ELFT HÍRADÓ

[Zsolt Kovacs <kovacszs AT caesar.elte.hu>]

ELTE Fizikai Intézet, Anyagfizikai Tanszék TANSZÉKI SZEMINÁRIUM
2019. március 26., kedd, 8:30-kor ELTE, Pázmány Péter sétány 1/A alatti 
épületében, 4.52 (Sas Elemér) teremben Victor Berdichevsky (Wayne State 
University, USA) Beyond classical thermodynamics: dislocation-mediated 
plasticity At the dawn of dislocation theory there was a hope that 
plasticity can be understood in terms of dislocation interactions 
similarly to what is done by statistical mechanics of colliding 
particles in gas dynamics. There was a fascinating challenge to learn 
how to deal with interactions of lines instead of points, but that 
seemed to be a pure technical issue. It turned out, however, that the 
major difficulty lies in a different place. One of the aims of this talk 
is to explain what the difficulty is and to make an attempt to treat it. 
It will be argued that the core problem is the drastic difference in 
geometries of dislocation phase space and phase space of classical 
statistical mechanics. Due to that thermodynamics of plasticity is quite 
different from classical thermodynamics of solids. Briefly, the major 
differences are as follows. First of all, several additional 
thermodynamic parameters come into play - dislocation polarization 
tensor, stress resistance tensor, entropy and temperature of 
microstructure. In a sense, polarization is similar to density in gas 
dynamics while stress resistance bears some properties of pressure. In 
contrast to gases where molecules keep moving when mass density is 
constant, in crystals the dislocation ensemble becomes frozen as soon as 
polarization tensor is fixed. Polarization tensor is the collective 
kinematic characteristic of dislocation ensembles on which external 
stresses work. Fixing polarization tensor makes the work zero. In order 
to deform a crystal, the applied stress must exceed the resistance 
stress. The resistance stress tensor and polarization tensor are linked 
to entropy and temperature of microstructure by some constitutive 
equations. Entropy and temperature of microstructure have simple 
physical meaning: temperature of microstructure is associated with 
average energy drop in slip avalanches, while rate of microstructure 
entropy is linked to the number of slip avalanches per unit time. An 
unusual peculiarity of dislocation-mediated plasticity is the decay of 
microstructure entropy in monotonic loading. Minden érdeklődőt szívesen 
látunk. A szemeszter szemináriumainak programja megtalálható a 
http://metal.elte.hu honlapon a Szemináriumok címszó alatt Kovács Zsolt