In situ observation of ternary eutectic growth in a directionally solidified Mo–Si–B alloy using high‐energy synchrotron X‐rays

Herein, synchrotron-generated high-energy X-ray is used to study growth behavior at the liquid–solid transition of multicomponent alloys during in situ directional solidification experiments at Deutsche Elektronen-Synchrotron (DESY), Hamburg, Germany. The unique “FlexiDS” sample environment is used to directly investigate crystal growth of a ternary eutectic Mo–17.5Si–8B alloy. During the directional solidification process, high-energy X-rays with a photon energy of 100 keV (λ ¼ 0.124 Å) are used in transmission to obtain Debye– Scherrer diffraction rings. The diffraction rings were obtained within the liquid phase, the liquid–solid interphase and the solidified eutectic crystals by scanning through the respective observation volume of interest. The results provide strong evidences for a coupled ternary eutectic growth of the phases MoSS, Mo3Si, and Mo5SiB2, which can be directly observed during in situ experiments for the first time.Projekt DEAL 202

Processing of Al–12Si–TNM composites by selective laser melting and evaluation of compressive and wear properties

Al-12Si (80 vol%)-Ti52.4Al42.2Nb4.4Mo0.9B0.06 (at.%) (TNM) composites were successfully produced by the selective laser melting (SLM). Detailed structural and microstructural analysis shows the formation of the Al6MoTi intermetallic phase due to the reaction of the TNM reinforcement with the Al-12Si matrix during SLM. Compression tests reveal that the composites exhibit significantly improved properties (∼140 and ∼160 MPa higher yield and ultimate compressive strengths, respectively) compared with the Al-12Si matrix. However, the samples break at ∼6% total strain under compression, thus showing a reduced plasticity of the composites. Sliding wear tests were carried out for both the Al-12Si matrix and the Al-12Si-TNM composites. The composites perform better under sliding wear conditions and the wear rate increases with increasing loads. At high loads, the wear takes place at three different rates and the wear rate decreases with increasing experiment duration

Laser powder bed fusion of Ti-22Al-25Nb at low and high pre-heating temperatures

Titanium alloys based on the orthorhombic Ti2AlNb phase are being considered as potential structural lightweight alloys since the early 1990s due to their favourable mechanical performance, i.e., balanced strength and ductility at room and high temperatures as well as high oxidation and creep resistance. With the emergence of additive manufacturing these alloys become particularly interesting again as the microstructures and properties differ considerably from conventionally processed materials. In our work, we consider the whole process chain including the powder production and explain the microstructure formation of the orthorhombic alloy Ti-22Al-25Nb and the effects of in situ and intrinsic heating during laser powder bed fusion with differing energy densities at low and high pre-heating temperatures by means of state-of-the-art characterization techniques such as in situ high energy synchrotron X-ray diffraction and advanced electron microscopy. Fast cooling rates during low-temperature LPBF lead to metastable weakly ordered β phase. For high-temperature LPBF a Widmanstätten microstructure was observed with lenticular O phase precipitates within the β matrix

Microstructural and micromechanical characterisation of TiAl alloys using atomic force microscopy and nanoindentation

Different microstructures were generated in the Ti–45Al–4.6Nb–0.2B–0.2C and Ti–45Al–1Cr alloys (at.%) by heat treatment. The microstructures were investigated using nanoindentation and atomic force microscopy whichwas compared with transmission electron microscopy. Topographic contrast is usually used for phase identification in the atomic force microscope. However, it was found that the topographic order of the phases changes with different microstructures and specimen preparations. Nanoindentation measurements provided local hardness values not obtainable by othermethods and enabled clear distinction of the phases. The hardness values can give information on surrounding microstructure and solid solution hardening. The mean lamellar spacing of the colonies was measured using both atomic force microscopy and transmission electron microscopy. Atomic force microscopy was found to be suitable to determine the spacing between 2/-interfaces offering the advantages of easier sample preparation and fewer specimens compared to evaluation by TEM analysis

Solid-Solid Phase Transformations and Their Kinetics in Ti-Al-Nb Alloys

The application of light-weight intermetallic materials to address the growing interest and necessity for reduction of CO$_2$ emissions and environmental concerns has led to intensive research into TiAl-based alloy systems. However, the knowledge about phase relations and transformations is still very incomplete. Therefore, the results presented here from systematic thermal analyses of phase transformations in 12 ternary Ti-Al-Nb alloys and one binary Ti-Al measured with 4–5 different heating rates (0.8 to 10 °C/min) give insights in the kinetics of the second-order type reaction of ordered (βTi)o to disordered (βTi) as well as the three first-order type transformations from Ti$_3$Al to (αTi), $ω_o$ (Ti$_4$NbAl$_3$) to (βTi)$_o$, and O (Ti$_2$NbAl) to (βTi)$_o$. The sometimes-strong heating rate dependence of the transformation temperatures is found to vary systematically in dependence on the complexity of the transformations. The dependence on heating rate is nonlinear in all cases and can be well described by a model for solid-solid phase transformations reported in the literature, which allows the determination of the equilibrium transformation temperatures

Synthesis of metal-intermetallic laminate (MIL) composites with modified Al3_3Ti structure and in situ synchrotron X-ray diffraction analysis of sintering process

Al$_3$Ti-based alloys attract exceptional attention due to their high specific mechanical properties. However, their application is still insufficient due to their low ductility and fracture toughness. Several approaches were previously proposed to address these problems. The first one is stabilization of the cubic modification of titanium trialuminide by alloying. Another approach consists in fabricating metal-intermetallic laminated composites (MIL). In this study, we combined both methods to synthesize the first MIL composite with cubic Al$_3$Ti interlayers. Copper additions were used to stabilize the cubic modification of Al$_3$Ti and produce a novel Ti-Al$_5$CuTi$_2$ MIL composite. First mechanical characterization by indentation tests showed that the binary Al$_3$Ti intermetallic tended to crack at a load of 0.2 kg while the fracture was not observed in the Al$_5$CuTi$_2$ layers at least at a load of 1 kg. These results are an indirect evidence of a higher ductility and fracture toughness of the composite with cubic Al$_3$Ti compared to tetragonal one. The sequence of the phase transformations in the Al-Ti-Cu system was studied using in situ synchrotron X-ray radiation diffraction. The formation of Al$_5$CuTi$_2$ occurred via several intermediate stages including eutectic melting of Al and Cu and the formation of binary AlCu and Al$_3$Ti compounds

Screening for O phase in advanced γγ–TiAl alloys

This is a screening study using high-energy X-ray diffraction measurements to determine whether an orthorhombicphase forms in $γ$-based TiAl alloys of different compositions. The 13 alloy compositions investigatedwere chosen to be either close to commercial alloys or to identify the effects of different single alloying elementson the formation of orthorhombic phase. The orthorhombic O phase with Cmcm structure was found in several ofthose $γ$-TiAl alloys after an aging heat treatment at 550 °C for 20 h. The presence of different β-stabilising elementssuch as niobium, tantalum, molybdenum or vanadium did promote the formation of orthorhombic phase,while micro alloying elements such as carbon or boron were neutral in this respect. Furthermore, a limit foraluminium was also found, below which the orthorhombic O phase is formed in the alloys investigated. This limitlies between 46 at.% and 47 at.%