INFLUENCE OF SAMPLE THICKNESS ON ISOTHERMAL CRYSTALLIZATION KINETICS OF POLYMERS IN A CONFINED VOLUME

Isothermal crystallization process of polymers in a confined volume was simulated in the case of instantaneous nucleation by use of the Monte Carlo method.The influence of sample thickness on some kinetic parameters of crystallization was quantitatively evaluated.It was found that there was a critical thickness value.Influence of thickness on the crystallization behavior was only found for samples of thickness near and less than the critical value.For thick samples the Avrami plot showed straight lines with a turning point at the late stage of crystallization due to the secondary crystallization.When the thickness was near or less than the critical value a primary turning point appeared in the Avrami plot at the very beginning of the crystallization process.A model was proposed to explain the mechanism of this phenomenon.According to this model the critical thickness value is related to the nucleation density or the average distance between adjacent nuclei,and the primary turning point is an indication of a transformation of crystal growth geometry from a three-dimensional mode to a two-dimensional one.Analysis of experimental results of PEO isothermally crystallized at 53.5℃ was consistent with the proposed model.

Isothermal Crystallization Kinetics of Biodegradable Poly(butylene succinate-co-terephthalate)

The poly （butylene sucinate-co-terphthalate）（PBST） copolyesters with differnet monocs of the PBST has been investigated by differential scanning calorimeter（DSC）.According to Avrami equation,the exponent n for all the copolyesters is independent of the crystallization temperature and content of BT units,and is close to 3,which probably corresponds to the heterogeneous nucleation and a 3-dimensional spherulitic growth.Polarized light micrographs confimed the result.

Investigation of Hot Deformation Behavior of SNCM8 Alloy Steel

Plastic flow behavior of the SNCM8 steel was investigated by performing hot compression tests within the temperature range of 850˚C to 1200˚C and strain rates of 0.01 s−1 to 10 s−1. Constitutive modeling based on dynamic recrystallization was established, in which Cingara equation was applied to represent work hardening up to peak stress and Avrami equation to describe dynamic softening beyond peak stress up to steady state. It was found that stress-strain responses predicted by the combined model fairly agreed with experimentally resulted curves for the particular conditions. The correlation coefficient (R) of 0.9485 and average absolute relative error (AARE) of 2.3614% was calculated for the modeled flow curves.

Microstructural evolution of a superaustenitic stainless steel during a two-step deformation process

Single-and two-step hot compression experiments were carried out on 16Cr25Ni6Mo superaustenitic stainless steel in the temperature range from 950 to 1150°C and at a strain rate of 0.1 s^（-1）. In the two-step tests, the first pass was interrupted at a strain of 0.2; after an interpass time of 5, 20, 40, 60, or 80 s, the test was resumed. The progress of dynamic recrystallization at the interruption strain was less than 10%. The static softening in the interpass period increased with increasing deformation temperature and increasing interpass time. The static recrystallization was found to be responsible for fast static softening in the temperature range from 950 to 1050°C. However, the gentle static softening at 1100 and 1150°C was attributed to the combination of static and metadynamic recrystallizations. The correlation between calculated fractional softening and microstructural observations showed that approximately 30% of interpass softening could be attributed to the static recovery. The microstructural observations illustrated the formation of fine recrystallized grains at the grain boundaries at longer interpass time. The Avrami kinetics equation was used to establish a relationship between the fractional softening and the interpass period. The activation energy for static softening was determined as 276 kJ/mol.

Continuous Cooling Transformation Behavior and Kinetic Models of Transformations for an Ultra-Low Carbon Bainitic Steel

The aim was to investigate transformation behavior and transformation kinetics of an ultra-low carbon bai- nitic steel during continuous cooling. Continuous cooling transformation （CCT） curves of tested steel were measured by thermal dilatometer and metallographic structures at room temperature were observed by optical microscope. Then transformation kinetic equation of austenite to ferrite as well as austenite to bainite was established by analyzing the relationship of lnln]-l/（1--f）] and lnt in the kinetic equation on the basis of processed experimental data. Finally, the measured and calculated kinetic behaviors of the steel during continuous cooling were compared and growth pat- terns of transformed ferrite and bainite were analyzed. Results showed that calculated result was in reasonable agree- ment with the experimental data. It could be concluded that the growth modes of transformed ferrite and bainite were mainly one dimension as the Avrami exponents were between 1 and 2.

Experimental and Data Fitting Guidelines for the Determination of Polymer Crystallization Kinetics

The crystallization kinetics of semicrystalline polymers is often studied with isothermal experiments and analyzed by fitting the data with analytical expressions of the Avrami and Lauritzen and Hoffman(LH)theories.To correctly carry out the analysis,precautions in both experiments and data fitting should be taken.Here,we systematically discussed the factors that influence the validity of the crystallization kinetics study.The basic concepts and fundamentals of the Avrami and LH theories were introduced at first.Then,experimental protocols were discussed in detail.To clarify the impact of various experimental parameters,selected common polymers,i.e.,polypropylene and polylactide,were studied using various experimental techniques(i.e.,differential scanning calorimetry and polarized light optical microscopy).Common mistakes were simulated under conditions when non-ideal experimental parameters were applied.Furthermore,from a practical point of view,we show how to fit the experimental data to the Avrami and the LH theories,using an Origin■App developed by us.