Phase field modeling of Spinodal decomposition in TiAlN

Abstract: TiAlN  thin  films  are  used  commercially  in  the  cutting  tool  industry  as  wear protection  of  the  inserts.  During  cutting,  the  inserts  are  subjected  to  high temperatures (~ 900  ° C and sometimes higher). The  objective of this work is to simulate the material behavior at such high temperatures. TiAlN has been studied experimentally at least for two decades, but no microstructure simulations have so far been performed. In this thesis two models are presented, one based on regular solution and one that takes into account clustering effects on the thermodynamic data. Both  models  include  anisotropic  elasticity  and  lattice  parameters  deviation from  Vegard’s  law.  The  input  parameters  used  in  the  simulations  are ab  initio calculations and experimental data.Methods for extracting diffusivities and activation energies as well as Young’s modulus  from  phase  field  results  are  presented.  Specifically,  strains,  von  Mises stresses,  energies,  and  microstructure  evolution  have  been  studied  during  the spinodal  decomposition of  TiAlN. It  has  been  found  that  strains  and  stresses  are generated during the decomposition i.e. von Mises stresses ranging between 5 and 7.5  GPa  are  typically  seen.  The  stresses  give  rise  to  a  strongly  composition dependent  elastic  energy  that  together  with  the  composition  dependent  gradient energy   determine   the   decomposed   microstructure.   Hence,   the   evolving microstructure depends strongly on the global composition. Morphologies ranging from isotropic, round domains to entangled outstretched domains can be achievedby  changing  the  Al  content.  Moreover,  the  compositional  wavelength  of  the evolved  domains  during  decomposition  is  also  composition  dependent  and  it decreases with  increasing  Al  content.  Comparing  the  compositional  wavelength evolution extracted from simulations and small angle X-ray scattering experiments show that the decomposition of TiAlN occurs in two stages; first an initial stage of constant  wavelength and  then  a  second  stage  with  an  increasing  wavelength are observed.  This  finding  is  characteristic  for  spinodal  decomposition  and  offers conclusive evidence that an ordering transformation occurs. The Young’s modulus evolution  for  Ti 0.33 Al 0.67 N  shows  an  increase  of  5%  to  ~398  GPa  during  the simulated decomposition.