ELFT HÍRADÓ

[Gubicza Jeno <gubicza AT ludens.elte.hu>]

                                        MEGHIVO

                        az ELTE TTK Altalanos Fizika Tanszek
                                    szeminariumara

A szokasostol eltero idopontban!!!

2001. december 14. pentek, 10.00.:

        Prof. Pierre Guyot (Institut National Polytechnique de Grenoble, 
France):

        Bulk Metallic Glasses: Thermal stability and Mechanical Properties.

Abstract:

Zr65-x-yTixAlyCu22Ni13 alloys solidify, when slowly cooled (rate from 10 to
200°K/s) in so-called bulk metallic glasses (BMG). 
The thermal stability (glass transition, crystallization) of such BMG, in
conjunction with their mechanical properties, are studied in a Balaton
research programm between Eotvos and INP-Grenoble, using various
complementary experimental techniques: DSC, X-Ray-synchrotron radiation
ESRF and neutron diffraction at ILL, TEM and HREM, compression and fracture
tests.
Some results are presented:
-1) the crystallization of Zr57Ti5Cu20 Al10Ni8
The various transformations occurring above the glassy transition
temperature during the glass heating, are first identified by DSC scans
coupled with diffraction and TEM observations. Unmixing occurring in the
liquid or the amorphous state, before the onset of crystallisation,
sometime invoked as a criterium of amorphisabiliy, is first ruled out in
the present alloy. Above Tg a first step of crystallization (425°C) appears
to be of the primary type, and corresponds to the formation of nanometric
crystals dispersed in a residual amorphous matrix. The metastability of
this state is discussed in terms of a thermodynamical model proposed by
Desré. At higher temperature the crystallization of Zr2Ni is completed.
-2) Mechanical behaviour
Different tests have been performed and discussed:
-hardness measurements at room temperature
-compression tests at constant strain rate at various temperatures between
Tg and the crystallization, coupled with microstructural observations
before and after deformation. Three regimes are observed: the elastic
domain followed, beyond a stress peak, by a softening stage (up to strain
of 0.3 at a strain rate of 10-4s-1), which is itself followed by a
pronounced hardening stage when the crystallization invades the specimen.
- the fracture surfaces at room temperature, showing shear bands and vein
patterns.
Some conclusions are drawn.



2001. december 14. pentek, 11.00.:


                        Myriam Nicolas (LTPCM-ENSEEG, France)
                                
Welding metallurgy of an Al-Zn-Mg alloy: influence of post-weld
heat-treatments

Abstract:

Welding of precipitation hardening aluminium alloys is increasingly used in
the transportation industry. The optimisation of the welding process
requires a good understanding of the microstructures generated by the rapid
temperature rise in the heat-affected zone. Welding problems in Al-Zn-Mg
alloys arise mainly because of the poor temperature stability of the
precipitates involving phenomena such as dissolution, coarsening and
transformation from metastable particles to more stable ones. The aim of
our study is to determine quantitatively the modification of the
microstructural features involved by arc-welding (MIG) of a commercial
Al-Zn-Mg alloy, especially in the heat-affected zone. 
First, a quantitative measurement (in terms of average size and volume
fraction) of hardening precipitation state just after welding was obtained
in each point of the heat-affected zone by Small-Angle X-ray Scattering
(SAXS) measurements under high flux carried out at the European Synchrotron
Radiation Facility (ESRF). Three initial states have been studied : T4
(containing mainly GP zones), T6 (containing mainly metastable eta'
precipitates) and T7 (containing mainly stable eta precipitates). Basically
the material just after welding is characterised by an unaffected zone far
from the weld nugget and a completely dissolved one in the vicinity of the
fusion zone. These two parts are separated by a transition zone where
dissolution and coarsening are in competition.
The capacity of the material to recover its initial properties through
post-weld heat-treatments was also investigated using microhardness and
SAXS. Both natural ageing (at room temperature) and T6 (at 150°C) and T7
(at 170°C) type heat treatments were performed on the three initial weld
structures. It was observed that, for the T6 and T7 initial states, the
transition zone properties are weaker due to coarsening of particles in
this zone, which involves a solute depletion. On the contrary, under
certain experimental conditions, a complete recovery can be achieved in the
case of the T4 base material.




Az eloadasok helyszine:         ELTE TTK Eszaki Tomb, Altalanos Fizika Tanszek
                                1117 Budapest, Pazmany Peter setany 1/A, 4. 
emeleti tanszeki
Tanacsterem.


        Minden erdeklodot szivesen latunk.

                                                
                                                        Gubicza Jeno