Cavity growth and filament formation of superplastically deformed Al 7475 Alloy

doi:10.1016/S0928-4931(00)00193-4

Materials Science and Engineering: A

Volume 298, Issues 1–2, 31 January 2001, Pages 235-244

https://doi.org/10.1016/S0928-4931(00)00193-4 Get rights and content

Abstract

Superplastic deformation induced cavitation of aluminium alloys usually results in the material performance degradation. In this study the cavitation behaviour of Al 7475 was characterised, using samples deformed at temperature ranging from 480 to 530°C and at an initial strain rate of 10⁻³s⁻¹. The results showed that the cavity growth rate parameter increased slightly as the test temperature increased. The cavitation ratio at fracture increased evidently with increasing temperature from 480 to 500°C, and reached a plateau for any further increase of temperature. Various morphological filaments were observed at cavities and fracture surface, as evidence of the presence of liquid phase along grain boundaries. The effects of test temperature and thermal history on cavitation were found to be closely related to the presence of the liquid phase. The presence of liquid phase will improve the ability of the materials to tolerate high volume fraction of cavities before fracture. On the other hand, when liquid phase is anisotropically distributed along grain boundaries, it will cause the preferential interlinkage of cavities along the weak grain boundaries, and result in corporate grain boundary sliding (CGBS). It is concluded that the critical factor is to achieve appropriate quantity, high property, and uniformly distributed liquid phase along grain boundary. This highlights a new clue in searching for an economical and practical way to alleviate cavitation.

Introduction

During the past two decades, much effort has been devoted to the development of superplastic forming (SPF) high strength aluminium alloys and forming techniques, since high strength aluminium alloys have wide applications in aerospace industry. Unfortunately, cavitation usually occurs in a wide range of metallic materials and metal matrix composites (MMCs) after SPF, especially in aluminium alloys, leading to the degradation of the overall properties of the post-SPF materials [1], [2]. It has been demonstrated that the mechanical properties of the materials would be significantly reduced when the cavity ratio exceeded approximately 1% [3]. This limits the useable strain range for SPF to a certain value at which the overall properties of the materials are not seriously degraded by cavitation, though many SPF materials can be strained to as high as 1000% or even more before fracture.

In order to alleviate the problem of cavitation, studies of the effects of superimposed pressure, hot isostatic presses (HIP), and prior-deformation heat treatments, etc., have been extensively carried out recently [4], [5], [6], [7]. The superimposed pressure and HIP methods need relatively high additional costs, and besides, or more importantly, the dimension of the forming components is limited using these techniques. From the practical point of view, prior-deformation heat treatment is an economic and applicable way to alleviate cavitation by altering the thermal history of the materials [5].

The thermal history plays an important role in the cavitation behaviour of the materials. The variation of cavitation behaviour for samples with different thermal history is usually accompanied by the changes of filament formation [8], [9]. The filament formation of superplastic deformed materials is frequently reported. It is generally thought as the evidence of the existence of liquid phase during the deformation [8], [9], [10], [11]. Cao et al. [8] suggested that the filament formation was more likely to be a viscous flow of a liquid-like substance originating at the grain boundaries. Blandin et al. [9] found that filaments present on the deformed samples of as-received materials but the filaments are no longer present on heat-treated ones.

The presence of liquid phase was expected to greatly change the properties of grain boundaries, and to influence the cavitation behaviour of the materials. The altering of thermal history through heat treatment could be a potential way to modify the role of liquid phase. However, little information is available on these aspects. The research therefore aimed to investigate the nature of liquid phase and its effect on cavity growth. It is also expected that the results of this research would be useful for the understanding of cavitation mechanism and the searching of economic and applicable methods to alleviate cavitation.

Section snippets

Experimental procedures

Commercial Al 7475 sheet of thickness 2.0 mm used in the present experiment has a chemical composition of (wt.%) 5.64 Zn, 2.34 Mg, 1.58 Cu, 0.19 Cr, 0.08 Fe, 0.05 Si, 0.02 Ti, 0.01 Mn and Al balance. The as-received sheet was in T4 condition (solution heat treated, quenched and natural aged). The tensile specimens were of 15 mm length, 4 mm width and 2 mm thickness in gauge parts, with sample cutting direction parallel to the rolling direction of the sheet. The surfaces of tensile sample gauge

As received condition of commercial Al 7475

The microstructure of as-received Al 7475 sheet showed that the material was fully recrystallised, Fig. 1. The grains were in pancake-shape, i.e. equiaxed shape in longitudinal-transverse (L-T) section and thin in short transverse (N) direction. Its average linear intercepts were 10 μm, 10.1 μm and 7.2 μm in the L, T and N directions, respectively. Precipitates could be found in the interior grain and along grain boundaries. The precipitates were generally about 0.5 μm or less in diameter. EDS

Conclusions

Filaments were observed at cavities and fracture surface as evidence of liquid phase along grain boundaries. The quantity and radius of the filament increased as testing temperature increases. Continuous filaments emerged on long and narrow cavities indicating that GBS was closely related to cavity growth.

The deformation temperature and thermal history significantly influence the filament quantity and composition. The presence of Mg and Zn enrichment is expected to improve the viscosity of the

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Cavity growth and filament formation of superplastically deformed Al 7475 Alloy

Abstract

Introduction

Section snippets

Experimental procedures

As received condition of commercial Al 7475

Conclusions

Acta Metall.

J. Mater. Proc. Tech.

Acta mater.

Acta mater.

Acta mater.

Acta mater.

Mater. Sci. Forum

Scripta Metallurgica et, Materialia

Acta Metall.

Acta Metall.

Acta Mater.

Acta Mater.

Acta Metall. Mater.

Acta metall.

Metall. Trans.

Metall Trans.

Aluminium

Mater. Sci. Tech.

Metall. Trans.