Materials Science Forum Vol. 690

Abstract: The addition of La to cp-Ti or Ti alloys, like Ti 6Al 4V, leads to the formation of short-breaking chips. Free-machinability is given by elementary La particles in the Ti matrix. The new alloy Ti 6Al 2Fe 1Mo 0.9La 0.5Cu was developed out of the Ti 6Al 4V alloy and exhibits free-machi­nability. However, this alloy offers poor castability due to the formation of hot cracks. Thermocalc® simulations dis­covered that Fe and Cu are broadening the solidification interval of the new alloy, which favors the formation of hot cracks. Therefore, the suitability of other β-stabilizing, like Mn and Cr alloying elements was investigated.

Abstract: Evaluation concepts for the Growth Restriction Factor, Q, in multicomponent alloys are discussed and illustrated for ternary Al-Si-Ti alloys involving precipitation of primary intermetallic phases.

Abstract: The present work focuses on a physical description of phenomena occurring during the heat treatment of a cold deformed aluminium sheet. The major practical interest lies in the prediction of microstructural changes and their impact on the yield strength. The material softening part was described due to thermally activated glide of dislocations. The precipitation kinetics are calculated by using the computational thermodynamics program MatCalc (MATerials CALCulator). The model was validated by comparison with experimental data from tensile tests of cold deformed and heat treated sheets. Finally, it was shown that the model can be used to predict the yield strength during heat treatment of Al-Mg-Si alloys.

Abstract: Lattice parameters, electronic and vibrational energies, enthalpies of formation at 0 and 298 K, the elasticity tensor components, C_ij, polycrystalline bulk, shear and Young’s moduli based on the Hill criteria were computed for LaMg, LaMg₂, LaMg₃, and La₅Mg₄₁, La₂Mg₁₇, and LaMg₁₂. The quasiharmonic approximation was used to compute the linear thermal expansion tensor, aij(T), and the constant pressure heat capacity, C_p(T). Elastic anisotropy was analyzed in terms of the Young’s moduli dependence on crystallographic direction. Results were compared with available experimental data and discussed in terms of the applications of materials from the La-Mg system.

Abstract: The combination of Twin-Roll-Casting (TRC) and subsequent rolling constitutes the most promising process chain for producing magnesium strips economically. Fast solidification (10 times faster than continuous casting) combined with partial deformation lead to a fine primary microstructure as well as less shrinking holes, pores, segregations and brittle precipitations which all together has a very positive effect on forming behavior of the initial material and quality of the final product. The paper elaborates on metallurgical processes in consideration of microstructure and texture results and on the obtained mechanical properties of TRC magnesium strips and finished strips. In addition, the influence of twin-roll-cast and rolling conditions on the mechanical properties will be discussed. The investigation has also been expanded to possible heat treatments and their influence. Concluding remarks will be made on results of rolling trail which were carried out on an industrial scale rolling mill, revealing that the production of hot rolled thin sheets of magnesium alloy AZ31 is possible with a very promising combination of strength and ductility.

Abstract: The behavior of inclusions in castings was studied using radioactive labeling of oxide particles located by Positron Emission Particle Tracking (PEPT). This uses the isotope ¹⁸F, which has a half-life of 110 minutes, and allows particle detection within an accuracy of a few mm. Alumina and glass particles with a size range of 110 to 600 μm were made radioactive by an ion-exchange/surface adsorption process involving irradiated water. Individual radioactive particles were placed in resin-bonded sand moulds at known initial positions, and the moulds were filled with Al alloy, causing the particle to be entrained into the metal stream during the casting process. A modular γ-ray positron camera was used to track the paths of the particles within the mould, demonstrating the applicability of the technique to the study of inclusion behaviour in castings.

Abstract: In this study the effects of different aging heat treatments on the properties and microstructure of a high strength, high toughness metastable β Ti, BTi-6554 (Ti-6Cr-5Mo-5V-4Al), have been compared. An initial β phase solution treatment was followed by aging at moderate temperatures in the α/β dual phase zone by either step aging directly from the solution treatment temperature or by quenching to room temperature prior to the aging heat treatment. The differing heat treatment methods have significant effects on the microstructure and mechanical properties.

Abstract: The effect of HIPping on the mechanical properties and microstructure of a cast Ti-6Cr-5Mo-5V-4Al alloy have been investigated. In the cast condition, the alloy predominantly consists of coarse β grains. In comparison to the HIPped samples, it exhibits poor mechanical properties, in particular low ductility, due to large amounts of remnant porosity. HIPping at 740°C substantially reduces the amount and size of porosity, while HIPping at 820°C and 900°C virtually eliminates all porosity, significantly improving the mechanical properties.

Abstract: The purpose of the work was to assess the impact of construction factors on weldability of MSRB alloy. One conducted transvarestraint test, which allowed to assess welds' susceptibility to cracking under the conditions of forced strain of the casting, Fisco test which simulates welding under the conditions of strong stiffening of the weld, as well as Houldcroft test simulating variable strain of the weld. One concluded that the strain of castings is characteristic of the process of pad welding and welding. Assessment of susceptibility of MSRB alloy to cracking under the conditions of forced strain allows to determine the width of the high-temperature brittleness range (HTBR), critical strain speed of the weld CSS and critical temperature strain intensity CST. These parameters are the criteria of hot cracking of welds from MSRB alloy, therefore, they are indicators of the assessment of the alloy's weldability. Castings from MSRB alloys with constant rigidity should be classified as easily weldable. On the other hand, variable rigidity of the casting, resulting from e.g. diverse thickness of the walls, causes significant increase in the alloy's susceptibility to hot cracking.