volume fraction of primary aluminides is about 13%, with maximum thickness and length of 10×80 μm. Difference in colour within the primary phase is attributed to differences in orientation. The eutectic aluminides are globular with an

Discussion

The research performed has shown that SPD of brittle cast Al–5% Fe by ECAP technique without failure is possible only if backpressure is applied. It was found that workability of the material increases with increase in the level of backpressure. Failure did not start even after an accumulated strain of 18.4 at the maximum backpressure applied. In the absence of backpressure during ECAP, samples cracked or broke after strains of 1–2. In accordance with [7] the hydrostatic component plays a

Conclusions

(1) Brittle cast Al–5 wt.% Fe alloy can be subjected to SPD to 18.4 without failure by applying ECAP with a backpressure not less than 275 MPa. Billets failed after two ECAP passes in the absence of backpressure.

(2) ECAP of the cast alloy led to strong refinement of structure and formation of a close-to-nanocomposite structure with a mean grain size of 325 nm in the aluminium-based matrix and small uniformly spread second phase particles of less than 10 μm.

(3) A supersaturated solid solution of

Acknowledgments

We acknowledge the principal financial support from the Australian Research Council (Linkage International Grant # LX0211114) and Monash University. This research has also been partially supported by the Russian Foundation for Basic Research (Grant # 01-03-32125) and an Independent International Association (INTAS Grant # 99-01741). The authors are grateful to Dr D. Bashlukov for the supply of raw material and Dr P.N.H. Nakashima for TEM observations.

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Ultrafine-grained Al–5 wt.% Fe alloy processed by ECAP with backpressure

Abstract

Introduction

Section snippets

Experimental material and procedures

Characterisation of optical microstructure

Discussion

Conclusions

Acknowledgments

J. NanoStruct. Mater.

Mater. Sci. Eng.

Scr. Mater.

The Phys. Met. Metallography