Project Number 54. Evaluation of near-surface deformed layers · Evaluation of near-surface...
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204 ADVANCED SURFACE ENGINEERING Project Number 54 – 1. Evaluation of near-surface deformed layers in model and practical alloys Research activities associated with understanding and control of alloy microstructure, particularly in the near-surface regions developed after manufacture and fabrication, including advanced joining processes, and after surface pretreatment and finishing is being pursued. EXECUTIVE SUMMARY An example of the near surface structure developed after pretreatment is illustrated in the high resolution transmission electron micrograph of the ultramicrotomed section of Figure 1, where alkaline etching of an AA6111 alloy has been utilized for cleaning and removal of any disturbed or altered layers. This treatment has resulted in enrichment of copper at the alloy surface, immediately below the residual alumina film that persists through the alkaline treatment. Figure 2 shows a recreated image of the enriched layer and the underlying bulk; this was obtained from Fast Fourier Transformation (FFT) of relevant regions of the bright field image. Such study is being used to identify the atom arrangement of the enriched layers and their copper contents, and relationship to the underlying structure. In this activity, in which Manchester is world leading, the extent of alloy enrichment and possible phase formation are grain orientation dependent. Further, knowledge of such layers, together with their significance in corrosion mechanisms, is being pursued actively. As an example, enrichments of copper may proceed from the bulk alloying level to approximately 45 at%, with very significant changes to the corrosion potential and, hence, driving forces for the corrosion processes. OUTLINE 2 nm 2 nm 4.2 84 4.2 12 4.82 -43 Plan (011) of Alα phase Figure 1. Transmission electron micrograph of an ultramicrotomed section of the alkaline etched AA 6111 alloy, showing interfacial enrichment of copper; the FFT of the bulk alloy is also shown
Transcript of Project Number 54. Evaluation of near-surface deformed layers · Evaluation of near-surface...
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� ADVANCED SURFACE ENGINEERING
Project Number 54 – 1. Evaluation of near-surface de formed layers in model and practical alloys
Research activities associated with understanding and control of alloy microstructure, particularly in the near-surface regions developed after manufacture and fabrication, including advanced joining processes, and after surface pretreatment and finishing is being pursued.
�EXECUTIVE SUMMARY
An example of the near surface structure developed after pretreatment is illustrated in the high resolution transmission electron micrograph of the ultramicrotomed section of Figure 1, where alkaline etching of an AA6111 alloy has been utilized for cleaning and removal of any disturbed or altered layers. This treatment has resulted in enrichment of copper at the alloy surface, immediately below the residual alumina film that persists through the alkaline treatment.
Figure 2 shows a recreated image of the enriched layer and the underlying bulk; this was obtained from Fast Fourier Transformation (FFT) of relevant regions of the bright field image. Such study is being used to identify the atom arrangement of the enriched layers and their copper contents, and relationship to the underlying structure. In this activity, in which Manchester is world leading, the extent of alloy enrichment and possible phase formation are grain orientation dependent. Further, knowledge of such layers, together with their significance in corrosion mechanisms, is being pursued actively. As an example, enrichments of copper may proceed from the bulk alloying level to approximately 45 at%, with very significant changes to the corrosion potential and, hence, driving forces for the corrosion processes.
�OUTLINE
2 nm2 nm
4.2
84
4.2
12
4.82
-43
Plan (011) of Alα phase
Figure 1. Transmission electron micrograph of an ultramicrotomed section of the alkaline etched AA 6111 alloy, showing interfacial enrichment of copper; the FFT of the bulk alloy is also shown
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In order to pursue rapidly the role of microstructure and the need for its control in advanced surface engineering, significant investment of time and resource (largely School derived) have been employed in exploiting various instrumental routes, resulting in their acquisition. These include scanning Kelvin probe microscopy (SKPM), complementing the existing scanned probe microscopies, rf-glow discharge optical emission spectroscopy, high resolution scanning electron microscopy (adding to the existing suite of instruments in the School) and a local electrochemistry facility. The last has been designed in association with Uniscan Instruments and is currently being assembled. The major goal now is to exploit the instruments in a complementary manner to determine the preferred microstructures for performance of engineered products. Professor Shimizu, from Keio University, is collaborating in several aspects of this study. Examples of the use of the facilities include the ability of rf-GDOES to generate elemental depth profiles through thick and thin coatings, probe adsorbed layers and to quantify elemental enrichments in association with RBS and MEIS. With specimen preparation through ultramicrotomy, sections of interest may be readily located and examined by high resolution SEM, with local chemistry examined through application of low kV, energy and angle selected backscattered electrons. Figure 3 shows a secondary electron micrograph of an ultramicrotomed block of a rectified AA 6111 alloy, revealing the presence of a fine grain, disturbed region after surface grinding.
Concerning the ultramicrotomy approach to specimen selection, low angle knives have been shown to generate surfaces of nanoscale roughness whilst limiting damage and associated contrast under low kV irradiation. For low kV examination, rapid insertion into the microscope column is also essential to restrict contamination and air-formed oxide growth on freshly cut surfaces. A more rapid generation of revealing sections is now possible by exploiting the low energy, high flux argon ion beam employed for GDOES.(Figure 4) Such an approach leaves little residual damage and is rapid; further, the approach has promise for serial sectioning which is to be exploited. Finally, specimens produced by conventional routes or the novel procedures highlighted here are also being examined by scanning Kelvin probe.
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0.23
0.19
0.22
1 nm1 nm
0.23
0.22
2 nm2 nm
Figure 2. Reconstruction of the atom arrangement of the bulk AA 6111 ally, showing the (110) orientation; reconstruction (100) of the enriched copper layer at the interface between the alloy and the residual alumina film developed during alkaline etching
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Figure 3. Secondary electron micrograph of near surface regions of a rectified AA 6111 alloy.
Figure 4.Secondary and backscattered electron micrographs of a coated titanium; the specimen was prepared by an argon ion plasma method. The BSE image, revealing detail of the W-Co coating also shows embedded alumina in the initially cleaned titanium alloy surface regions (in association with Professor K Shimizu).
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microscopy to reveal surface potential differences and to highlight the influence of local microstructure on prediction of corrosion (Figure 5). With commissioning of the local electrochemical facility, the electrochemical responses from fine regions can also be elaborated. Together these will provide a unique capability to identify the influence of microstructure in engineered materials and their treated surfaces on corrosion and its control through further surface alteration.
Figure 5. Backscattered scanning electron micrograph of an ultramicrotomed block of a model A-4.5% Fe alloy, revealing Al3Fe constituent particles. The SKPM image reveals the surface potential difference between the matrix and the constituent particles.
5 µm
Complementary approaches have been developed for the interrogation of the influence of microstructure on performance particularly corrosion. With novel and practical alloys this study will be progressed further to provide mechanisms of corrosion and its prediction.
�ACHIEVEMENTS
PUBLISHED PAPERS
1. The effect of near-surface microstructure on the corrosion resistance of aluminium alloys, X. Zhou, Y. Liu, G. E. Thompson, G. M. Scamans and A. Asfeth, 16th International Corrosion Congress, Beijing, China (2005).
2. Ultra-fast elemental depth profiling analysis of oxide films on aluminium with RF GDOES, P. Chapon, C. Tauziede, G. E. Thompson and J. Malherbe, ASST IV, Beaune, France (2006).
