TEM/APT/SAXS/WAXS on Precipitation

1. Transmission Electron Microscopy (TEM) is a versatile and powerful technique for characterizing precipitation in metallic materials.

• Historically, TEM has played a pivotal role in advancing our understanding of precipitation phenomena and precipitation hardening. Over the past decades, it has also contributed significantly to the optimization of material performance through precipitation engineering. With the integration of modern aberration-corrected technologies, TEM has become an indispensable tool for precipitation-related research, offering comprehensive capabilities that span morphological (imaging), chemical (spectroscopy), structural (diffraction), three-dimensional (tomography), and environmental (in situ) analyses, reaching resolutions down to the Ångström scale.
• In our recent review paper (tao-zhou-et-al-2022.pdf), we summarize how advancements in TEM-based techniques, sample preparation methodologies, and data analysis algorithms have substantially enhanced the capabilities of (S)TEM for quantitative characterization of precipitation in metals. These developments continue to expand the frontiers of nanoscale materials research and enable more precise engineering of precipitation processes for improved mechanical performance.

Figure 1. A schematic illustrating the plethora of TEM capabilities for analyzing precipitate characteristics, of importance for various properties of metallic materials.

• Due to the inherent projection nature of (S)TEM imaging, accurately quantifying the volume fraction and number density of embedded precipitates within a metallic matrix remains a significant challenge. In this work (tao-zhou-et-al-2018.pdf), we demonstrate how combining thin-foil sample thickness measurements (obtained via EELS or CBED) with EDS mapping enables quantitative analysis of the volume fraction and number density of nanoscale precipitates. This integrated approach enhances the reliability of precipitation characterization.

Figure 2. Integrating EELS/CBED and STEM-EDS mapping for the quantification of volume fraction and number density of embedded precipitates. 

2. Atom probe tomography is a technique capable of 1) 3D atom-by-atom mapping with near atomic spatial resolution; 2) Equal detection efficiency to all elements and isotropes; and 3) A powerful combination of spatial and chemical sensitivity (down to ppm level). 

Figure 3. EIKOS-UV APT instrument equipped with both laser and voltage evaporation modes, located at Hultgren laboratory, MSE, KTH.

• In this work (tao-zhou-et-al-2021.pdf), A Cu precipitation-mediated austenitic transformation during ageing treatment of a precipitation hardening stainless steel is revealed through atom probe tomography, in situ synchrotron X-ray diffraction and computational thermodynamics and kinetics. The austenitic transformation is proposed to occur through the pathway: Cu precipitation at the martensite/retained austenite interfaces or at martensite lath boundaries → partitioning of austenite stabilizing elements towards interfaces of the Cu precipitates → reverted austenite formation.

Figure 4. APT 3D reconstruction of precipitates and elemental distribution in a 20 h aged specimen at 500 °C: (a) the overall view of the measured volume with elemental distribution and isoconcentration analysis of Cu, Ni, and Mn; (b) the distribution of Cu atoms for a selected part of the measured volume; (c) 10 at% Cu isoconcentration analysis, showing the absence of Cu precipitates in the Cu-rich bulky phase; and (d) 10 at% Cu + 7 at% Ni isoconcentration analysis, showing the lamellar structure.

• In this work (ze-sheng-et-al-2021.pdf), the chemical compostion, mean radius, volume fraction, and number density of Cu precipitates a a function of ageing time for a maraging stainless steel was quantified by APT. 

Figure 5. APT 3D reconstructions of aged specimens of Alloy A with 10 at.%Cu isoconcentration, red spheres indicating Mn atoms and green spheres indicating Ni atoms

3. In situ simultaneous SAXS/WAXS with furnace setup at Swedish Materials Science beamline P21.2, PETRA III

Figure 6. Schematic of the in situ simultaneous SAXS/WAXS measurement setup using a furnace sample environment at beamline P21.2, PETRA III.

Figure 7. The experimental setup for simultaneous SAXS and WAXS measurements using a furnace sample environment at the PETRA III Swedish Materials Science beamline P21.2.

• In this study (tao-zhou-et-al-2025.pdf), we investigate the precipitation kinetics of Cu in a maraging stainless steel during high-temperature thermal treatments in the fully austenitic state. This provides direct evidence that Cu precipitation can occur in the austenite phase of martensitic or ferritic steels. The kinetics of Cu precipitation in austenite are examined at 700 and 800 °C using in situ synchrotron small-angle and wide-angle X-ray scattering, complemented by atom probe tomography investigations to analyze the precipitates, particularly their chemistry, following heat treatment. The resulting experimental data, which include the evolution of size, volume fraction, number density and chemical composition, are used to inform precipitation kinetics modelling using the Langer-Schwartz-Kampmann-Wagner (LSKW) approach coupled with CALPHAD thermodynamic and kinetic databases. The simulations accurately capture the experimental data by adjusting the interfacial energy in an inverse modelling approach. The insight that Cu precipitation occurs in austenite and subsequently in martensite paves the way for design of hierarchical structures with a bi-modal particle size distribution of Cu precipitates with varying crystal structures and compositions. Additionally, the validated LSKW modelling approach establishes a foundation for designing Cu-alloyed high-performance steels, taking into account various manufacturing routes.

Figure 8. Integrating in situ simultaneous SAXS/WAXS and post-morten APT has proved the Cu precipitation in the high-temperature austenite phase of martensitic/ferritic steels.

4. SAXS/WAXS, APT, TC-PRISMA, etc. on gamma prime precipitation in Ni-based superalloys.

In this work, we quantitatively investigates the γ′ precipitation behaviour and mechanical properties of the Alloy 925 (UNS N09925) material system to elucidate its resilience to compositional variations and aging treatments. Quantitative analysis of the γ′ volume fraction, implementing a novel analysis procedure for WAXS data proposed in this work, shows that minor changes to the Ti and Al contents result in changes of up to 2 vol.% in the γ′ phase during aging. In addition, the introduction of a second aging step at lower temperatures increases the γ′ fraction by approximately 2.5 vol.%. SAXS captures not only the progressive coarsening with increasing temperature and time during one- and two-step aging, resulting in precipitate diameters ranging from 4 to 22 nm, but also the compositional effect on precipitate size. The experimentally obtained quantitative data for the γ′ precipitates allow the prediction of the yield strength increase associated with precipitation strengthening as well as the validation of precipitation kinetics modelling. 

Figure 9. Integrating SAXS, WAXS, APT and TC-PRISMA for the study of gamme prime precipitation in Ni-based alloy 925