Metals Science Research and Consulting


by Dr. Stan T. Mandziej

 

 

Enhancing ductility of Al-alloys by grain refinement

 

Stan T. Mandziej

Advanced Materials Analysis, Enschede, NL

E-mail: stanamanl@cs.com

 

Abstract.

Pure light metals and single-phase alloys have not been the best candidates for achieving ultrafine-grained microstructures, thus the use of precipitation-hardening multi-phase alloys has been advised for the SPD experiments. Low- or medium temperature deformation of such alloys results in strain localization in shear bands associated with generation of adiabatic heat, which affects recovery and precipitation processes responsible for the final grain size. For GleebleTM physical simulator the MaxStrain device was developed which allows generating ultrafine microstructures by accumulated multiple compressive strains executed at various strain rates during programmed thermal cycles with adequate control of process temperature. This paper deals with processing of Al-6061 wrought alloy and the Al-319 cast alloy by the MaxStrain device and describes obtained microstructures.

 

Keywords: SPD, Al-alloys, Gleeble, MaxStrain, metallography

 

1.      Introduction

Manufacturing of ultra-fine-grained materials is now a hot issue in materials science. Since more than two decades an explosive boom has been observed in development of techniques producing kilogram(s) of nano-grained materials as well as publishing results obtained by these techniques, most of which applying severe plastic deformation with self-recovery or post-deformation recovery. In the noise created by this explosion was omitted that the nano-structured metal alloys exist in World’s technology since ages and these have been used from ancient swords and mediaeval arms till more recent high-carbon martensitic tool steels, medium-tempered spring steels and patented near-eutectoidal wires, altogether produced annually in thousands of tons. Soon this initial run for manufacturing a kilogram of nano-material was replaced by better defined challenging tasks, like how to make the nano-structured materials further processable and afterwards stable during exploitation, as well as more efforts have been directed towards development of better compositions for this last purpose. The ultimate grain refinement of single-phase alloys often leads to fast deterioration of mechanical properties or consistency of the material due to release of deformation energy stored in it [1]. Also the Hall-Petch relation [2] appeared to have limits – below certain grain size further refinement results in softening and decrease of strength [3]. Thus more attention was recently given to multi-phase alloys in which the dynamic recovery and recrystallization processes coincide with strain-induced precipitation. For light-metal alloys important appeared observing adiabatic heating effects and their influence on self-recovery as such alloys usually have high strain-rate sensitivity. The most frequently used for light metal alloys ECAE (equal channel angular extrusion) technique forces alloys to deform predominantly by shear localized in deformation bands, which process is known as coinciding with generation of adiabatic heat [4]. Formation of these shear bands and generation of heat is typical for the high strain rate deformations. ECAE processes usually apply low strain rates, however they force the material to deform along preferential shear directions. Most of the ECAE devices do not allow measuring the temperature in the highest deformation zone of the processed material, while additionally friction conditions between the workpiece and die remain unknown. As long as the adiabatic shear bands persist, the uniform grain size cannot be achieved, so for this the saturation strain must be exactly determined, while in the ECAEd material the amount of accumulated strain is usually given as a number of passes with unknown true strain in each pass. To study all associated processes of SPD in more detail a tool was developed – the MaxStrain device combined with Gleeble simulator [5]. 

 






The principle of the MaxStrain is to deform from two perpendicular directions the central portion of the bar-like sample while restraining its ends. Without the restraint the material would flow uni-directionally (A), while with restraint it flows transversely (B). 

The sample mounted like in this figure rotates 90 degrees between each hit, accumulating in its central portion compressive strains. The MaxStrain executes multiple such two-directional plane-strain compressions to reach accumulated strains of 50 and more, limited only by the ductility of processed alloy at particular deformation conditions.


Characteristic of plane st
rain deformation is localization of strain in deformation bands and in the centrazone of the sample. This is the reason that maximum of adiabatic heat is generated within the deformation bands shown in this picture.
Therefore the temperature in MaxStrain is measured  by thermocouple inserted under angle from the edge of mounting portion towards thecentre of sample, so the tip of thermocouple comes near to the axis of most deformed zone.
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Full article from Nano-SPD-6 conference, Metz, France, 2014, entitled: 

"Enhancing ductility of Al-alloys by grain refinement",

available from: amatemlab@gmail.com, upon request. 


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References:

[1] Byrne J G 1965: Recovery, Recrystallization and Grain Growth, Macmillan, New York.

[2] Information on: http://en.wikipedia.org/wiki/Grain boundary strengthening.

[3] Conrad H & Narayan J 2000: On the grain size softening in nanocrystalline materials. Scripta Materialia 42(11), pp.1025–30.

[4] Bai Y L & Dodd B 1992: Adiabatic Shear Localization – Occurrence, Theories and Applications, Pergamon Press, Oxford UK.

[5] Information on: http://gleeble.com/index.php/products/maxstrain.html.