Texture evolution in the presence of precipitates in Fe−3% Si alloy

Texture evolution by grain growth in the presence of MnS or MnS and AIN precipitates in Fe−3% Si alloy was investigated by X-ray diffraction (vector method) and ECP (selected area electron channelling pattern). Texture evolved by grain growth varied with the intensity of the inhibitors in the following manner: when an inhibitor intensity is higher than a critical value, the orientation N having the highest I_CΣi value in the matrix becomes major orientation after grain growth. Here the I_CΣi value denotes an intensity of orientations with Σi type coincidence orientation relationship with a given orientation N (except Σ1 and Σ3 type coincidence orientations). When an inhibitor intensity is weaker than the critical value, the orientation (N) having the highest P_CNΣi value in the matrix becomes major orientation after grain growth. Here the P_CNΣi value denotes an product of a intensity of the given orientation (I_N) and the I_CΣi value. With an weaker inhibitor level, I_N being more dominant and with a stronger inhititor level, I_CΣi being more dominant under the same P_CNΣi value. The inhibitor effect of P_CNΣi1 value increases with increasing in the inhibitor intensity, where P_CNΣ1 is a product of the intensity of the given orientation (I_N)     

The precursor effect of the athermal βaihω transformation in Ti-9.62Fe alloy

Ti-9.62Fe alloy was quenched from 1000°C into ice water. The athermal ω phase has been observed at room temperature. In the as-quenched alloy the resistivity, Mössbauer recoil-free fraction, and X-ray diffraction all show anomalous variations with temperature. The anomalous variations existing in a wide range of temperatures below RT are found to be reversible. The explanation of the anomalous phenomena is based on the instability of the β phase, which shows the localized soft modes. The instability can be referred to as a precursor effect of ω transformation. One of the reasons that the precursor effect can be observed in a wide range of temperatures below RT is due to the slightly higher oxygen concentration and as a result the oxygen atoms suppress the formation of the ω phase. The partially filled 3d-band of this alloy may have some influence on the precursor effect of the ω transformation.     

Overview no. 63 Non-equilibrium grain boundary segregation of boron in austenitic stainless steel—IV. Precipitation behaviour and distribution of elements at grain boundaries

The distribution of elements and the precipitation behaviour at grain boundaries have been studied in boron containing AISI 316L and “Mo-free AISI 316L” type austenitic stainless steels. A combination of microanalytical techniques was used to study the boundary regions after cooling at 0.29–530°C/s from 800, 1075 or 1250°C. Tetragonal M₂B, M₅B₃ and M₃B₂, all rich in Fe, Cr and Mo, precipitated in the “high B” (40 ppm) AISI 316L steel whereas orthorhombic M₂B, rich in Cr and Fe, was found in the “Mo-free steel” with 23 ppm B. In the “high B steel” a thin (<2 nm), continuous layer, containing B, Cr, Mo and Fe and having a stoichiometry of typically M₉B, formed at boundaries after cooling at intermediate cooling rates. For both types of steels a boundary zone was found, after all heat treatments, with a composition differing significantly from the bulk composition. The differences were most marked after cooling at intermediate cooling rates. In both types of steel boundary depletion of Cr and enrichment of B and C occurred. It was found that non-equilibrium grain boundary segregation of boron can affect the precipitation behaviour by making the boundary composition enter a new phase field. “Non-equilibrium phases” might also form. The synergistic effect of B and Mo on the boundary composition and precipitation behaviour, and the observed indications of C non-equilibrium segregation are discussed.     

A mass-spectrometric study of the thermodynamics of the Fe−Cu and Fe−Cu−C(sat) systems at 1600°C

The Knudsen cell-mass spectrometer combination has been used to study the Fe?Cu and Fe?Cu?C_(sat) alloys at 1600°C. Activity coefficients in the Fe?Cu system are closely represented by the equations $$\begin{gathered} \ln \gamma _{Fe} = 1.86N_{Cu}^2 + 0.03, (0< N_{Fe}< 0.7) \hfill \\ \ln \gamma _{Cu} = 2.25N_{Fe}^2 - 0.19, (0.7< N_{Fe}< 1.0) \hfill \\ \end{gathered} $$ with an uncertainty in the quadratic terms of about 5 pct. For the iron-rich carbon-saturated alloys, the activity coefficient of copper is given by the equation $$\ln \gamma _{Cu} = 2.45(N'_{Fe} )^2 + 0.3N'_{Fe} + 0.03, (0< N'$$ to within an uncertainty of about 10 pct. N _Fe ^′ represents the fraction N_Fe/(N_Fe+N_Cu), etc. The activity coefficient of iron in this region is found to be essentially constant at 0.69±0.05.     

Equilibrium boron distribution between Fe–C–Si–Al melt and boron-bearing slag

HSC 6.1 Chemistry software (Outokumpu) and a simplex–lattice experiment design are employed in thermodynamic modeling of the equilibrium boron distribution between steel containing 0.2% C, 0.35% Si, and 0.028% Al (wt % are used throughout) and CaO–SiO₂–Al₂)₃–8% MgO–4% B₂O₃ slag over a broad range of chemical composition at 1550 and 1600°C. For each temperature, mathematical models (in the form of a reduced third-order polynomial) are obtained for the equilibrium boron distribution between the slag and the molten metal as a function of the slag composition. The results of simulation are presented as graphs of the composition and equilibrium distribution of boron. The slag basicity has considerable influence on the distribution coefficient of boron. For example, increase in slag basicity from 5 to 8 at 1550°C decreases the boron distribution coefficient from 160 to 120 and hence increases the boron content in the metal from 0.021% when L _B = 159 to 0.026% when L _B = 121. In other words, increase in slag basicity favorably affects the reduction of boron. Within the given range of chemical composition, the positive influence of the slag basicity on the reduction of boron may be explained in terms of the phase composition of the slag and the thermodynamics of boron reduction. Increase in metal temperature impairs the reduction of boron. With increase in temperature to 1600°C, the equilibrium distribution coefficient of boron increases by 10, on average. On the diagrams, we see regions of slag composition with 53–58% CaO, 8.5–10.5% SiO₂, and 20–27% Al₂O₃ corresponding to boron distribution coefficients of 140–170 at 1550 and 1600°C. Within those regions, when the initial slag contains 4% B₂O₃, we may expect boron concentrations in the metal of 0.020% when L _B = 168 and 0.023% when L _B = 139.     

Formation of Fe16N2 and development of magnetic anisotropy in a FeN alloy

The effect of an applied stress on formation of Fe₁₆N₂ in a FeN alloy has been examined by use of a magnetic torque meter and a scanning electron microscope. Stress-assisted preferential nucleation of Fe₁₆N₂ and formation of metastable primary clusters have been detected by measuring magnetic anisotropy. It is found that uniaxial magnetization along the cube axis is not achieved in the early stage of precipitation, particularly during low temperature aging, but is achieved only after full aging at a higher temperature. On the basis of the analyses of magnetic torque curves, the possible structures of the primary clusters are proposed and compared with the microscopic observations of the precipitates.     

Electronic and structural studies of grain boundary strength and fracture in Ll2 ordered alloys—II. On the effect of third elements in Ni3Al alloy

It was proposed in the Paper I that the grain boundary of the polycrystalline Ni₃Al compound is intrinsically weak due to the heterogeneous bonding environment at the grain boundary region. In this paper, the effect of the substitutional third elements on the mergranular embrittlement of the Ni₃Al compound was systematically investigated at room temperature. Various kinds of third elements ranging from groups IIIa to Vb in the periodic table were selected and alloyed in the Ll₂ structure range. First the phenomenological aspects such as the mechanical tests, metallographic and fractographic observations were described. It was shown that the third elements (Mn and Fe) which have the similar electronic chemical bonding nature with the Ni atom prohibited the grain boundary fracture when they substitute for the Al site. Several factors responsible for these phenomena were considered. As a consequence, the value of the valency difference between the third element and constitutive solvent atom substituted by the third element seems to control the grain boundary strength of the ternary Ni₃Al compound. The grain boundary is strengthened when the value of the valency difference is positive. The possible fracture mechanism was proposed, based on the crystal structures connected with the electronic chemical bonding nature at the grain boundary region. The modification in that the electronic chemical bonding environment of the grain boundary region becomes more homogeneous by the alloying makes the grain boundary stronger.     

Effect of the Al–Zr–Y master alloy composition on the modifying effect in the Al–4% Cu alloy

The results of modifying the Al–4% Cu alloy with test Al–1.04% Zr–0.70% Y and Al–1.23% Zr–0.39% Y master alloys are reported; the Zr-to-Y atomic-percentage ratio in the alloys is 1.41 and 3.08, respectively. The effect of small amounts of Zr and Y (from 0.1 to 0.3%) added in the form of test ternary master alloys of different compositions and binary Al–Zr and Al–Y master alloys on the grain refinement in the Al–4% Cu alloy has been studied. The structure of the initial alloy is characterized by pronounced directional solidification of the α phase. As the Zr + Y content increases, the columnar-crystal zone decreases and the equiaxed-crystal zone increases; at a (Zr + Y) content of 0.326%, only equiaxed crystals ~200 μm in size are present in an ingot. When Zr and Y are added with binary master alloys, the macrostructure of the modified Al–4% Cu alloys indicates that columnar crystals grow until their contact at the center of the ingot, and their growth is independent of the amount of added Zr and Y.     

The effect of elastic anisotropy on the migration of intergranular liquid films and the precipitation of a liquid phase in a CoCu alloy

When a Co20Cu (wt%) alloy prepared by liquid phase sintering at 1300°C is heat-treated at 1150°C, the intergranular liquid films and grain boundaries migrate, leaving behind a new solid solution with reduced Cu content. The phenomenon is identical to that observed previously in MoNi. The calculated driving force for the migration is almost equal to the coherency strain energy in the solute diffusion zone ahead of the migrating boundaries. The migrating liquid films show faceting with curved planes and edges. Their shape closely resembles the growth shapes predicted on the basis of the coherency strain energy with anisotropic elastic constants. During the heat-treatment, liquid precipitates form within grains and at grain boundaries, and they also show faceting, which is consistent with orientation dependent coherency strain energy in the diffusion zone around the precipitates.     

Observations of the effect of an applied stress on the morphology of discontinuous precipitation in a CuCd alloy

The influence of an applied tensile stress on the morphology and growth rate of the discontinuous precipitation product in a Cu−3.8 wt% Cd alloy was first reported by Sulonen. In this contribution, we investigate and further quantify a number of factors associated with the role of applied stress in this alloy, including absolute and relative growth rates of interfaces with normals parallel to and perpendicular to the tensile axis, and the morphological stability of discontinuous precipitation front as a function of the applied stress. Once morphological instability has occurred, the amplitude and wavelength of the protuberences formed depends on the value of the applied stress, and on the angle between the average interface normal and the tensile axis. It is suggested that the process is best viewed as one of transformation-assisted viscoelastic deformation.     

An evaluation of the flow behavior during high strain rate superplasticity in an Al−Mg−Sc alloy

An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10⁻² s⁻¹. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673 K. An analysis of the experimental data for the equal-channel angular (ECA)—pressed samples shows consistency with conventional superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary diffusion in pure Al and interdiffusion in Al−Mg solid solution alloys. MINORU NEMOTO, formerly Professor, Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University.     

Activities in the system LiF−NaF−AlF3

Measurements have been made of the contents of Na and Li in Al in equilibrium with the molten fluorides at 1020 °C. The theory to calculate the activities of the three constituents is derived. Across the Li₃AlF₆-Na₃AlF₆ section the activity coefficients {ie409-1} are given in terms of mol fractionsN _i by $$ \begin{gathered} \log \gamma _{{\text{AIF}}_{\text{3}} } = - 3.034 + 3.342N_{LiF} - 0.848(N_{LiF} )^2 \hfill \\ \log \gamma _{{\text{NaF}}} = - 0.246 - 1.114N_{LiF} - 0.283(N_{LiF} )^2 \hfill \\ \log \gamma _{LiF} = 0.158 - 0.266N_{LiF} - 0.283(N_{LiF} )^2 \hfill \\ \end{gathered} $$ Across the LiF?Na_2.5AlF_5.5 section the activity coefficients for 0≤N _LiF≤0.45 are nearly constant at log \Gg_LiF \t~ 0.1, log \Gg_NaF \t~ 0.4, and log \Gg_{AIF 3} \t~ -2.6.The vapor over these melts is a mixture of LiAlF₄ and NaAlF₄, the pressures being given byp _{LiAlF 4}/bar=0.78a _LiF·a _{AlF 3} andp _{NaAlF 4}/bar=56.2a _NaF·a _{AlF 3}. Combination of these equations with those for the activity coefficients reproduces the maximum observed in the total pressure in the Li₃AlF₆?Na₃AlF₆ section. The increase in pressure observed when Li₃AlF₆ is added to Na₃AlF₆ is due, not to the appearance of LiAlF₄ in the gas, but to the increased pressure of NaAlF₄ following the rise in AlF₃ activity.     

Formation regularities of grain-boundary interlayers of the α-Ti phase in binary titanium alloys

The microstructure of polycrystalline alloys of titanium with chromium (2, 4, and 5.5 wt %), cobalt (2 and 4 wt %), and copper (2 and 3 wt %) is investigated. Series of prolonged isothermal annealing of these materials are performed in a temperature range from 600 to 850°C (in vacuum). Annealing temperatures fall in two-phase regions α(Ti,Me) + β(Ti,Me) of phase diagrams Ti–Cr, Ti–Co, and Ti–Cu. Temperature dependences of the fraction of grain boundaries β(Ti,Me)/β(Ti,Me) completely “wetted” by interlayers of the second solid phase α(Ti,Me) and average contact angle are measured. The results of microstructural investigations showed that the type and concentration of the second component in the alloy strongly affect the formation of equilibrium grain-boundary interlayers. A nonmonotonic temperature dependence of the fraction of grain boundaries completely wetted by interlayers of the second solid phase in the absence of ferromagnet–paramagnet phase transformations in the volume is revealed for the first time.