| 1. |
- Bock, Florian, 1994-
(författare)
-
Combining ab‐initio and machine learning techniques for theoretical simulations of hard nitrides at extreme conditions
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis I focus on combining the high accuracy of first-principles calculations with modern machine learning methods to make large scale investigations of industrially relevant nitride systems reliable and computationally viable. I study the electronic, thermodynamic and mechanical properties of two families of compounds: Ti1−xAlxN alloys at the operational conditions of industrial cutting tools and ReNx systems at crushing pres-sures comparable to inner earth core conditions. Standard first-principles simulations of materials are usually carried out at zero temperature and pressure, and while many state-of-the-art approaches can take these effects into account, they are usually accompanied by a substantial increase in computational demand. In this thesis I therefore explore the possiblities of studying materials at extreme conditions using machine learning methods with extraordinary efficiency without loss of calculational accuracy. Ti1−xAlxN alloy coatings exhibit exceptional properties due to their inherent ability to spinodally decompose at elevated temperature, leading to age-hardening. Since the cubic B1 phase of Ti1−xAlxN is well-studied, available high-accuracy first-principles data served as both a benchmark and data set on which to train a machine learning interatomic potential. Using the reliable moment tensor potentials, an investigation of the accuracy and efficiency of this approach was carried out in a machine learning study. Building upon the success of this technique, implementation of a learning-on-the-fly (active learning) methodology into a workflow to determine accurate material properties with minimal prior knowledge showed great promise, while maintaining a computational demand up to two orders of magnitude lower than comparable first-principles approaches. Investigations of properties of industrially lesser desired, but sometimes present hexagonal alloy phases of Ti1−xAlxN are also included in this thesis, since knowledge and understanding of all competing phases can help guide development toward improving cutting tool lifetime and performance. Furthermore, while w-Ti1−xAlxN may not be able to compete with its cubic counterpart in terms of hardness, it shows promise for other applications due to its electronic and elastic properties. Metastable ReNx phases are high energy materials due to their covalent N-N and Re-N bonds, leading to exceptional mechanical and electronic properties. Just like diamond, the hardest and arguably most famous metastable mate-rial naturally occurring on earth, they are stabilized by extreme pressures and high temperatures, but can be quenched to ambient conditions. Understanding the formation and existence of these non-equilibrium compounds may hold the key to unlocking a new generation of hard materials. In this thesis, all currently known phases of ReNx compounds have been investigated, encompassing both experimentally observed and theoretically suggested structures. Investigations of the convex hulls across a broad pressure range were carried out, coupled with calculations of phonons in the proposed crystals to determine both energetic and dynamical stability. Overall, the studies included in this thesis focused mainly on investigation of the ground state of ReN2 at higher pressure, where experimental results were deviating from earlier theoretical predictions. Additional research focused on specifically exploring properties and stability of novel ReN6 at synthesis conditions using the active learning workflow to train an interatomic potential.
|
|
| 2. |
- Broering Chaar, Ana Beatriz, 1990-, et al.
(författare)
-
The Effect of Cathodic Arc Guiding Magnetic Field on the Growth of (Ti0.36Al0.64)N Coatings
- 2019
-
Ingår i: Coatings. - : MDPI. - 2079-6412. ; 9:10
-
Tidskriftsartikel (refereegranskat)abstract
- We use a modified cathodic arc deposition technique, including an electromagnetic coil that introduces a magnetic field in the vicinity of the source, to study its influence on the growth of (Ti0.36Al0.64)N coatings. By increasing the strength of the magnetic field produced by the coil, the cathode arc spots are steered toward the edge of the cathode, and the electrons are guided to an annular anode surrounding the cathode. As a result, the plasma density between the cathode and substrate decreased, which was observed as a lateral spread of the plasma plume, and a reduction of the deposition rate. Optical emission spectroscopy shows reduced intensities of all recorded plasma species when the magnetic field is increased due to a lower number of collisions resulting in excitation. We note a charge-to-mass ratio decrease of 12% when the magnetic field is increased, which is likely caused by a reduced degree of gas phase ionization, mainly through a decrease in N2 ionization. (Ti0.36Al0.64)N coatings grown at different plasma densities show considerable variations in grain size and phase composition. Two growth modes were identified, resulting in coatings with (i) a fine-grained glassy cubic and wurtzite phase mixture when deposited with a weak magnetic field, and (ii) a coarse-grained columnar cubic phase with a strong magnetic field. The latter conditions result in lower energy flux to the coating’s growth front, which suppresses surface diffusion and favors the formation of c-(Ti,Al)N solid solutions over phase segregated c-TiN and w-AlN.
|
|
| 3. |
- Calamba, Katherine, et al.
(författare)
-
Effect of nitrogen vacancies on the growth, dislocation structure, and decomposition of single crystal epitaxial (Ti1-xAlx)N-y thin films
- 2021
-
Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 203
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of varying nitrogen vacancies on the growth, microstructure, spinodal decomposition and hardness values of predominantly single crystal cubic phase c-(Ti1-xAlx)N-y films was investigated. Epitaxial c-(Ti1-xAlx)N-y films with y = 0.67, 0.79, and 0.92 were grown on MgO(001) and MgO(111) substrates by magnetron sputter deposition. High N vacancy c-(Ti1-xAlx)N-0.67 films deposited on MgO(111) contained coherently oriented w-(0001) structures while segregated conical structures were observed on the films grown on MgO(001). High resolution STEM images revealed that the N-deficient growth conditions induced segregation with small compositional fluctuations that increase with the number of N vacancies. Similarly, strain map analysis of the epitaxial c-(Ti1-xAlx)N-y (001) and (111) films show fluctuations in strain concentration that scales with the number of N vacancies and increases during annealing. The spinodal decomposition coarsening rate of the epitaxial c-(Ti1-xAlx)N-y films was observed to increase with decreasing N vacancies. Nanoindentation showed decreasing trends in hardness of the as-deposited films as the N vacancies increase. Isothermal post-anneal at 1100 degrees C in vacuum for 120 min revealed a continuation in the increase in hardness for the film with the largest number of N vacancies (y = 0.67) while the hardness decreased for the films with y = 0.79 and 0.92. These results suggest that nitrogen-deficient depositions of c-(Ti1-xAlx)N-y films help to promote a self-organized phase segregation, while higher N vacancies generally increase the coherency strain which delays the coarsening process and can influence the hardness at high temperatures.
|
|
| 4. |
- Chen, Yu-Hsiang, et al.
(författare)
-
Enhanced thermal stability and fracture toughness of TiAlN coatings by Cr, Nb and V-alloying
- 2018
-
Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 342, s. 85-93
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of metal alloying on mechanical properties including hardness and fracture toughness were investigated in three alloys, Ti 0.33Al0.50(Me) 0.17N (Me = Cr, Nb and V), and compared to Ti0.50Al0.50N, in the as-deposited state and after annealing. All studied alloys display similar as-deposited hardness while the hardness evolution during annealing is found to be connected to phase transformations, related to the alloy’s thermal stability. The most pronounced hardening was observed in Ti0.50Al0.50N, while all the coatings with additional metal elements sustain their hardness better and they are harder than Ti0.50Al0.50N after annealing at 1100 °C. Fracture toughness properties were extracted from scratch tests. In all tested conditions, as-deposited and annealed at 900 and 1100 °C, Ti0.33Al0.50Nb0.17N show the least surface and sub-surface damage when scratched despite the differences in decomposition behavior and h-AlN formation. Theoretically estimated ductility of phases existing in the coatings correlates well with their crack resistance. In summary, Ti0.33Al0.50Nb0.17N is the toughest alloy in both as-deposited and post-annealed states.
|
|
| 5. |
- Magnuson, Martin, 1965-, et al.
(författare)
-
Interface bonding of Zr1−xAlxN nanocomposites investigated by x-ray spectroscopies and first principles calculations
- 2020
-
Ingår i: Physical Review Research. - College Park, MD, United States : American Physical Society. - 2643-1564. ; 2:1
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic structure, chemical bonding, and interface component in ZrN-AlN nanocomposites formed byphase separation during thin film deposition of metastable Zr1−xAlxN (x = 0.0, 0.12, 0.26, 0.40) are investigatedby resonant inelastic x-ray scattering, x-ray emission, and x-ray absorption spectroscopy and compared to firstprinciples calculations including transitions between orbital angular momentum final states. The experimentalspectra are compared with different interface-slab model systems using first principles all-electron full-potentialcalculations where the core states are treated fully relativistically. As shown in this work, the bulk sensitivity andelement selectivity of x-ray spectroscopy enables one to probe the symmetry and orbital directions at interfacesbetween cubic and hexagonal crystals. We show how the electronic structure develops from local octahedralbond symmetry of cubic ZrN that distorts for increasing Al content into more complex bonding. This results inthree different kinds of bonding originating from semicoherent interfaces with segregated ZrN and lamellar AlNnanocrystalline precipitates. An increasing chemical shift and charge transfer between the elements takes placewith increasing Al content and affects the bond strength and increases resistivity.
|
|
| 6. |
- Moreno, Maiara, et al.
(författare)
-
Adhesive wear of TiAlN coatings during low speed turning of stainless steel 316L
- 2023
-
Ingår i: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 524-525
-
Tidskriftsartikel (refereegranskat)abstract
- The wear behavior of TiAlN coatings during turning of stainless steel 316L at low cutting speeds (60–120 m/min) was investigated using scanning electron microscopy. In this speed range, the coatings fail by fracture due to an adhesive wear mechanism. The fracture of the coating is described in detail, including the strong influence of Al-content and cutting speed on the rate of wear. Low Al-content (x ≤ 0.23) coatings showed worse wear resistance than high Al-content (x ≥ 0.53) samples. Less substrate is exposed when the cutting speed is increased, because of reduced adhesive wear. The TiN and Ti0.77Al0.23N coatings are severely worn for all cutting speeds while Ti0.47Al0.53N and Ti0.38Al0.62N remain essentially unaffected at the highest speed. The difference in wear behavior is interpreted as a difference in the fracture toughness of the coatings.
|
|
| 7. |
- Moreno, Maiara, 1993-, et al.
(författare)
-
Strain and phase evolution in TiAlN coatings during high-speed metal cutting : An in operando high-energy x-ray diffraction study
- 2024
-
Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 263
-
Tidskriftsartikel (refereegranskat)abstract
- We report on phase and strain changes in Ti1-xAlxN (0 ≤ x ≤ 0.61) coatings on cutting tools during turning recorded in operando by high-energy x-ray diffractometry. Orthogonal cutting of AISI 4140 steel was performed with cutting speeds of 360–370 m/min. Four positions along the tool rake face were investigated as a function of time in cut. Formation of γ-Fe in the chip reveals that the temperature exceeds 727 °C between the tool edge and the middle of the contact area when the feed rate is 0.06 mm/rev. Spinodal decomposition and formation of wurtzite AlN occurs at the positions of the tool with the highest temperature for the x ≥ 0.48 coatings. The strain evolution in the chip reveals that the mechanical stress is largest closest to the tool edge and that it decreases with time in cut for all analyzed positions on the rake face. The strain evolution in the coating varies between coatings and position on the rake face of the tool and is affected by thermal stress as well as the applied mechanical stress. Amongst others, the strain evolution is influenced by defect annihilation and, for the coatings with highest Al-content (x ≥ 0.48), phase changes.
|
|
| 8. |
- Rogström, Lina, 1983-, et al.
(författare)
-
Structural changes in Ti1-xAlxN coatings during turning : A XANES and EXAFS study of worn tools
- 2023
-
Ingår i: Applied Surface Science. - : Elsevier. - 0169-4332 .- 1873-5584. ; 612
-
Tidskriftsartikel (refereegranskat)abstract
- Structural changes in Ti1-xAlxN coated tool inserts used for turning in 316L stainless steel were investigated by XANES, EXAFS, EDS, and STEM. For coarse-grained fcc-structured Ti1-xAlxN coatings, with 0 ≤ x ≤ 0.62, the XANES spectrum changes with Al-content. XANES Ti 1s line-scans across the rake face of the worn samples reveals that TiN-enriched domains have formed during turning in Ti0.47Al0.53N and Ti0.38Al0.62N samples as a result of spinodal decomposition. The XANES spectra reveal the locations on the tool in which the most TiN-rich domains have formed, indicating which part of the tool-chip contact area that experienced the highest temperature during turning. Changes in the pre-edge features in the XANES spectra reveal that structural changes occur also in the w-TiAlN phase in fine-grained Ti0.38Al0.62N during turning. EDS shows that Cr and Fe from the steel adhere to the tool rake face during machining. Cr 1s and Fe 1s XANES show that Cr is oxidized in the end of the contact length while the adhered Fe retains in the same fcc-structure as that of the 316L stainless steel.
|
|
| 9. |
- Salamania, Janella, 1992-, et al.
(författare)
-
Elucidating dislocation core structures in titanium nitride through high-resolution imaging and atomistic simulations
- 2022
-
Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 224
-
Tidskriftsartikel (refereegranskat)abstract
- Although titanium nitride (TiN) is among the most extensively studied and thoroughly characterizedthin-film ceramic materials, detailed knowledge of relevant dislocation core structures is lacking. Byhigh-resolution scanning transmission electron microscopy (STEM) of epitaxial single crystal (001)-oriented TiN films, we identify different dislocation types and their core structures. These include, besidesthe expected primary a/2{110}h110i dislocation, Shockley partial dislocations a/6{111}h112i and sessileLomer edge dislocations a/2{100}h011i. Density-functional theory and classical interatomic potentialsimulations complement STEM observations by recovering the atomic structure of the different disloca-tion types, estimating Peierls stresses, and providing insights on the chemical bonding nature at the core.The generated models of the dislocation cores suggest locally enhanced metal–metal bonding, weakenedTi-N bonds, and N vacancy-pinning that effectively reduces the mobilities of {110}h110i and {111}h112idislocations. Our findings underscore that the presence of different dislocation types and their effects onchemical bonding should be considered in the design and interpretations of nanoscale and macroscopicproperties of TiN.
|
|
| 10. |
- Salamania, Janella, 1992-, et al.
(författare)
-
High-resolution STEM investigation of the role of dislocations during decomposition of Ti1-xAlxNy
- 2023
-
Ingår i: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456. ; 229
-
Tidskriftsartikel (refereegranskat)abstract
- The defect structures forming during high-temperature decomposition of Ti1-xAlxNy films were investigated through high-resolution scanning transmission electron microscopy. After annealing to 950 °C, misfit edge dislocations a/6〈112〉{111} partial dislocations permeate the interface between TiN-rich and AlN-rich domains to accommodate lattice misfits during spinodal decomposition. The stacking fault energy associated with the partial dislocations decreases with increasing Al content, which facilitates the coherent cubic to wurtzite structure transition of AlN-rich domains. The wurtzite AlN-rich structure is recovered when every third cubic {111} plane is shifted by along the [211] direction. After annealing to 1100 °C, a temperature where coarsening dominates the microstructure evolution, we observe intersections of stacking faults, which form sessile locks at the interface of the TiN- and AlN-rich domains. These observed defect structures facilitate the formation of semicoherent interfaces and contribute to hardening in Ti1-xAlxNy.
|
|
