Microstructure Evolution of a New Precipitation-Strengthened Fe–Al–Ni–Ti Alloy down to Atomic Scale

Ferritic materials consisting of a disordered matrix and a significant volume fraction of ordered intermetallic precipitates have recently gained attention due to their favorable properties regarding high-temperature applicability. Alloys strengthened by Heusler-type precipitates turned out to show promising properties at elevated temperatures, e.g., creep resistance. The present work aims at developing a fundamental understanding of the microstructure of an alloy with a nominal composition of 60Fe–20Al–10Ni–10Ti (in at. %). In order to determine the microstructural evolution, prevailing phases and corresponding phase transformation temperatures are investigated. Differential thermal analysis, high-temperature X-ray diffraction, and special heat treatments were performed. The final microstructures are characterized by means of scanning and transmission electron microscopy along with hardness measurements. Atom probe tomography conducted on alloys of selected heat-treated conditions allows for evaluating the chemical composition and spatial arrangement of the constituent phases. All investigated sample conditions showed microstructures consisting of two phases with crystal structures A2 and L21. The L21 precipitates grew within a continuous A2 matrix. Due to a rather small lattice mismatch, matrix–precipitate interfaces are either coherent or semicoherent depending on the cooling condition after heat treatment

Charakterisierung der Mikrostruktur von Yttrium-stabiliziertem Zirconiumoxid Wärmedämmschichtsystemen gewachsen auf Saphire-Einkristallen

The present work systematically addresses, for the first time, the characterization of interfaces between thermal barrier coatings (TBCs) and the underlying thermally grown aluminum oxide (TGO) using a model system comprising yttria-stabilized zirconia (YSZ) deposited by electron-beam physical vapor deposition (EB-PVD) on basal plane sapphire. The results provide new insight into the initial stages of TBC growth and especially on the development of texture and the influence of crystallographic orientation relationships at the YSZ/sapphire interface. The YSZ films investigated were deposited at moderate-to-low rates on stationary and rotated substrates, to assess the effects of changing the vapor incidence pattern on the microstructures of interface and coating. A second model system comprising YSZ seeds prepared by solution precursor methods on similar sapphire substrates was used to investigate the potential for modifying texture and column microstructure by templating the growth. The microstructure of the coating and the interface were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), High-Resolution TEM (HRTEM). TBCs deposited on stationary substrates resulted in a dense columnar microstructure with a roof-top morphology and strong out-of-plane fiber texture. It was shown that the texture is templated by the substrate and, because most grains are equally competitive, gives rise to columns with persistent small diameters. Moreover, although the substrate promotes a strong out-of-plane epitaxy, it is rather non-selective with respect to the in-plane orientation, resulting in the observed fiber texture. A periodic variation in the vapor incidence pattern upon rotation yields a less dense, porous microstructure with a larger column diameter than those coatings on stationary substrates. Moreover, in addition to the out-of-plane texture the coatings also exhibit a preferred in-plane orientation with the other axes in-plane, parallel and perpendicular to the rotation axis. Porosity, morphology and texture are strongly correlated to the geometrical effect of rotation. TEM revealed again a common out-of-plane axis in the grains near the interface with multiple in-plane orientations but much less random than on the stationary substrates. Near the interface 83% of the grains exhibit one of the three possible variants of the [ ]S || [001]YSZ orientation relationship, each rotated 120° from one another around the [001]YSZ axis. The other 17% of the grains have intermediate orientations between these variants. The above coatings represent special cases of the mechanism of evolutionary selection, in which columns do not evolve from a spatially random array of nuclei on the substrate, but rather from an initial population with a strong out-of-plane orientation. In the absence of rotation most of the grains are equally competitive and, after a short initial period where stray grains are eliminated, most of the columns propagate with little variation in the diameter and crystallographic orientation. Upon rotation, only the orientation that allows all facets of the column tip to receive equal amount of flux may survive the evolutionary selection mechanism. Hence, a much smaller fraction of columns is selected, and their diameter increases until a steady state condition corresponding to the bi-axially textured film is developed. Because growth under rotation occurs at a slower rate, the grain structure at the interface also evolves during deposition, promoting those orientations that reduce the interfacial energy with the substrate. This results in the development of preferred interfacial variants after the nucleation stage, with a mechanism largely independent from the evolutionary selection process occurring at the growth front. The HRTEM investigations confirm the origin of the epitaxy right at the interface and also the absence of any inter-phases. TBCs deposited on seeded, rotating sapphire substrates show textures much less developed with some influence of the orientation of the seeds but with no evidence of epitaxial growth of the vapor deposited material on the seeds. The results reveal that the surface roughening resulting from the incorporation of seeds has a strong effect in delaying the evolution of the texture in spite of the presence of “favorable” sites. The texture in this case evolves in the more conventional manner, with rotation playing a predominant role over surface templating effects.Die vorliegende Arbeit charakterisierte erstmals systematisch die Mikrostruktur an der Grenzfläche zwischen Wärmedämmschichten (Thermal Barrier Coatings TBC) und dem darunter thermisch aufwachsenden Aluminiumoxid (Thermally Groth Oxyde TGO) am Bsp. des Modellsystems Yttria-stabilisertes Zirkoniumoxid (YSZ) abgeschieden durch Elektronenstrahlverdampfen (Electron Beam Physical Vapor Deposition EB-PVD) auf der Basalebene (0001) von Saphireinkristallen. Die Ergebnisse bieten einen neuen Einblick in das Anfangsstadium des TBC-Wachstums, insbesondere der Texturentwicklung und des Einflusses der kristallographischen Orientierungsbeziehung an der YSZ/Saphir-Grenzfläche. Die untersuchten YSZ-Filme wurden auf stationären und rotierenden Substraten abgeschieden, um in Abhängigkeit des Materialflusses die Mikrostruktur der Grenzfläche an der Beschichtung zu studieren. Ein zweites Modellsystem, wofür sich auf dem Substrat YSZ-Keime befinden (hergestellt durch Precursor-Methoden auf Saphireinkristallen), erlaubt durch das Wachstum in Zeitschritten die Untersuchung der Veränderung der Textur und der Säulenmikrostruktur. Die Charakterisierung der Mikrostruktur erfolgte durch Röntgendiffraktometrie (XRD), Rasterelektronenmikros-kop, Transmissionselektronenmikroskop (TEM), und Hochauflösende TEM (HRTEM). TBC, abgeschieden auf stationären Substraten, zeigten eine dichte, säulenartige Mikrostruktur mit einer dachförmigen Morphologie und ausgeprägter Fasertextur. Es wurde gezeigt, dass die Fasertextur durch die Kristalloberfläche der Substratoberfläche bedingt ist, weil die meisten Körner vergleichbar konkurrierend sind. Die Säulen des TBC wachsen mit einem gleichmäßigen schmalen Durchmesser auf. Außerdem fördert das Substrat ein ausgeprägtes out-of-plane Kristallwachstum, d.h. es ist nicht selektiv im Vergleich zur in-plane Orientierung, woraus die beobachtete Fasertextur resultiert. Eine periodische Veränderung im Materialfluss während der Rotation führt zu einer weniger dichten, porösen Mikrostruktur mit großen Durchmessern der Säulen des TBC, verglichen zu dem unter stationären Bedingungen beschichteten Substraten. Außerdem zeigte das TBC zusätzlich zu der beobachteten out-of-plane Fasertextur eine parallel und rechtwinklig zur Rotationsachse bevorzugte in-plane Kristallorientierung mit einer -Achse in der Kristallebene. Porosität, Morphologie und Textur sind stark von der Rotationsgeometrie während des Elektronenstrahlverdampfens abhängig. TEM bestätigte nochmals die out-of-plane-Achse in den zur Grenzfläche benachbarten Körnern, wobei mehrere in-plane Kristallorientierungen mit einer geringen Anzahl als bei den stationären Substraten statistisch verteilt sind. An der Grenzfläche weisen 83% der Körner eine der drei möglichen Varianten der [10 0]S||[001]YSZ auf, jede gegenüber der anderen um 120° um die [001]YSZ Kristallorientierung gedreht. Der verbleibende Anteil der Körner von 17% hatte zwischen diesen Varianten liegende Kristallorientierungen. Das zuvor beschriebene TBC stellt einen speziellen Fall der Auswahl des Wachstumsmechanismus dar, in welchen die Säulen des TBC nicht ausgehend von einer statistischen Verteilung der Kristallorientierung der Keime auf dem Substrat aufwachsen, sondern von einer anfänglichen Verteilung mit einer ausgeprägten out-of-plane Kristallorientierung. Durch die Rotationsgeometrie sind die meisten Körner im Wachstumsmechanismus miteinander konkurrierend, wobei nach einer kurzen Anfangsperiode in der abweichende Körner ausscheiden, sich Säulen des TBC mit einer geringen Variation des Durchmessers und der kristallographischen Orientierung sich ausbilden. Während des Rotierens verbleibt der Wachstumsmechanismus, wo die Kristallorientierung aller Facetten an der Säulenspitze des TBC unter einen gleichmäßigen Materialfluss wächst. Deshalb wird nur ein geringerer Anteil der Säulen ausgewählt, und der Durchmesser steigt bis ein Gleichgewichtszustand an. Der Gleichgewichtszustand entspricht dem entwickelten zwei-axialen texturierten YSZ-Film. Da während der Rotation die Materialflussrate gering ist, veränderte sich während des Abscheidens die Kornstruktur an der Grenzfläche, wobei Kristallorientierungen die zur Reduktion der Oberflächenenergie mit dem Substrat führen bevorzugt werden. Diese Ergebnisse zum bevorzugten Wachstum an ausgewählten Kristallorientierungen nach der Keimbildungsphase, mit einem Mechanismus weitgehend unabhängig von der Wachstumsauswahl finden an der wachsenden YSZ-Oberfläche statt. HRTEM-Untersuchungen bestätigten das anfängliche epitaktische Wachstum an der Grenzfläche und das nicht vorhanden sein einer weiteren Grenzflächenphase. TBC, das auf mit Keimen geimpften, rotierenden Saphir-Einkristallen aufwuchs, zeigte eine weit geringere Texturierung, wie es die Verteilung der Kristallorientierung der Keime vermuten lässt. Die Ergebnisse zeigten, dass die aus Aufbringen der Keime folgende Oberflächenrauhigkeit eine Texturentwicklung an den bevorzugten Kristallorientierungen stark verzögert. Die Texturierung geschah in diesem Fall auf einen mehr konventionellen Weg, wo die aus der Rotationsgeometrie folgende Oberflächenabschirmung einen wichtigen Einfluss hatte

Effect of carbon addition on solidification behavior, phase evolution and creep properties of an intermetallic β\beta-stabilized γ\gamma-TiAl based alloy

Improving mechanical properties of advanced intermetallic multi-phase γ-TiAl based alloys, such as the Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is limited by compositional and microstructural adaptations. A common possibility to further improve strength and creep behavior of such β-solidifying TiAl alloys is e.g. alloying with β-stabilizing substitutional solid solution hardening elements Nb, Mo, Ta, W as well as the addition of interstitial hardening elements C and N which are also carbide and nitride forming elements. Carbon is known to be a strong α-stabilizer and, therefore, alloying with C is accompanied by a change of phase evolution. The preservation of the solidification pathway via the β-phase, which is needed to obtain grain refinement, minimum segregation and an almost texture-free solidification microstructure, in combination with an enhanced content of C, requires a certain amount of β-stabilizing elements, e.g. Mo. In the present study, the solidification pathway, C-solubility and phase evolution of C-containing TNM variants are investigated. Finally, the creep behavior of a refined TNM alloy with 1.5 at.% Mo and 0.5 at.% C is compared with that exhibiting a nominal Ti-43.5Al-4Nb-1Mo-0.1B alloy composition

Evolution of the ω\omegao{_o} phase in a β\beta-stabilized multi-phase TiAl alloy and its effect on hardness

The intermetallic b-stabilized Ti–43.5Al–4Nb–1Mo–0.1B alloy (in at.%), termed TNM alloy, is designed to be used at elevated temperatures,typically up to 750 C. To understand the evolution of the microstructures during heat treatments and subsequent creep tests,an understanding of the phase transformations and decomposition reactions that occur is necessary. The present study deals with thedevelopment and growth mechanism of the ω${_o}$ phase, which forms in the b${_o}$ phase during static annealing treatments and creep testsperformed at 750, 780 and 800 C using an applied stress of 150 MPa. In situ high-energy X-ray diffraction experiments were conducted to investigate the decomposition behaviour of the ω${_o}$ phase during heating as well as to determine its dissolution temperature. Highresolutiontransmission electron microscopy was used to study the coarsening of xo grains during creep. The chemical compositionof b${_o}$ and ω${_o}$ was determined by means of energy dispersive X-ray microanalysis. In particular, the impact of the Mo content on thegrowth of the ω${_o}$ grains within the b${_o}$ matrix was investigated. Additionally, nanohardness measurements in c, a${_2}$, b${_o}$ and (b${_o}$ + ω${_o}$) grainswere performed by cube corner indentation. The results show that b${_o}$ is the hardest phase in the TiAl–Nb–Mo alloy system when finelydispersed xo precipitates are present

In situ small-angle X-ray scattering study of the perovskite-type carbide precipitation behavior in a carbon-containing intermetallic TiAl alloy using synchrotron radiation

Intermetallic c-TiAl based alloys of the latest generation, e.g. TNM alloys with a nominal composition of Ti–43.5Al–4Nb–1Mo–0.1B(in at.%), exhibit the potential to be used in modern high-performance combustion engines due to their low density, high strength andcreep resistance as well as their good oxidation properties at elevated temperatures. Alloying with C can further improve the hightemperatureperformance by both solid solution hardening and/or carbide formation. In this study, starting from a supersaturatedTNM–1C alloy the precipitation behavior and thermal stability of perovskite-type carbides Ti3AlC during isothermal annealing andensuing re-heating to 1200 C are quantified by means of an in situ small-angle X-ray scattering experiment using synchrotron radiation.Complementarily, the formed hierarchical structures on the nano-scale, i.e. p-type carbide precipitates within ultra-fine c-lamellae of thea2/c-colonies, were investigated by means of monochromatic high-energy X-ray diffraction in combination with scanning and transmissionelectron microscopy. Additionally, an explanation of an obtained diffraction phenomenon is given, i.e. streak formation that iscaused by the very small lamellar spacing of the c-phase within the a2/c-colonies. It was also found that the geometrically well-definednanostructure allows a correlation between the c-lath thickness and a characteristic dimension of p-type carbides

Evidence of an orthorhombic transition phase in a Ti-44Al-3Mo (at.%) alloy using in situ synchrotron diffraction and transmission electron microscopy

Alloying of a binary system with an additional element often leads to the formation of new phases. In this work, a $γ$-TiAl based alloy with 3 at.% molybdenum was investigated, which was quenched from 1450 °C. Upon reheating,the formation of an orthorhombic phase was observed with the help of in situ high-energy X-ray diffraction.This phase formed at 600 °C and vanished again at 720 °C, and acts as a transition phase between the $α_2$ and the $γ$ phase. Such a transition phase has not been observed before in this type of alloy. Additionally, transmission electron microscopy was used to study the microstructure of selected sample states on a submicrometerlevel. The orthorhombic phase formed fine lamellae inside the $α_2$ phase and the $α_2$′ martensite

Advanced β-Solidifying Titanium Aluminides – Development Status and Perspectives

After almost three decades of intensive fundamental research and development activities intermetallic titanium aluminides based on the -TiAl phase have found applications in automotive and aircraft engine industries. The advantages of this class of innovative high-temperature materials are their low density as well as their good strength and creep properties up to 750°C. A drawback, however, is their limited ductility at room temperature, which is reflected by a low plastic strain at fracture. This behavior can be attributed to a limited dislocation movement along with microstructural inhomogeneity. Advanced TiAl alloys, such as β-solidifying TNM™ alloys, are complex multi-phase materials which can be processed by ingot or powder metallurgy as well as precision casting methods. Each production process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat-treatments. The background of these heat-treatments is at least twofold, i.e. concurrent increase of ductility at room temperature and creep strength at elevated temperature. In order to achieve this goal the knowledge of the occurring solidification processes and phase transformation sequences is essential. Therefore, thermodynamic calculations were conducted to predict phase fraction diagrams of engineering TiAl alloys. After experimental verification, these phase diagrams provided the base for the development of heat treatments to adjust balanced mechanical properties. To determine the influence of deformation and kinetic aspects, sophisticated ex- and in-situ methods have been employed to investigate the evolution of the microstructure during thermo-mechanical processing and subsequent multi-step heat-treatments. For example, in-situ high-energy X-ray diffraction was conducted to study dynamic recovery and recrystallization processes during hot-deformation tests. Summarizing all results a consistent picture regarding microstructure formation and its impact on mechanical properties in TNM alloys can be given