Solid Solution Hardening of Intermetallic Compounds

An electron work function based mechanism for solid solution hardening

In our work, the model of self-consistent electron–deformation–diffusion effects in thin films grown on substrate with the mismatch of lattice parameters of the contacting materials is constructed. The proposed theory self-consistently takes into account the interaction of the elastic field (created by the mismatch of lattice parameters of the film and the substrate, and point defects) with the diffusion processes of point defects and the electron subsystem of semiconductor film. Within the framework of the developed model, the spatial distribution of deformation, concentration of defects, conduction electrons and electric field intensity is investigated, depending on the value of the mismatch, the type of defects, the average concentrations of point defects and conduction electrons. It is established that the coordinate dependence of deformation and the concentration profile of defects of the type of stretching (compression) centers, along the axis of growth of the strained film, have a non-monotonic character with minima (maxima), the positions of which are determined by the average concentration of point defects. It is shown that due to the electron–deformation interaction in film with a lattice parameter mismatch, the spatial redistribution of conduction electrons is observed and n-n+ transitions can occur. Information about the self-consistent spatial redistribution of point defects, electrons and deformation of the crystal lattice in semiconductor materials is necessary for understanding the problems of their stability and degradation of nano-optoelectronic devices operating under conditions of intense irradiation.

The influence of solute atoms on the athermal component of the flow stress, determined by means of dip-tests (incremental unloading), has been investigated at room temperature and slightly above in binary Ta-Re, Ta-Mo, Ta-W, Ta-Hf, Ta-Zr, and Ta-Nb alloys and in ternary Ta-W-Re, Ta-W-Mo, Ta-W-Hf, and Ta-W-Nb alloys. Binary athermal substitutional solid solution hardening in tantalum is linear up to high concentrations of solute and is dominated by the atomic size misfit parameter, in agreement with the authors’ recent model for binary athermal solid solution hardening in bcc metals at temperatures where the Peierls stress is still important. In this model, solid solution hardening is caused by interactions of solute atoms having a size misfit with polarity reversing kinks and constrictions in 〈111〉 screw dislocations. The observed solid solution hardening in the ternary alloys is well described by the authors’ phenomenological model for multicomponent solid solution hardening.

Deuterated potassium dihydrogen phosphate (K(DxH1-x) 2PO4) crystals with different deuteration levels (x=0, 0.51, 0.85) were grown by conventional cooling method from deuterated solutions at Shandong University. Positron annihilation spectroscopy has been widely used to the study on micro-defects of semiconductors and other materials, which is very sensitive to the crystal structure, defect types, defect concentrations, and so on. In this paper, positron annihilation spectroscopies (positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy), combined with X-ray diffraction (XRD) are used to investigate micro-defects characterization in K(DxH1-x) 2PO4 crystals. Influences of deuteration degree on the crystal structure characteristics, defect types and concentrations are discussed. It can be concluded from XRD experiments that the lattice parameters of a and b increase with the increase in deuteration levels, while no obvious change occurs on the lattice parameter c. KH2PO4(KDP) crystals at low deuteration level and high deuteration level could be regarded as low deuterium-doped KDP crystal and low hydrogen-doped DKDP crystal respectively. It is indicated that the higher the replacement ratio in the crystals, the weaker the diffraction peak they show. Positron annihilation lifetimes increase clearly in the highly-deuterated KDP crystals. It is found that neutral interstitial defects and oxygen defects in the KDP crystal increase with increasing deuteration degree. And these types of defects can be attributed to lattice distortion effect. From positron annihilation lifetime results we can arrive at another conclusion that the compound defects will form and defects concentration is declined, when hydrogen vacancies, K vacancies and substitutional impurity defects continue to react by means of association reactions. These phenomena suggest that high deuteration plays a significant role in promoting association reaction of internal defects in the crystals. Furthermore, the polymerization reaction of the clusters and micro-cavities continue to occur, therefore defect concentrations will show a constant decrease. Doppler broadening spectra show that the internal defects in the crystals increase integrally with an increase of deuteration level; this agrees well with the results of positron annihilation lifetime. Moreover, Doppler broadening spectra indicate that the proportional change of these defects is synchronous and consistent with the actuality. To sum up, our experimental results suggest that the defect reaction is weak in low degree of KDP crystal deuteration growth (less than 50%), while reaction is enhanced in the high degree of deuteration growth (higher than 50%).

Defect structure in oxygen-excess manganese-magnesium ferrite

Defect structure of β-LiAl

Solute in solution hardens the basal planes but causes solid solution softening on the prismatic (and possibly pyramidal) planes in dilute (< 0.5 at.% Zn) Mg-Zn alloys. Solid solution softening lowers the strain-hardening rate and increases the ductility of the alloys with respect to the pure Mg metal. In the concentrated (0.5 ∼ 2.4 at.% Zn) alloys, solid solution hardening of the basal planes is extensive but it is not clear whether solid solution softening of the secondary slip systems still occurs. Therefore, solid solution effects on the strain hardening rate and ductility of cast polycrystalline Mg-Zn alloys, with Zn contents between 0 and 2.4 at.% have been studied. A constant grain size was obtained in all alloys by adding a small amount of Zr to the melt. The strain-hardening rate is low for dilute concentrations increasing monotonically above 1 at.% Zn. The ductility goes through a maximum at very low concentrations of Zn, decreasing for higher concentrations. This suggests that the solid solution hardening gradually offsets the solid solution softening effects at high concentrations of Zn.

MoSi2 has recently been investigated as a potential material for high temperature structural applications. It has excellent oxidation resistance up to 1700°C, a high melting temperature, 2030°C, and a brittle-to-ductile transition temperature at 900-1000°C. WSi2 is isomorphous with MoSi2 and has a body-centered tetragonal unit cell of the space group 14/mmm. The lattice parameters are a=3.20 Å and c=7.84 Å for MoSi2 and a=3.21 Å and c=7.88 Å for WSi2. Therefore, WSi2 was added to MoSi2 to improve its strength via solid solution hardening. The purpose of this study was to investigate the slip systems in polycrystalline MoSi2/WSi2 alloys.

Ni and Ni-W binary alloys are basis for nickel based superalloys. For most nickel based superalloys, strengthening mechanisms include both solid solution hardening and precipitation hardening. W is a vital element to create solid solution hardening and to improve the creep strength. In spite of its wide usage to strengthening of high temperature alloys, the mechanisms for solid solution hardening are not fully quantified. From the assumption that it is due to the attraction of solute atoms to dislocations and formation of Cottrell atmosphere to slow down the motion of dislocations, a fundamental model has been formulated previously. In the present paper, the model is expanded by taking the stacking fault energy and strain induced vacancies into account. Important parameters in the model are the variation of the lattice constant and the shear modulus with alloying content. Models for these variations have been formulated as a function of solute content. Another important parameter is the maximum interaction energy between the dislocations and the solutes. The model can satisfactorily predict both the large difference in creep rate between pure Ni and Ni-W alloys and the comparatively smaller differences between the three investigated Ni-2W, Ni-4W and Ni-6W alloys.

The application of dilatometry for studies of non-stoichiomstric defucts in oxides

First-principles electronic structure calculations have been performed for defect structure in non-stoichiometric CoAl and CoTi. In order to determine the type of constitutional defects, the compositional dependence curves both of formation energies and of lattice parameters are obtained by the calculations employing supercells in various sizes. The defect formation energies are calculated with taking into account the compositional dependence of the chemical potential. The calculated results suggest that the Co vacancy is the dominant thermal-excitation defect even in the Co-rich side near the stoiciometry in CoTi. Intermetallic compounds with the B2 structure have attracted considerable attention as promising candidates for high-temperature materials. One of the important features of intermetallic compounds with the B2 structure is having a wider range of compositions than other phases. In order to compensate the deviation from the stoichiometry, constitu- tional defects are introduced. Several properties such as mechanical and magnetic properties, therefore, show com- positional dependence originating from the constitutional defects. In addition, thermally excited defects play an important role in the diffusion of atoms and therefore the understanding of the defects are required for the alloy design and optimization of heat treatment. Electronic structure calculations using supercell models have been performed for non-stoichiometric B2 intermetallic compounds to understand the defect structure. The size of the supercell is one of the important factors for electronic structure calculations including defects. The defect formation energies in FeAl were calculated using the 8, 16 and 32 atoms supercells. 1) The calculations with the 16 and 54 atoms supercells were performed for convergence tests of the effective formation energies and the structural relaxation of the constitutional defects in NiAl. 2) In our previous work, we

Introduction All-solid-state batteries (ASSBs) utilize non-flammable solid electrolytes, offering enhanced safety. In addition, the application of lithium metal anodes is expected to further improve energy density. Among solid electrolytes, oxide-based solid electrolytes have attracted increasing attention due to their higher stability under atmospheric conditions compared to sulfide-based solid electrolytes. However, oxide-based solid electrolytes exhibit lower ionic conductivity compared to sulfide-based and liquid electrolytes, leading to inferior performance on high-rate cycling.We focused on a novel oxide-based solid electrolyte, lithium lanthanum niobium oxyfluoride (Li1.25La0.58Nb2O6F, LLNOF) [1], which has a bulk ionic conductivity of 7 mS cm-1, which is the highest among oxide-based solid electrolytes reported so far. However, the electrolyte with a high relative density is difficult to be prepared, and the residual voids in the electrolyte easily allow lithium dendrite formation, leading to short circuits. Consequently, improving the sintered density remains an important issue for the LLNOF electrolyte to be used in practical cells. We have reported that the co-addition of alumina (Al2O3) [2] and lithium tungstate (Li2WO4) [3] as sintering aids increased the relative density of lithium lanthanum zirconium tantalate (Li6.4La3Zr1.4Ta0.6O12, LLZT) [4]. In this study, we investigated the effects of various sintering aids on the densification and ionic conductivity of the LLNOF electrolyte. Experimental LLNOF powder used in this experiment was provided by DENSO Corporation [1]. Sintering aids such as Li2WO4, Al2O3, Li2ZrO3, Li2MoO4, and Li2.2C0.8B0.2O3 (LBCO)[5] were used, with each added at 2 wt.%. The LLNOF powder and sintering aids were mixed using a planetary ball mill. A powder of 0.5 g was then pressed into a green pellet at 98 MPa. The green pellet was wrapped in copper foil and sintered at 1000 ℃ for 6 h under Ar atmosphere. The sintered pellet was dry polished to adjust the thickness at 1.0 mm, and Au blocking electrodes were sputtered on both sides of the pellet for conductivity measurements. AC impedance measurements were performed at an applied voltage of 10 mV over the frequency range from 7 MHz to 1 Hz in the temperature range 60 to -20°C. Results and Discussion Table 1 summarizes the relative density, ionic conductivity, and lattice parameters of LLNOF electrolytes with and without sintering aids. The highest relative density (92.7%) was obtained when Li2WO4 was added. However, the bulk ionic conductivity decreased to 2.73 mS cm-1, which was lower than that of LLNOF without sintering aids. The increase in relative density can be attributed to the fact that Li2WO4 has a melting point (742°C) lower than the sintering temperature, leading to the formation of a liquid phase that enhances sintering density.Figure 1 shows the relationship between lattice parameters and ionic conductivity. The ionic conductivity decreased with a decrease in the lattice parameter except for Li2ZrO3. The highest ionic conductivity was observed for the sample prepared without sintering aids. This can be attributed to elemental substitution within LLNOF caused by the addition of sintering aids. Figure 2 shows the cross-sectional SEM images of LLNOF pellets. The pellets prepared with Li2WO4 and LBCO, which generate a liquid phase, exhibit relatively larger grain sizes and fewer voids, resulting in higher relative densities. On the other hand, the sample with Al2O3 has a relatively higher number of residual pores within the grains, which is considered to have led to a decrease in relative density. These results suggest that liquid-phase sintering facilitates the densification of LLNOF.

Hardening behavior of Pt–Ti microalloys

3 - Vacancy and defect structures in metal oxides

Crystalline materials exhibit long-range ordered lattice unit, within which resides nonperiodic structural features called defects. These crystallographic defects play a vital role in determining the physical and mechanical properties of a wide range of material systems. While computer vision has demonstrated success in recognizing feature patterns in images with well-defined contrast, automated identification of nanometer scale crystallographic defects in electron micrographs governed by complex contrast mechanisms is still a challenging task. Here, building upon an advanced defect imaging mode that offers high feature clarity, we introduce DefectSegNet - a new convolutional neural network (CNN) architecture that performs semantic segmentation of three common crystallographic defects in structural alloys: dislocation lines, precipitates and voids. Results from supervised training on a small set of high-quality defect images of steels show high pixel-wise accuracy across all three types of defects: 91.60 ± 1.77% on dislocations, 93.39 ± 1.00% on precipitates, and 98.85 ± 0.56% on voids. We discuss the sources of uncertainties in CNN prediction and the training data in terms of feature density, representation and homogeneity and their effects on deep learning performance. Further defect quantification using DefectSegNet prediction outperforms human expert average, presenting a promising new workflow for fast and statistically meaningful quantification of materials defects.