Crystallographic Analysis, ‘a Forensic Tool’: from Revealing Metallurgical Micromechanisms to ...

Описание: 2026-01-15 Lecture by dr. Habil Yudong Zhang.

Abstract:
As both a Research Engineer and Dr. Habil., I lead a dual mission: advancing metallurgical research while managing technical services for Transmission Electron Microscopy (TEM). By leveraging the crystallographic expertise that is a hallmark of our group at LEM3, I integrate orientation-related analysis into both my primary research and daily technical operations. In this lecture, I will present three case studies that demonstrate the analytical power and versatility of this expertise.

The first example shows the formation mechanism of the sigma phase in a CoCrV medium-entropy alloy (MEA), driven by partial dislocation activity during annealing. The alloy possesses low SFE and belongs to the non-sigma formation family at equilibrium. However, when annealed from the cold-rolled state, nano-sized sigma particles were precipitated ultra-rapidly with heating (20°C/s). Detailed crystallographic analysis allowed us to resolve the structure similarity of the partially dislocated FCC regions to the sigma structure. This alignment elucidates the transformation mechanism that facilitates the ultra-rapid formation of the sigma phase. Consequently, the crystallographic texture of the sigma phase is governed by the orientation of the most active slip planes via the FCC {111} to sigma {001} heredity.

The second example demonstrates how crystallographic analysis resolved the micro mechanisms of a high-temperature α phase in a TiAl alloy during hot compression. In this thermomechanical treatment, the recrystallization experienced three steps: (ⅰ) stress-induced grain boundary bulging and formation of symmetrical-tilt low-angle boundaries characterized by〈0001〉disorientation axis induced by prismatic slip; (ⅱ) evolution of low-angle boundaries into asymmetrical-tilt boundaries characterized by 〈101x〉disorientation axis by local basal slip, or tilt-twist boundaries characterized by 〈112y〉disorientation axis through rotational grain boundary sliding, resulting in the formation of subgrains from the boundary bulges; (ⅲ) the detachment of the subgrains and the mixing of the detached subgrains by grain boundary sliding. The three steps happened continuously and repeatedly from the boundary regions toward grain interior till the completion of recrystallization.

The third example presents our newly developed software-assisted crystallographic approach for achieving specific beam orientations for TEM operation. It is known that TEM imaging relies on specific orientation of the incident electron beam relative to the sample. Traditionally, achieving optimal alignment has relied on empirical trial-and-error, requiring user expertise and considerable time. To overcome this limitation, we worked out a new method supported by a dedicatedly developed module in in-house ATEX-software. This method leverages the determined crystal orientation, expressed by Euler angles relative to the sample holder. It establishes the geometric relations between the incident beam, the desired diffraction vector ɡ (for two-beam conditions) or a zone-axis (for on-axis imaging) and the tilt/rotation axes of the holder. Using this information, the software provides precise tilt and rotation instructions to efficiently reach the desired beam orientation. Unlike conventional methods, this approach significantly reduces the alignment effort, typically requiring no more than two tilts of the sample holder.