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

Abstract

Intermetallic γ-TiAl based alloys of the latest generation, e.g. TNM alloys with a nominal composition of , exhibit the potential to be used in modern high-performance combustion engines due to their low density, high strength and creep-resistance as well as their good oxidation properties at elevated temperatures. Alloying with C can further improve the high-temperature performance both by solid solution hardening and/or carbide formation. In this study, starting from a supersaturated TNM-1C alloy the precipitation behavior and thermal stability of Perovskite-type carbides Ti 3 AlC during isothermal annealing at 750 °C and ensuing re-heating to 1200 °C is quantified by means of an in-situ small-angle X-ray scattering (SAXS) experiment using synchrotron radiation. Complementary, the formed hierarchical structures on the nano-scale, i.e. p-type carbide precipitates within ultra-fine γ-lamellae of the α 2 /γ-colonies, were investigated by means of monochromatic high-energy X-ray diffraction (HEXRD) in combination with transmission electron microscopy (TEM). Additionally, an explanation of an obtained diffraction phenomenon is given, i.e. streak formation which is caused by the very small lamellar spacing of the γ-phase within the α 2 /γ-colonies. It was also found that the geometrically well-defined nanostructure allows a correlation between the γ-laths thickness and a characteristic dimension of p-type carbides.The experiments were carried out at the HZG high-energy materials science beamline HEMS (P07). A sketch of the experimental setup is depicted in Fig. 1. The monochromatic beam with a mean energy of 100 keV is guided through the sample which is situated in a modified quenching and deformation dilatometer device DIL 805 A/D from Bähr Thermoanalyse, Germany. The diffraction patterns are recorded with a Perkin Elmer XRD 1622 flat panel detector in experimental hutch EH 3, whereas a mar345 Image Plate detector located behind in EH 4 is used to record the signal from small-angle scattering. The cylindrical sample with a size of 5 mm in diameter and 10 mm in length was heated inductively. The SAXS data were recorded with a sample-to-detector distance of 11.654 m and a frame rate of 20 images per hour. Slits made of pure tungsten minimized undesired scattering and, thus, ensured a low background. Data evaluation was performed by means of the program Fit2D with respect to azimuthal integration [1,2] followed by a fitting procedure of the derived scattering curves with the program SANSFit based on a least-square method iteration as conducted in Ref. [3]. More details about the experimental conditions and the obtained results are given in the original full-length paper in [4].

Thermal stability and precipitation strengthening of fully lamellar Ti-45Al-5Nb-0.2B-0.75C alloy

“…The driving force for the precipitation process is the limited solubility of carbon, and as generally observed on other alloys, the shape, size and distribution of precipitates are the main factors determining the mechanical properties. In spite of the previous studies which dealt with precipitation of carbides in TiAl alloys [10,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28], information about the microstructural stability of carbon added alloys at higher temperatures and longer times is still lacking in the literature. Therefore, this article aims to study the long-term microstructural stability of the Ti-45Al--5Nb-0.2B-0.75C (at.%) alloy and effect of microstructural changes on mechanical properties during longterm ageing at temperatures of 750, 850 and 950…”

“…However, most of the carbon added alloys include both types of carbides [21][22][23]. The driving force for the precipitation process is the limited solubility of carbon, and as generally observed on other alloys, the shape, size and distribution of precipitates are the main factors determining the mechanical properties.…”

“…Several studies have been focused on the improvement of mechanical properties and creep resistance in the carbon-doped TiAl alloys, and it has been proven that improvement of mechanical properties relates to the solid solution hardening and precipitation hardening [13][14][15][16][17][18][19][20][21][22]. Also, the addition of carbon to lamellar TiAl--based alloys can also reduce lamellar space effectively [19].…”

“…• C [20][21][22] and incoherent Ti 2 AlC (H-type) carbides for higher carbon contents at higher temperatures or at longer times. However, most of the carbon added alloys include both types of carbides [21][22][23].…”

In Situ Characterization Techniques Based on Synchrotron Radiation and Neutrons Applied for the Development of an Engineering Intermetallic Titanium Aluminide Alloy

“…Carbon and Si are known to act as solid solution strengtheners [40,41]. In addition to this, precipitates (cubic Ti 3 AlC, hexagonal Ti 2 AlC or Ti 5 Si 3 ) tend to form during high-temperature creep experiments [42][43][44]. Detailed knowledge of the associated precipitation processes, however, is still needed for their deliberate application to improve a material's high-temperature properties.…”