Ultra Fine Structured Metallic Materials
About the project
This project is funded by the Flemish gouvernment through the IWT
(Flemish institute for the stimulation of the scientific
technological research in industry). It is part of the STWW
research program (STWW = strategic technologies for well-being and
prosper), section technology and economy.
The starting date is Decemder 1st, 1999. It is a two-year project.
Ultra fine structured (UFS) materials
Ultra fine structured materials are polycrystals with a grain size
below about 1 µm. The term nanostructured materials is used for
UFS materials with a grain size between 2 nm and 100 nm, and the
term sub-micron materials for grain sizes between 100 nm and 1 µm.
Besides dramatic increases in strength and toughness, significant
changes in physical properties have been observed in UFS materials
when compared with the corresponding conventionally produced
coarser grain counterparts. These properties include: thermal
expansion coefficient, magnetic susceptibility, saturation
magnetisation.
Motivation
The search for materials exhibiting higher performance is
nowadays an important topic in research and development of new
metallic materials. Indeed the demand for better use of resources,
less energy consumption and more efficient production of materials
can only be fulfilled when less material is used in particular
applications. This project aims to improve the mechanical
performance of metallic materials by reducing the grain size to a
sub-micron level.
Strength is an important aspect of the mechanical performance. The
strengthening of metals can be obtained in several ways, for
example by solid solution hardening, work hardening, precipitation
hardening or grain refinement. Grain refinement is technologically
attractive because it generally does not adversely affect
ductility and toughness, contrary to most other strengthening
methods. For this mechanism, the Hall-Petch equation relates the
yield stress sigmay to the average grain size d:
sigmay =
sigma0 + k / SQRT(d)
Metals obey the Hall-Petch equation over several orders of
magnitude in grain size, which means that their strength increases
when their grain size is reduced. Recently, it has been found that
the equation no longer holds for grain sizes below a certain
critical grain size, where the strength can even decrease with
decreasing grain size. The critical grain size is material
dependent and is estimated to be about 50 nm for Cu. Since this
project aims for an optimal combination of strength and ductility,
grain sizes of around 100 nm are aimed for.
The materials that will be worked with are low and high carbon
steels as well as low and high alloyed steels, because of their
importance in high strength applications.
Processing routes
There are many routes to produce UFS materials. Out of these,
the two following are the most relevant for the project: the
powder metallurgical route and the technique of Severe Plastic
Deformation (SPD).
Powder metallurgical route
Two approaches can be followed:
- One can start with nanosized powder and try to consolidate
it following conventional powder metallurgical consolidation
techniques, avoiding excessive growth of the starting powder
(grain) size. This mostly implies that the processing should be
done at relatively low temperature. Also the handling of extreme
fine nanopowders leads to complex manipulation techniques.
- Another route is the formation of nanostructures by mechanical
attrition (sometimes generally called Mechanical Alloying
processes). During the process of (dry) mechanical attrition, a
reduction of the average grain size is observed from typically 50
µm (cf. powder particle size) to sizes ranging from a few to 50
nm. This “internal” refining process results from the creation of
high angle grain boundaries within the particles during the
milling process. The resulting microstructure is very similar to
the structure obtained starting from nanocrystalline powder (see
first method).
Powder metallurgical techniques for the production of UFS
metals at the present are mainly applied in laboratory conditions.
Severe Plastic Deformation (SPD)
Severe plastic deformation can be obtained by a special set-up
of thermomechanical processing, such as rolling or wire-drawing or
extrusion at high reduction rates or with a combination of
different steps during cold deformation.
Another rather new technique of SPD occurs by Equal Channel
Angular Processing (ECAP). It is shown that simple shear can be
considered as a near ideal deformation method for structure and
texture formation in metalworking. ECAP is a special process
employed to realise this method. It has the advantage of producing
extra-large, strictly uniform and unidirectional deformations
under relatively low pressure and load. And this without
macroscopic shape change.
There are many potential and promising applications of this
deformation method in materials synthesis and processing. It
should be pointed out that breakdown of cast ingots, consolidation
and bonding of powders and grain refinement by ECAP with
subsequent recrystallisation treatments can be used to produce
submicron-grained structures in various materials.
The general principle of the method is shown in Fig 1. The tool is
a block with two intersecting channels of identical
cross-sections. A well-lubricated billet of almost the same
cross-section is placed into one of the channels, and a punch then
extrudes it into the second channel. Under these conditions the
billet will move inside the channels as a rigid body, and
deformation is achieved by simple shear in a thin layer at the
crossing plane of the channels. In this way the complete billet
(except small end regions) is deformed in the same uniform manner.
After the extrusion stroke, the punch retreats and the deformed
billet is withdrawn from the second channel.
The process can easily be repeated a number of times in the same
tool. For this case, the total strain intensity is N (amount of
passes) times the single shear deformation. The process can also
be extended from Extrusion to Drawing. In that case it is
important to know the limit die geometry between angular shear and
angular bending.
Fig. 1: Equal channel angular processing (a), element
transformation under simple shear (b) and experimental distortion
of co-ordinate grid (c).
Source: V.M. Segal, Materials Science and Engineering A17,
1995, 157-164.
Objectives
Scientific objectives
The main scientific objective is to develop metallic alloys
with an ultra fine microstructure (typical grain size is below 1
µm), which possess unique mechanical properties (e.g. extreme
strength with acceptable toughness).
Generally, the ductility is decreased when the yield stress is
increased by mechanical processing. This process should allow to
produce material that has such fine grain structure (grain size
below 1 µm) that the yield stress is significantly increased,
according to the Hall-Petch relation, without loss of ductility.
Therefore, the more specific objectives are the following:
- Development of processing routes to produce UFS metals,
starting from powder material or from bulk alloys
- Better understanding of the processing actions and the
development of the ultra fine structure
- Better understanding of the nature of the ultra fine structure
and its influence on the mechanical properties (e.g. validity of
the Hall-Petch relation)
Technological objectives
The main technological objective is to build up the processing
equipment to produce UFS metallic materials. This processing
equipment for the bulk route will consist of a special die
construction, allowing pure shearing of the material. Part of the
technology will also consist of determining the optimal
lubrication conditions.
The next step is the assessment of the upscaling potential of the
developed processes; both the powder and the bulk route are
conceived to be applicable on larger scale. Finally, the
integration within current metal transformation technologies will
be evaluated.
Research challenges
- The project aims, in the end, for industrial application of
the developed techniques. That is why it aims to develop ECAD (D =
drawing) rather than ECAE (E = extrusion). ECAE is an easier and
better explored method to obtain severe plastic deformation, but
it is a batch process, whereas drawing can be used in a continuous
process. Moreover ECAP has been applied for pure metals but no
experience is described in literature about multiphase materials,
especially low and high carbon steels.
- Before the production of nano-crystalline materials, it was
known that the strength of polycrystalline materials follows the
Hall-Petch law (HP) given by the empirical formula:
sigmay =
sigma0 + k / SQRT(d)
where k and sigma0 are material constants, d is the average grain
dimension and sigmay the yield stress. This law is no longer valid
for extreme fine grain sizes. Other strengthening mechanisms
become dominant. Those mechanisms are associated with the
stability of the dislocation loops. It is thus important to
determine the sigmay -grain size (d)
relation for the different
materials and to determine the stability of the relation depending
on the initial processing. The problem that will appear here will
be tackled by modelling.
- The strengthening mechanisms are only effective at low and
moderate temperatures, but they become inoperative at high
temperatures due to the onset of thermally activated processes,
such as creep, recovery, recrystallisation and grain coarsening.
These thermally activated processes (diffusion, grain growth) have
the effect of degrading the strength quite rapidly. It must thus
be explored how stable the improved mechanical properties are as
function of time with temperature as the main parameter.
- Upscaling of the explored technology to industrial production level. This is considered as the last but probably most challenging step in the production of UFS metals for industrial applications.
This project will mainly concentrate on point 1 and 2.
Partners
- Department of Materials Science and Engineering (MTM)
Faculty of Applied Sciences, University of Leuven
People involved: Etienne AERNOUDT (co-ordinator), Joke DE
MESSEMAEKER, Ludo FROYEN, Christophe MOONS, Jan VAN
HUMBEECK
- Laboratory for Iron and Steelmaking (LISm)
Faculty of Applied Sciences, University of Gent
People involved: Katrien DE WIT, Yvan HOUBAERT, Leo KESTENS
User group
- OCAS, Sidmar N.V.
People involved: Roger HUBERT, Dirk VANDERSCHUEREN
- Steel Cord, Bekaert N.V.
People involved: Frank DEBRUYNE, Ignace LEFEVER
- Bodycote - IMT N.V.
People involved: Louis BUEKENHOUT
Status
This page will report on the current status of the project.
Contact us
For further information or comments on the project or web site,
please contact
Joke De Messemaeker
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