248 Prakt. Met. Sonderband 46 (2014)
powder metallurgy and the IO process are used mostly. However, for non-porous large preparati
billets the 10 is more suitable. 10 is a diffusion-controlled process involving selective with alun
reactions of a less noble solute or second phase particles with oxygen (also nitrogen or
carbon) diffusing in from the surface [3,4]. The problem that occurs by dispersion
strengthening with the |O process is to achieve a sufficient depth of the internal oxidation
zone (I0Z) in a technologically acceptable time [4]. However, the IO process can be
accelerated in the alloys with thermodynamically metastable microstructures containing
high concentration of defects in the crystal lattice. Especially grain boundaries and
dislocations enable faster diffusion of oxygen.
In the praxis severe plastic deformation can be used as a mean for obtaining a very high
concentration of defects in the copper crystal lattice. For achieving a high degree of plastic
deformation different techniques have been developed, such as high pressure torsion
(HPT) and equal channel angular pressing (ECAP) [5]. Of these, ECAP is an especially
attractive process because the overall billet dimensions do not change during processing
and high shear strains can be accumulated by repeated extrusion through the die [6,7,8].
Significant progress has been made in the understanding of the fundamental properties
and microstructures of the ECAPed materials by using theoretical analysis and
experimental methods. The influence of severe plastically deformed material on IO was Fig. 1:
investigated rarely. In fact the combination of ECAP process and IO has not yet been Co
studied and no information is available.
This paper describes the influence of highly metastable microstructure on the mechanism
and kinetic of the 10 process. The size, distribution and shape of the oxide particles with 3. RES
increasing depth of IOZ were also analysed. :
After inte
2. EXPERIMENTAL DETAILS particles
phase Ci
For the experimental activities we used Cu-0.4%Al alloy. Billets with a cross-section of 10 esos) N
x 10 mm and 50 mm in length were made by vacuum melting, mould casting and of the di
calibration rolling. diffusion :
For deformation experiments the ECAP tool has been designed. Many parameters were During th
taken into consideration regarding billet shape, size and maximum work load. The tool aluminiun
consists of two intersecting channels of the same cross section (10x10 mm?) that meet at because
the angle ® = 90°. The geometry of the tool provides that the material is deformed by a
simple shear at ideal, frictionless conditions. A simple standard press with 60 metric tons solubility
capacity was used. The ECAP pressings were carried out at room temperature using route precipitat
"A" where the billet is not rotated in any direction’. The billet was lubricated with motor oil and in the
(OLMALINE SF/CD 20W50). A schematic illustration of the ECAP process with starting shapes, f
and obtained microstructure: is shown in Figure 1. the billet |
Samples of dimensions 10 x 10 x 5 mm were taken transverse to pressing direction of the 102
before, after first and after fourth ECAP pass. These samples were internally oxidized. The Increase
IO procedure was performed in a mixture of equal parts of copper oxide, copper metal and conseque
Al,O3 powders enclosed in a glass ampoule and held at 1173K for 30min, 60min and of the all
300min. This procedure allowed a partial pressure of oxygen equal to the decomposition depths of
pressure of copper oxide and maintained the saturation concentration of oxygen in the Moreover
surface of the billet for longer annealing times. of the hur
The depth of IOZ was measured with optical microscopy (OM) on the cross-section of the for growir
sample. The samples for OM were prepared with standard metallographic methods and
etched with FeCls. The size, distribution and shape of the oxide particles was analysed
with the scanning electron microscope (SEM) Sirion 400 NC. The metallographic
