Microstructure and grain boundary evolution in titanium thin films

Korngrenzen (KG) spielen eine entscheidende Rolle bei der Bestimmung der physikalischen und chemischen Eigenschaften von polykristallinen Materialien. In dieser Arbeit untersuchen wir die atomare Struktur von KG in Titan. Eine neuartige bikristalline Dünnschicht wurde mit gepulstem Magnetronsputtern auf SrTiO$_3$ (001)-Substraten bei 600 °C synthetisiert. Textur und Mikrostruktur der Schichten werden zunächst mit Hilfe der Rasterelektronenmikroskopie analysiert. Alle beobachteten KG wurden als $\Sigma$13 [0001] indiziert. Weitere Aufnahmen mit dem Rastertransmissionselektronenmikroskop zeigten, dass die KG häufig in symmetrischen $\bar {7}$520- und $\bar {4}$310-Facetten und vereinzelt in asymmetrischen 10$\bar {1}$0 // 2$\bar {1}$$\bar {1}$0-Facetten vorliegen. Dabei folgen die symmetrischen KG dem Modell der Struktureinheiten, werden jedoch durch die Zugabe von Fe verändert. Insgesamt werden in dieser Arbeit verschiedene neuartige Verfahren zur Abscheidung von Ti-Dünnschichten und die Analyse der auftretenden KG dargestellt

Influence of substrates and e-beam evaporation parameters on the microstructure of nanocrystalline and epitaxially grown Ti thin films

International audienceTitanium thin films were deposited on silicon nitride (SiNx) coated Si, NaCl, and sapphire substrates varying the deposition conditions using e-beam evaporation to investigate thin film growth modes. The microstructure and texture evolution in dependence of substrate, deposition rate, film thickness, and substrate temperature were studied using X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. Thin films obtained on SiNx and NaCl substrates were nanocrystalline, while the films deposited on sapphire transformed from nanocrystalline to single crystalline at deposition temperatures above 200 °C. Predominantly, a surface plane orientation of (0002) was observed for the single crystalline films due to the minimization of surface energy. The orientation relationship of epitaxial single crystalline films grown on C-plane sapphire substrate is found to be (0002)Ti (0006)sapphire, (1120)Ti (0330)sapphire. In this orientation relationship, both the total surface and strain energy of the film are minimized. The results were complemented by resistivity measurements using the four-point probe method reporting an increase from 60 mohm cm to 95 mohmcm for single crystalline and nanocrystalline films, respectively

Micro-mechanical deformation behavior of heat-treated laser powder bed fusion processed Ti-6Al-4V

Industrial implementation of heat-treated Laser Powder Bed Fusion (L-PBF) processed Ti-6Al-4 V components requires a thorough understanding of the plastic deformation mechanisms to predict the part performance in safety-critical environments. Here, we study the micro-mechanical deformation behavior of a heat-treated L-PBF processed Ti-6Al-4 V by in-situ uniaxial tensile loading, during which high-resolution strain fields were monitored by Scanning Electron Microscope (SEM) based Digital Image Correlation (DIC). SEM-DIC revealed: (i) the transformed beta phase accommodates higher strain than the primary alpha phase; (ii) strain accumulation in primary alpha occurs primarily at the interface regions where the Al content is lower; and (iii) needle-shaped secondary alpha precipitate in the transformed beta creates strain localization pathways that bridge the interfacial strain bands. Based on the in-situ deformation behavior, recommendations are made on microstructure tailoring and alloy design to prevent strain localization and enhance the quasi-static mechanical properties of L-PBF processed titanium alloy components

Melt pool signatures of TiN nanoparticle dry-coated Co25Cr25Fe25Ni25 metal powder in laser-powder-bed-fusion

Metal powders in laser-powder-bed-fusion (L-PBF) often exhibit cohesive flow resulting from interparticle adhesion. Nanoparticle dry-coating can improve powder flowability and promote powder layer densification. A Co25Cr25Fe25Ni25 metal powder (20–90 µm) is dry-coated with TiN particles with a diameter of 16 nm at low concentrations of up to 69 ppm. The dynamic angle of repose decreased and bulk powder density increased compared to the uncoated state from 49 ° and 4.67 g/cm3 to 29 ° and 4.81 g/cm3 with dry-coating of TiN, respectively. UV/Vis spectroscopy showed negligible alterations by TiN additions on the powder light absorption. The powder modifications strongly affected their corresponding processability in L-PBF and reduced the melt pool signatures of the in situ detected confocal single-color pyrometer signal as well as ex situ measured melt pool depth and width. With increasing flowability, a significant decrease in thermal emission and melt pool size was observed. The results demonstrate the impact of powder flowability and bulk powder density on the quality of L-PBF parts when particle interactions are actively modified

Novel class of amorphous/crystalline high entropy alloys multilayer thin films with superior mechanical properties and thermal stability

The design of high performance structural materials is always pursuing combinations of excellent yet often mutually exclusive mechanical properties and thermal stability. Although crystal-glass composite alloys provide better ductility compared to fully amorphous alloys, their thermal stability is poor, due to heterogeneous nucleation at the crystal-glass interface [1, 2]. Here, we present a new strategy to develop thermally stable, ultrastrong and deformable crystal-glass nanocomposites through a thermodynamically guided alloy design approach, which mimics the mutual stabilization principle known from symbiotic ecosystems. We realized this in form of a model Cr-Co-Ni (crystalline)/Ti-Zr-Nb-Hf-Cr-Co-Ni (amorphous) laminate composite alloy (see Figure) [3]. The symbiotic alloy has an ultrahigh compressive yield strength of 3.6 GPa and large homogeneous deformation of ~15% strain at ambient temperature, values which surpass those of conventional metallic glasses and nanolaminate alloys (see Figure). Furthermore, the alloy exhibits ~200 K higher crystallization temperature (TX> 973 K) compared to that of the original TiZrNbHf-based amorphous phase. The elemental partitioning among adjacent amorphous and crystalline phases leads to their mutual thermodynamic and mechanical stabilization, opening up a new symbiotic approach for stable, strong and ductile materials [3].References:[1] W. Guo et al., Acta Mater. 80 (2014) 94-106.[2] D.B. Miracle, O.N. Senkov, Acta Mater. 122 (2017) 448-511.[3] M. Ghidelli et al., Submitted to Materials Today (2021

Symbiotic crystal-glass alloys via dynamic chemical partitioning

International audienc