NCAL: Quantifying Crystallographic Texture and Phase Fraction

Summary

Most engineering metals are composed of tiny individual granules (grains) sharing a common crystal structure but having different crystallographic orientations relative to neighboring grains.  The size, shape, crystal structure (phase), and the distribution of orientations often change as the material is heated and/or deformed during manufacturing processes. Accurate quantification of the crystallographic texture (or preferred orientation), phase fraction, and their evolution with deformation are needed to predict how metals behave as they are manufactured and how the resulting products perform in service.

Description

Predicting the deformation and force response of a polycrystalline material with the level of accuracy needed by today’s manufacturers requires knowledge of the aggregate deformation and force response of all the grains in the material.  Polycrystalline materials will respond differently depending on the types and amounts of phases present, on the deformation mechanisms each phase permits, and how neighboring grains interact with one another.  In addition, the phases and/or deformation mechanisms present in the structure may change as a function of deformation or temperature.  The polycrystal orientations are referred to as the crystallographic texture of a material.  Crystallographic texture can range from uniform - a random collection of orientations, to preferred - where many grains have a similar orientation, to single crystals where all parts of the material share the same orientation. 

In many models of structures,  the material properties are assumed to be constant over all length scales (continuum), not vary by sample orientation (isotropic), and have a phase fraction that does not vary as a function of deformation or load.  To improve model fidelity, the variation of material properties by sample orientation (anisotropy) needs to be included.  Historically, anisotropy has been incorporated with empirical approximations.  However, to obtain even higher fidelity models, multi-scale details beyond the continuum approximation are needed, such as dependence on grain shape and orientation (anisotropy) and the phase fraction response to deformation or load.  Accurate measurements of the crystal orientations, phases, and how they are affected by deformation are key to validation of the different models.

This project focuses on quantifying the evolution of phase fraction and crystallographic texture as materials are deformed in ways that mimic important manufacturing processes.  The NCAL: Multiaxial Material Performance and NCAL: Plasticity Modeling projects also leverage these measurements.  We are currently focused on advanced high-strength steels which require specific distributions of phases to achieve demanding performance targets.  However, our techniques and knowledge can be applied to a wide range of other materials (e.g. Al, Ti, Mg). 

 

Texture quantification and uncertainty

A variety of measurement techniques and analysis methods are currently used to quantify crystallographic texture.  However, the lack of documentary standards and reference materials makes it difficult to quantitatively compare texture measurements obtained by different methods.  Uncertainty metrics are also typically unquantified, with error sources arising from the physics of the measurement method, such as the number of grains sampled and assumptions used to interpret the results.  As such, current practice is limited to making qualitative comparisons between results from different methods.  

Active Work

• Cross comparison between measurement techniques such as neutron diffraction (NCNR), XRD, EBSD
• Advanced techniques for local orientation changes (ESRF), and development of standards or good practice guides (IUCr Diffraction Microstructure Imaging)

Phase quantification, bias errors, and uncertainty

In parallel with the crystallographic texture measurement efforts, we are addressing phase fraction measurements that face similar challenges in the lack of documentary standards and reference materials.  Diffraction techniques are commonly used to quantify phase fractions.  If there is a significant degree of preferred orientation in the material being measured, phase fraction measurements may exhibit bias as preferred orientation and phase fraction both affect the diffraction measurement.  In the phase fraction community, a good practice is to mill or grind the specimen into a powder before measuring to reduce crystallographic texture.  However, this process involves changes in temperature and deformation, which may change the phase content and make the resulting material unrepresentative of the actual materials of interest.  Some advanced high-strength steels, such as transformation induced plasticity (TRIP) steels, are known to be particularly susceptible to phase transformation.  New methods that allow the materials to be measured accurately in the as received state are needed.

Active Work

• Development of methods for phase quantification for application to textured materials, such as sampling schemes that mitigate the effect of texture or methods to correct for texture
• Explicit assessment of the bias errors caused by crystallographic texture
• Investigation of alternatives to diffraction methods for phase quantification (i.e. magnetic, mechanical, etc.)

Austenite Calculator

Assessing uncertainties in phase quantification is a multidisciplinary effort, requiring knowledge of the material being quantified, nuances of X-ray diffraction, and statistics.  To aid practitioners, we are developing an “Austenite Calculator” that includes multiple sources of uncertainty and displays the phase quantification measurement result with guidance on the largest sources of uncertainty affecting the analysis.  

Active Work

• Development of Austenite Calculator (see https://github.com/usnistgov/AusteniteCalculator)
• Alpha release planned summer of 2025

Major Accomplishments

• Development and evaluation of sampling schemes to mitigate texture effects in phase fraction measurements
• Development and deployment of an Austenite Calculator web application
• Creation of a method to express uncertainty in Orientation Distribution Functions