Lattice defects in HPT processed fcc nanometals studied by differential scanning calorimetry

Die vorliegende Dissertation befasst sich mit der Untersuchung der Natur, Anordnung und Dichte der durch Severe Plastic Deformation (SPD) generierten Gitterdefekte in Nanomaterialien. Da diese Defekte in wichtigem Zusammenhang mit den aussergewöhnlichen physikalischen Eigenschaften von SPD Nanomaterialien stehen, ist ihre Analyse für das Verständnis dieser Eigenschaften von grosser Bedeutung. Eine sehr repräsentative SPD Methode ist die Hochdrucktorsion (High Pressure Torsion - HPT), da mit ihr der Verformungsgrad und der hydrostatische Druck kontrolliert eingestellt werden können. In dieser Arbeit wurden Proben von Cu und Ni verschiedener Reinheit mittels HPT bei verschiedenen hydrostatischen Drucken unterschiedlich stark verformt, bevor daran Messungen mittels Differential Scanning Kalorimetrie (DSC) vorgenommen wurden. Während in HPT Cu verformungsinduzierte Leerstellenagglomerate und Versetzungen nachgewiesen wurden, konnten in HPT Ni auch Einfach- bzw. Doppelleerstellen beobachtet werden. Die Gesamtkonzentration an Leerstellen war in Nickel höher als in Cu, was eher auf den Einfluss der homologen Verformungstemperatur als auf denjenigen der Stapelfehlerenergie zurückzuführen ist. Allgemein nehmen die Konzentrationen der verformungsinduzierten Gitterdefekte mit höherem Verformungsgrad und höherem hydrostatischen Druck zu. Für Ni wurde gezeigt, dass die thermische Stabilität der Gitterdefekte mit zunehmender Verunreinigung zunimmt. Für die Bestimmung der Aktivierungsenthalpien (Q) wurde die Kissinger Methode angewandt. Die Aktivierungsenthalpien der Peaks in Ni betragen Qvac = 0.65 eV und Qdisl = 0.95 eV, im Einklang mit der Interpretation, dass diese Peaks die Ausheilung von Einzel/ Doppelleerstellen bzw. die von Leerstellenagglomeraten und Versetzungen repräsentieren. Für den einzigen Ausheilpeak in HPT Cu ergibt sich eine Änderung von Q = 0.78 eV bis Q = 0.48 eV je nach HPT induziertem Verformungsgrad, in Übereinstimmung mit der Variation der Ausheiltemperatur. Da die Grösse der Aktivierungsenthalpie betreffend deren Abhängigkeit vom Verformungsgrad eindeutig mit der Grösse der lokalen inneren Spannungen korreliert, können letztere als Ursache der Änderung der Aktivierungsenthalpien mit dem Verformungsgrad angenommen werden. Im Rahmen dieser Dissertation wurden auch Pd Proben mit Wasserstoff versetzt und anschliessend einer HPT Verformung bei tiefen Temperaturen unterzogen. Die DSC Untersuchung zeigte die Bildung hoher Konzentrationen von Leerstellen-Wasserstoff Agglomeraten an (7*10-4), und ausserdem auch die Stabilisierung von Versetzungen mittels Wasserstoff, wie begleitende elektronenmikroskopische Untersuchungen ergeben haben.The current PhD thesis is concerned with the investigation of the nature, distribution and density of lattice defects in nanomaterials which have been processed by Severe Plastic Deformation (SPD). Since these defects have some importance for the exceptional physical properties of SPD nanomaterials, their careful analysis is to enable a better understanding of these properties. As a representative technique for SPD nanostructures, that of High Pressure Torsion (HPT) has been used because of its capability to accurately control strain and hydrostatic pressure. Samples of copper and nickel were deformed by HPT at different hydrostatic pressures to different shear strains, and subjected to different DSC investigations. While in HPT-processed Cu only vacancy agglomerates and dislocations were found, in HPT-processed Ni also single/double vacancies could be observed. The total concentration of vacancies including those of agglomerates was higher, indicating the influence of homologous processing temperature rather than that of stacking fault energy. Generally, the concentrations of all lattice defects increased with increasing strain and pressure applied. Concerning Ni, additional impurities have shown to stabilize the lattice defects resulting in an increased annealing temperature. Activation enthalpies Q were determined by DSC using Kissinger's method. For Ni, the activation enthalpies of the two annealing peaks were determined as Q_vac = 0.65 eV and Q_disl = 0.95 eV, respectively, indicating the annihilation of single or double vacancies, and that of dislocations and vacancy agglomerates, respectively. For the one annealing peak found in HPT Cu, Q amounts from Q = 0.78 eV down to 0.48 eV as a function of shear strain applied, in correspondence with the change of peak temperature. Due to the obvious correlation with the strain dependence of external and local internal stresses, the strain dependence of Q, can be attributed to the local internal stresses governing the annihilation of dislocations and/or vacancy agglomerates. In addition, pure palladium was hydrogenated, subsequently deformed by HPT and analyzed by DSC. For comparison hydrogen-free HPT processed samples were also investigated. The results, for the first time, gave evidence for the formation of vacancy-hydrogen clusters caused by HPT. The vacancy concentration produced in Pd-H by this method is with 7*10-4 extraordinarily high. Similar to the effect of impurities in Ni, hydrogen seems to stabilize vacancies but also other HPT induced lattice defects as it can be concluded from the increase in peak temperatures and from concomitant observations by transmission electron microscopy

Controlling the Formation of Sodium/Black Phosphorus IntercalationCompounds Towards High Sodium Content

The solid-state synthesis of pure sodium-black phosphorus intercalation compounds (Na-BPICs) has been optimized in bulk for two stoichiometric ratios. Specifically, in-situ X-Ray diffraction (XRD) allowed the precise identification of the optimal temperature range for the formation of Na-BPICs: 94°C–96°C. Moreover, as the undesired formation of Na3P takes place at this very same range, we succeeded in introducing a new synthetic route based on a fast-thermal ball milling implementation that results in the bulk production of BPIC without Na3P in 9 out of 10 cases. Finally, by combining XRD, Raman spectroscopy, and DFT calculations we developed a new structural model for Na-based BPICs showing an increase of BP’s unit cell with Na atoms incorporated in every second layer. These results will pave the way for the large-scale synthesis and application of pure BPICs, which are of great interest in fields such as optoelectronics or energy storage

Controlling the formation of sodium/black phosphorus intercalated compounds towards high sodium content

The solid-state synthesis of pure sodium-black phosphorus intercalation compounds (Na-BPICs) has been optimized in bulk for two stoichiometric ratios. Specifically, in-situ X-Ray diffraction (XRD) allowed the precise identification of the optimal temperature range for the formation of Na-BPICs: 94 °C–96 °C. Moreover, as the undesired formation of Na3P takes place at this very same range, we succeeded in introducing a new synthetic route based on a fast-thermal ball milling implementation that results in the bulk production of BPIC without Na3P in 9 out of 10 cases. Finally, by combining XRD, Raman spectroscopy, and DFT calculations we developed a new structural model for Na-based BPICs showing an increase of BP's unit cell with Na atoms incorporated in every second layer. These results will pave the way for the large-scale synthesis and application of pure BPICs, which are of great interest in fields such as optoelectronics or energy storage.PNICTOCHEM 804110 (G.A.)PID2019-111742-GA-I00CIDEGENT/2018/00

Hot deformation characteristic and strain dependent constitutive flow stress modelling of Ti + Nb stabilized interstitial free steel

Abstract An effort has been made to establish a relation between Zener–Hollomon parameter, flow stress and dynamic recrystallization (DRX). In this context, the plastic flow behavior of Ti + Nb stabilized interstitial free (IF) steel was investigated in a temperature range of 650–1100 °C and at constant true strain rates in the range 10−3–10 s−1, to a total true strain of 0.7. The flow stress curves can be categorized into two distinct types, i.e. with/without the presence of steady-state flow following peak stress behavior. A novel constitutive model comprising the strain effect on the activation energy of DRX and other material constants has been established to predict the constitutive flow behavior of the IF steel in both α and γ phase regions, separately. Predicted flow stress seems to correlate well with the experimental data both in γ and α phase regions with a high correlation coefficient (0.982 and 0.936, respectively) and low average absolute relative error (7 and 11%, respectively) showing excellent fitting. A detailed analysis of the flow stress, activation energy of DRX and stress exponent in accord with the modelled equations suggests that dislocation glide controlled by dislocation climb is the dominant mechanism for the DRX, as confirmed by the transmission electron microscopy analysis

Exceptional strengthening of biodegradable Mg-Zn-Ca alloys through high pressure torsion and subsequent heat treatment

In this study, two biodegradable Mg-Zn-Ca alloys with alloy content of less than 1 wt % were strengthened via high pressure torsion (HPT). A subsequent heat treatment at temperatures of around 0.45 Tm led to an additional, sometimes even larger increase in both hardness and tensile strength. A hardness of more than 110 HV and tensile strength of more than 300 MPa were achieved in Mg-0.2Zn-0.5Ca by this procedure. Microstructural analyses were conducted by scanning and transmission electron microscopy (SEM and TEM, respectively) and atom probe tomography (APT) to reveal the origin of this strength increase. They indicated a grain size in the sub-micron range, Ca-rich precipitates, and segregation of the alloying elements at the grain boundaries after HPT-processing. While the grain size and segregation remained mostly unchanged during the heat treatment, the size and density of the precipitates increased slightly. However, estimates with an Orowan-type equation showed that precipitation hardening cannot account for the strength increase observed. Instead, the high concentration of vacancies after HPT-processing is thought to lead to the formation of vacancy agglomerates and dislocation loops in the basal plane, where they represent particularly strong obstacles to dislocation movement, thus, accounting for the considerable strength increase observed. This idea is substantiated by theoretical considerations and quenching experiments, which also show an increase in hardness when the same heat treatment is applied.ISSN:1996-194