Danqi Wang

Cellular Precipitation at a 17-7 PH Stainless Steel Interphase Interface During Low-Temperature Nitridation

AbstractCellular precipitation of Cr-rich nitrides was observed at an austenite–ferrite interface in 17-7 PH stainless steel after low-temperature nitridation. Fine-scale lamellar rocksalt-structured nitride (MN1−x, M: randomly distributed Fe, Cr, and Al) was identified at the interfaces between austenite and ferrite by local-electrode atom-probe tomography and transmission electron microscopy. The small size and spacing of the nitride lamellae reflect the low mobility of substitutional atoms under the conditions of low-temperature nitridation. Nitrides of the same structure were formed within the ferrite grain as extremely small particles. The face-centered cubic nitride precipitates in the Bain orientation relationship with the ferrite.

Tackling Characterization Challenges in High Deformation/Stress Steel Alloys Using Transmission Kikuchi Diffraction (TKD)

Although steels have been extensively studied, the application of traditional characterization methods to investigate the microstructure still poses significant challenges. One example is White Etched Areas (WEA) that are microstructural alternations in bearings induced by dynamic loading conditions [1]. Another example is ‘white layers’ at machined steel surfaces, which are generated by hard turning processes [2]. Both involve formation of nanostructured features at the surface that may lead to significant influence on surface-initiated damage, such as corrosion, fatigue and wear surface deformation. These highly deformed regions have grains that range in size from a few nanometers to 100nm and may consist of small pockets of retained austenite. In some cases preexisting carbides are no longer present in the deformed regions. In other processes such as low temperature carburization/nitridation, the challenges are not as much the structural refinement but primarily the very high level of lattice deformation and formation of nanometer size nitrides [3]. Here as well, the large stresses and very high level of interstitial alloying can result in local phase transformation that is not easily identified by scanning electron microscopy (SEM) or conventional transmission electron microscopy (TEM) without extensive effort. In these materials, sample preparation adds to the characterization challenge. Preserving the original microstructure without introducing any mechanical damage during preparation is critical. At the same time, producing adequate samples for investigating these nanometer scale features demands sample thickness and quality similar to high resolution TEM.

Orientation Mapping by Precession Transmission Electron Microscopy

Precession transmission electron microscopy (PTEM) is currently a very “hot” topic [1]. One of its major applications is orientation mapping. While conventional electron backscattered diffraction (EBSD) and transmission Kikuchi diffraction (TKD) [2] rely on Kikuchi pattern analysis, PTEM (ASTAR system by Nanomegas) acquires micro-diffraction patterns for orientation information. Therefore, even though both methods work in most samples, it is possible that for certain samples PTEM may have an advantage over the EBSD/TKD technique.

“Colossal” Interstitial Supersaturation in Delta Ferrite in 17-7 PH Stainless Steels After Low-temperature Nitridation

A carbon-induced spinodal decomposition of delta ferrite to nanometer-scale Cr-rich and Fe-rich alpha ferrite phases was observed in the weak-contrast regions in high-resolution scanning TEM (STEM) [1]. In addition, an extremely high dislocation density was observed in the decomposed regions, which is consistent with the hypothesis that carbon segregation to dislocation cores effectively delays carbide precipitation and makes possible the “colossal” carbon supersaturation.

Colossal Carbon Supersaturation of Delta Ferrite in 17-7 PH Stainless Steel

Low processing temperatures allow an interstitially-hardened case to be formed on the alloy with no carbide formation [1]; a “colossal” carbon supersaturation can be achieved in austenitic stainless steels such as the 316L grade [1, 2]. In addition to the notable mechanical property improvements [3, 4], such interstitially-hardened stainless steels show surprising improvements in corrosion resistance in marine environment [5].