Microstructural stability of ultrafine grained cold sprayed 6061 aluminum alloy

Highlights

• UFG cold spray 6061 layer was analyzed by in situ hot-stage TEM.

• Locations and temperatures of different microstructural changes were observed.

• Y direction was found to preserve UFG structures after heating up to 450 °C.

• In Z direction, the UFG's were not resistant to grain coursing.

Abstract

The microstructural stability of ultrafine grained (UFG) cold spray 6061 aluminum deposits produced by high pressure cold spray were investigated by in situ heating to a fully annealed state via a hot-stage transmission electron microscope (TEM). It was possible to observe the precise locations and temperatures of different microstructural changes, like dislocation movement and other restoration processes. Even after heating up to the annealing temperature for this alloy, the deposited layer in the perpendicular direction was found to preserve the UFG structures, which were the result of different recrystallization mechanisms caused by the high strains present during cold spraying. Extensive solute segregation at the grain boundaries acted as an obstruction for grain boundary migration in this direction, thereby preventing grain growth. However, in the direction parallel to the deposited surface, the UFGs were not resistant to grain coursing like the other direction, since the grain boundaries had much less solute segregation.

Introduction

Recently, considerable efforts have been devoted to fabricate bulk ultrafine grained (UFG) materials without porosity and a better combination of ductility and strength by imposing severe plastic strain [1], [2], [3], [4], [5], [6]. A high degree of deformation coupled with the ability to tailor admixtures of powders during cold spraying (CS) can produce dense homogeneous or functionally graded microstructures unattainable by traditional P/M methods as well as UFG or even nano structure [5], [6], [7]. Often UFG and as-deformed structure, which is believed to be thermo-dynamically metastable due to excess amounts of various lattice defects and elastic distortions [8], can work together to provide excellent strength at room temperature, however, for many applications a more thermodynamically stable microstructure with better ductility would be preferred. Hence, thermally activated restoration processes, e.g. recovery, recrystallization and grain growth, that arise during annealing to high temperatures, are of interest here. In many cases, precisely controlled recovery and recrystallization of cold sprayed or severely deformed structures have been used to augment ultrafine grain formation [9], [10], [11], [12].

There is also some indirect evidence of grain boundary (GB) segregation in UFG materials processed by cold work [13], [14], [15], [16], [17]. GB segregation is an extremely sensitive phenomenon because it may affect significantly material properties like corrosion resistance, mechanical behavior, or thermal stability [17], [18], [19]. It is thus of great importance to examine the mechanisms of the deformation-induced solute redistribution and concurrent grain boundary migration that was observed in the as-sprayed deposition of this investigation.

Non-isothermal heat treatments, especially the use of low heating rates, are frequently encountered during thermo-mechanical processing cycles, and hence are of importance for industrial purposes. During non-isothermal heat treatment, recovery and precipitation are likely to occur, generating complex interactions with recrystallization. Precipitation can also strongly influence the recrystallization behavior. The precipitation of dispersoids in the course of an annealing treatment can hinder or even suppress recrystallization [20], [21], [22]. Liu et al. [23] attributed the occurrence of large, inhomogeneous grain sizes in AA3105 to the occurrence of concurrent precipitation. Bampton et al. [20] also reported low heating rates should be employed in order to obtain maximum benefit from the fine grain processing technique. It has been demonstrated that the extent of recovery and precipitation also influence the recrystallization temperature [24].

The present study was undertaken in an effort to understand the recovery and recrystallization processes and associated changes in the microstructure during heating and annealing of a highly deformed 6061 alloy layer deposited by high pressure CS. Finding the exact temperatures for the processes involved during heating are important for microstructural control. In this case, an in situ hot-stage TEM investigation was found to be a suitable and powerful tool to study and probe the complex interactions and thermal stability of the various inhomogeneous as-deposited microstructures, during non-isothermal heating and annealing.