Current Research: Micromechanics of
deformation and fracture of Mg and Al (gravity and HPDC) casting alloys;
Pseudoelastic behaviour of Mg-Al alloys; Hardness behaviour of Mg alloys;
Strengthening effects in single crystals of Mg-Al and Mg-Zn; Hall-Petch
effects in Mg-Zn and Mg-Al alloys; Creep of HPDC Mg alloys. Short Range
Order and precipitation in binary Mg alloys. Microstructure of Al-Cu-Si-Mg
casting alloys. Environmental issues related to light alloys in
transportation. Methods in Materials Selection.
Some examples of what I do:
Damage Mechanisms
Some of
the oldest unresolved questions in cast Al-Si-Mg alloys refer to the
respective roles played by the dendrite cell size and the size/shape of the
Si particles on the material's ductility. Metallography provides a clue:
Dendrites divide the material into boxes,
as grain boundaries do. Following MF Ashby's theory of plastically
non-homogeneous materials, it is easy to show that small dendrites should
produce more strain hardening, enhancing particle cracking, hence lowering
the ductility. On the other hand, Si particles tend to be longer when the
dendrites are large. Long Si particles trap more dislocations, causing more
strain hardening and, because they are easier to crack, less ductility.
These two opposing tendencies in the strain hardening and damage rate
result in a maximum in the ductility at intermediate cell sizes. See the
predicted (red lines and circles) and measured (blue circles) ductility in
the figure below.
The tensile ductility
of alloy A356 as a function of the dendrite cell size. (Cáceres, C.H., Griffiths, J.R., "Damage by the Cracking of Si
Particles in an Al-7Si-0.4Mg Casting Alloy", Acta
Materialia, 44, 25-33, 1996)
The
calculated lines and points are from a model developed with John Griffiths (CSIRO, Pinjarra
Hills). This work combines Weibull statistics
with Brown and Stobbs' theory of dispersion
strengthening, and merited reviews at the invited lectures to the 1995 and
2000 Meetings of the American Foundry Society by Prof John Berry from Mississippi State University.
The bump in the ductility at intermediate dendrite sizes is present in
both, alloy A356 and A357
Quality Index
People in the
casting industry often use Quality Index charts to compare alloys and
processes. These clever charts, invented by Michel Drouzy,
Sylvain Jacob and Michel Richard in the 70's, tell you whether the material’s
properties change for the better (or otherwise) when you modify the process
or the chemistry.
In 1997 I
developed an analytical method for creating the charts and gave physical
meaning to the QI by referring the alloy's quality to the necking onset
strain (see Chart). This mathematical trick landed me the Best Paper Award
by the International Journal of Cast Metals Research in 1998 and merited a
dedicated review at the 2000 AFS invited lecture by Sylvain Jacob.
The
original QI charts were intended only for alloy A356, but the analytical
method extends them to any 
material. In joint efforts with a number of
people, the charts have been applied to Al-Cu
and Al-Si-Cu alloys.
A Quality Index chart.
Blue
lines: empirical lines (as determined by Drouzy,
Jacob and Richard., 1980); Red lines: calculated.
Twinning
in Mg
Recent work (with
Andrew Blake) and Pavel Lukac from Charles University,
was on the hardening caused by {10-12} twinning in Mg. Using the
Kocks-Mecking-Estrin phenomenological analysis,
we showed that twinning hardly introduces any hardening. The reason? Due to
their small intrinsic shear strain (~0.12), {10-12} twins in Mg are way too
big, unlike in other metals where because of their large shear strain they
are bound to remain small. Thus, any grain partition effects in Mg are
quickly overrun by the athermal forest hardening that develops once the
profuse twinning stage is over.
This is shown by the figure: where the lines represent the
dislocation hardening. Twinning accounts for the hardening inside the pink area
near the origin. [Cáceres, C.H., P. Lukác
and A. Blake, "Strain Hardening due to Twinning in Pure Magnesium
", Philos. Mag. A, 2008, 88, 991-1003] DOI: http://dx.doi.org/10.1080/14786430701881211
3D
FIB reconstruction of the intermetallics in HPDC Mg-Al alloys
These beautiful
images are the result of painstaking work by AV Nagasekhar, a Postdoctoral
fellow working with me in 2008-2010 (Currently with Carpenter Technology, US). The
percolating intermetallics introduce a strong elastic constraint on the Mg
matrix near the casting surface, accounting for a large share of the so
called skin effect. MPEG files
showing the full 3D structures are available at: http://dx.doi.org/10.1016/j.matchar.2010.06.007
Earlier Work
During
the 1980's I worked on serrated flow in aluminium alloys and, jointly with
David Wilkinson from McMaster University, on superplastic materials, creep
fracture of alumina and processing of ceramic powders. The work on
serrated flow and superplasticity merited reviews by Neuhäuser
and Schwink, and Mukherjee, respectively, in the
series Materials Science and Technology (Editors: RW Cahn, P Haasen and EJ Kramer; VCH, New York, 1991), Vol. 6:
Plastic deformation of materials, pp. 221 and 416-433.