Academic literature on the topic 'Spinodal Decomposition'

Spinodal decomposition is a phase separation mechanism within the miscibility gap. Its importance in case of Fe-Cr system, the basis of the whole stainless steel family, stems from a phenomenon known as the “475oC embrittlement” which results in a ruin of mechanical properties of ferritic, martensitic and duplex stainless steels. This work is aimed at a better understanding of the phase separation process in   the Fe-Cr system. Alloys of 10 to 55 wt.% Cr , each five percent, were homogenized to achieve fully ferritic microstructure and then isothermally aged at 400, 500 and 600oC for different periods of time ranging from 30min to 1500 hours. Hardness of both homogenized and aged samples were measured by the Vickers micro-hardness method and then selected samples were studied by means of Transmission Electron Microscopy (TEM).  It was observed that hardness of homogenized samples increased monotonically with increasing Cr content up to 55 wt.% which can be attributed to solution hardening as well as higher hardness of pure chromium compared to pure iron.  At 400oC no significant change in hardness was detected for aging up to 1500h, therefore we believe that phase separation effects at 400oC are very small up to this time. Sluggish kinetics is imputed to lower diffusion rate at lower temperatures. At 500oC even after 10h a noticeable change in hardness, for alloys containing 25 wt.% Cr and higher, was observed which indicates occurrence of phase separation. The alloy with 10 wt.% Cr did not show change in hardness up to 200h which suggests that this composition falls outside the miscibility gap at 500oC. For compositions of 15 and 20 wt.% Cr only a small increase in hardness was detected even after 200h of aging at 500oC, which could be due to the small amounts of α´ formed. However, it means that alloys of 15 wt.% Cr and higher are suffering phase separation. For compositions inside the miscibility gap, hardening effect is a result of phase separation either by nucleation and growth or spinodal decomposition. To distinguish between these two mechanisms, TEM studies were performed and we found evidence that at 500oC the Fe-25 wt.% Cr sample decomposes by nucleation  and growth  while that of 35 wt.% Cr  shows characteristics of the spinodal mechanism. For compositions inside the miscibility gap, with increasing Cr content up to 40% the change in hardness generally increased and for 45% and higher it always decreased. This suggests that the composition range corresponding to the spinodal region at 500oC is biased towards the Fe-rich side of the phase diagram. At 600oC only samples of 25, 30 and 35 wt.% Cr were studied because according to the previous studies, the spinodal boundary is most probably located in this composition range. However, no change in hardness was observed even up to 24h. We believe that this means the miscibility line lies below 600oC for alloys containing 35 wt.% Cr and lower. Further investigations are needed to confirm and explain this result.

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.

Spinodal decomposition is a phenomenon which hinders the usage of ferrite containing steels under certain conditions. With a better understanding and knowledge of spinodal decomposition new ferritic and duplex stainless steels could be designed. The project is divided into two parts, an experimental part and a theoretical part. The aim of the theoretical part was to compare simulated values of the amplitude and wavelength of spinodal decomposition, obtained from solving the Cahn-Hilliard equation with other experimental values. The experimental part is performed as to determine the chemical diffusion coefficient and the mobility of Fe and Cr in the binary Fe-Cr system for lower temperatures using the Boltzmann-Matano method from chemical profiles of different samples. Another purpose with the project is to determine if the methods and the approach which are taken is viable and useful for calculating diffusion coefficients and if it can be used in studies to come. The simulations were in good agreement with other data at higher time intervals, but deviates at lower times. The calculated interdiffusion coefficients were in all cases lower than the expected values and as such the conclusion that is drawn is that the methods which are used could need further development.<br>Spinodalt sönderfall är ett fenomen som hindrar användningen av stål som innehåller ferrit under vissa förutsättningar. Med en större förståelse och mer kunskap om spinodalt sönderfall så kan nya ferritiska och duplexa rostfria stål utformas. Projektet är uppdelat i två delar, en experimentell del och en teoretisk del. Syftet med den teoretiska delen var att jämföra simulerade värden av amplituden och våglängden för spinodalt sönderfall, vilka erhölls från att lösa Cahn-Hilliard ekvationen med andra experimentella värden. Den experimentella delen utförs för att bestämma kemiska diffusionskoefficienter för Fe och Cr i det binära Fe-Cr system för lägre temperaturer med hjälp av Boltzmann-Matano metod från kemiska profiler av olika prover. Ett annat syfte med projektet är att ta reda på om de metoder och tillvägagångssätt som tas är livskraftiga och användbara för beräkning av diffusionskoefficienter och om den kan användas i studier framöver. Simuleringarna var i god överensstämmelse med annan experimentel data vid högre tidsintervall, men avviker vid lägre tider. De beräknade Interdiffusion koefficienterna var i samtliga fall lägre än de förväntade värdena och som sådan, dras slutsatsen att de metoder som används kan förbättras.

Time resolved small angle x-ray scattering (SAXS) using synchrotron radiation was applied to the study of the kinetics of spinodal decomposition (SD) in an AlZn binary alloy at critical composition quenched into the immiscible region. These millisecond time scale measurements, performed at the National Synchrotron Light Source (Brookhaven National Labs., N.Y.), constitute the first direct experimental verification in a binary alloy of the theory proposed by Langer, Bar-on and Miller in 1975 for SD. A scheme based on the composition distribution functional is proposed to account for the decomposition taking place during the quench. The interatomic mobility, a free energy gradient coefficient and two coefficients that suffice to determine a coarse-grained (intensive) free energy have been obtained in the framework of this theory. The mobilities obtained compare well with tracer diffusion measurements reported in literature. A dependence of the coarse-grained free energy coefficients on the coarse-graining length is found and a procedure is proposed to uniquely choose the values of these coefficients based on the predicted integrated intensity from the equilibrium concentrations and on the measured integrated intensities.<br>Late-stage coarsening regimes were also investigated. In these regimes, growth exponents higher than the value 1/3 predicted by the Lifshitz-Slyozov-Wagner theory are obtained. These higher values, comprised between 0.40 and 0.45 are consistent with predictions that alloys in which elastic effects are important can present a transition regime from a $t sp{1/3}$ growth law to a $t sp{1/2}$ law. The structure factors do not quite scale. They also present a shoulder at high wavevectors, a feature not reported before in metallic alloys.