First-principles study of the ternary effects on the plasticity of $$\upgamma $$-TiAl crystals

We studied the effects of important ternary elements, such as Cr, Nb, and V, on the plasticity of $$\upgamma $$ γ -TiAl crystals by calculating the point defect formation energy and the change in the generalized stacking fault energy (GSFE) surface from first-principles calculations. For all three elements, the point defect formation energies of the substitutional defects are lower in the Ti site than in the Al site, which implies that substitution on the Ti site is energetically more stable. We computed the GSFE surfaces with and without a substitutional solute and obtained the ideal critical resolved shear stress (ICRSS) of each partial slip. The change in the GSFE surface indicates that the substitution of Ti with Cr, Nb, or V results in an increase in the yield strength because the ICRSS of the superlattice intrinsic stacking fault (SISF) partial slip increases. Interestingly, we find that Cr substitution on an Al site could occur owing to the small difference between the substitutional defect formation energies of the Ti and Al sites. In that case, the reduction of ICRSSs of the SISF partial slip and twinning would lead to improved twinnability. We discuss the implications of the computational predictions by comparing them with experimental results in the literature.

Efficient Calculation Methods for the Diffusion Coefficient of Interstitial Solutes in Dilute Alloys

In the example of oxygen diffusion in dilute ferritic iron alloys it is shown that the calculation of the diffusion coefficient can be separated into a contribution related to the migration in the interaction region between oxygen and the substitutional solute and a part related to diffusion in pure body centered cubic (bcc) Fe. The corresponding diffusion times are determined by analytical expressions using Density-Functional-Theory (DFT) data for the respective binding energies. The diffusion coefficient in the interaction region must be determined by atomistic kinetic Monte Carlo (AKMC) simulations with DFT values for the migration barriers as input data. In contrast to previous calculations, AKMC simulation must only be performed for one concentration of the substitutional solute, and the obtained results can be employed to obtain data for other concentrations in a very efficient manner. This leads to a tremendous decrease of computational efforts. Under certain conditions it is even possible to use analytical expressions where merely DFT data for the binding energies are needed. The limits of applicability of the presented calculation procedures are discussed in detail. The methods presented in this work can be generalized to interstitial diffusion in other host materials with small concentrations of substitutional solutes.

The Effect of Soaking on Segregation and Primary-Carbide Dissolution in Ingot-Cast Bearing Steel

In this work, segregation in the cast and hot worked structure, as well as the effects of soaking on macro and micro segregation, in hypereutectoid bearing steel produced by ingot casting were studied. Samples were selected from ingots that where either as cast or soaked for twenty hours. Two similar bearing steel grades were used for this investigation. For the as cast ingot, samples were selected from both A-segregation channel regions and the matrix region. Samples were also selected from hot-worked bars originating from ingots that had been soaked for four hours or twenty hours. Micro and macro examinations of the microstructures were conducted and compared. In addition, a segregation analysis of the substitutional solute elements was performed using EDX equipment mounted on a Scanning Electron Microscope (SEM). EMPA mapping of the composition pattern in the bulk, as well as the carbides, was conducted. Precipitation of M3C, M2C, and M6C was observed. The carbides at A-segregation channels were found to have a different morphology to those precipitated in the bulk matrix. After soaking at 1200 °C for 4 h, all the primary carbides are dissolved.

Theoretical study of the substitutional solute effect on the interstitial carbon in nickel-based alloy

The carbon binding in nickel-based alloy with 3d, 4d and 5d transition metal solutes are investigated by using first-principles methods.

Solute–Solute Interaction In α IRON: The Status QUO

An overview is presented on the interaction of substitutional solutes with carbon and nitrogen in α iron, which is an important factor in controlling the properties of steels. Starting from a simple model of trapping of the interstitial solute atoms by substitutional solute atoms, the principles of experimental methods for quantitative studies are described, focussing on the Snoek relaxation and solubility measurements, and the knowledge acquired by such experiments is reviewed. An account of recent theoretical approaches to the interaction is also given.

Determination of Substitutional-Interstitial Interaction from Chemical Potentials of Interstitials in the Steel Matrix

The interaction between interstitially diffusing atoms and substitutional solute atoms, acting as trapping sites, causes a non-negligible influence on the diffusion process itself and, consequently, on many aspects of alloys, such as phase transformations, solubility, precipitation of carbides and nitrides etc. The most important quantity in this treatment is the so-called trapping enthalpy (depth of trap), which has been used in several approaches in literature over the last century. However, the determination of the trapping enthalpy so far relies on approximations or assumptions on the one hand (statistical approaches, quasi chemical approach) or is significantly limited due to high complexity (ab initio approaches) on the other hand. The model introduced in this paper illustrates a rigorous and efficient thermodynamically-based concept utilizing only the dependence of the chemical potential of the interstitial component on the chemical composition of the alloy. Such a dependency is available in a very precise form from CALPHAD thermodynamic databases. Using the most recent databases available, the trapping enthalpies of carbon and nitrogen at various solute atoms (trapping sites) are evaluated for austenitic and ferritic steels. Good agreement with previous literature results is observed. The flexibility of the concept allows also for the treatment of trapping in a multi-component system, where different types of solute atoms are responsible for different depths of traps.

Serrated Flow in 316LN Austenitic Stainless Steel

Serrated flow behavior of the 316LN austenitic stainless steel was investigated through tensile tests at initial strain rates of 2×10-5 to 10-4 s-1 at temperatures ranging from room temperature to 1048 K. Serrated flow occurred at room temperature and 6981048K at the strain rate of 2×10-4 s-1, as well as at temperatures of 623673 K at the strain rate of 2×10-5 s-1. Type A, A+B, C and E serrations appeared. The activation energy for the occurrence of serrated flow at high temperatures was about 327 kJ/mol. The dynamic strain aging caused by the interaction between substitutional solute Cr atoms and moving dislocations is considered as the mechanism of serrated flow at the temperatures higher than 973 K.