The Effect of Spatial Heterogeneities on Transformation kinetics in Amorphous Alloys
Abstract
Transformation in materials under high supersaturation or deep undercooling is often used to generate novel microstructures based upon stable or metastable phases with nanoscale sizes and ultrahigh particle densities that exhibit superior structural performance. Increasingly sophisticated diagnostic techniques and analysis models that enable determination of local structure have established local structural heterogeneities with sizes from a few to 10’s of atoms as characteristic features. Current kinetics models that are based on a uniform structure cannot represent the kinetic influence of structural heterogeneities and will not reveal new transformation behavior. Amorphous alloys offer an effective system for the separate study of the influence of local heterogeneities on the nucleation and growth mechanisms without the influence of excess vacancies or dislocations that confound the kinetics in crystalline materials. Thus, amorphous alloys provide a good model system for the systematic study of the influence spatial heterogeneities on the crystallization reaction. We propose to address the fundamentals of the influence of local heterogeneities on transformation behavior by a systematic study of their influence on the primary crystallization reaction in amorphous alloys. Our prior work on amorphous Al alloys has established that the primary crystallization reaction yields ultrahigh densities from 1021 to 1023 m-3 of Al nanocrystals (10-20 nm in diameter) in an amorphous matrix. Structural analysis by fluctuation electron microscopy and kinetics measurements by differential scanning calorimetry, DSC, nanocrystal density determination and high rate nanocalorimetry demonstrate that the naonocrystallization reaction is controlled by a heterogeneous nucleation catalyzed by medium range order (MRO) heterogeneities and is strongly affected by transient (i.e. non-steady state) kinetics. In our previous program we have developed a new nucleation kinetics model based upon MRO catalyzed Al nanocrystal formation. We have also established that minor solute substitution by Cu has a dramatic effect on the crystallization behavior in terms of the nanocrystal densities and transient behavior that is related to local spatial heterogeneities Similarly, the initiation of shear bands is of critical importance since they control the mechanical behavior (i.e. ductile vs. brittle) in amorphous alloys. The proposed effort will focus on the testing of the new nucleation model to describe the effects of heterogeneities induced by solute additions on primary crystallization. This will be complemented by detailed structural analysis to establish the effect of solute substitution on the local MRO heterogeneities. The confluence of kinetics analysis and local structure determination provides an effective probe of the influence of spatial heterogeneities on transformation behavior. The primary crystallization in Fe-based glasses such as Fe-B-Si and Fe-B-Nb is of critical importance in optimizing the magnetic properties. The application of our nucleation model to the Fe-based glasses will serve as a test of the generality of the model. In order to advance the study of shear band nucleation we will utilize iso-loading experiments to determine load, P, time, T, transformation diagrams (PTT) describing the nucleation kinetics and the effect of relaxation conditions. The outcome of the systematic kinetics measurements, structural determinations and model analysis will form the basis for a new paradigm for transformation kinetics and microstructure evolution in materials with spatial heterogeneities. The analysis will also provide useful guidance for the control of spatial heterogeneities to design new microstructures
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 31, 2020
- Source ID
- N000142012704
Entities
People
- John H. Perepezko
Organizations
- Office of Naval Research
- United States Navy
- University of Wisconsin System