Optical properties and spectral emissivities at 632.8 nm in the titanium-aluminum system

The spectral emissivities, refractive indices, and extinction coefficients of liquid titanium-aluminum alloys were measuredvs temperature by He-Ne laser ellipsometry at 632.8 nm. Results were obtained on overheated liquids and on liquids undercooled by as much as 350 K. The experiments were conducted under containerless conditions using electromagnetic levitation and heating supplemented by CO₂ laser beam heating. The undercooled melts exhibited spectral emissivities that were nearly independent of temperature, and emissivities varied slightly with temperature for the stable liquids. The spectral emissivities showed a maximum at near-equiatomic compositions and a minimum at a composition near pure liquid aluminum. The temperature coefficients of emissivity were negative for near-equiatomic compositions and positive for aluminum-rich alloys. The emissivity was determined to be independent of temperature at a composition of approximately 75 at. pct Al. This was termed the “X-point composition,” because it is the composition at which, for a wavelength of 632.8 nm, the spectral emissivityvs composition isotherms cross. Significant errors in noncontact temperature measurements would result if the spectral emissivities were estimated from a linear interpolation between the pure elements. Formerly with the Department of Materials Science and Engineering, Vanderbilt University     

Temperature and compositional dependencies of normal spectral emissivities at 632.8 nm for solid Cu-Ni alloys—Ellipsometric measurements and formulation of empirical prediction equation

Normal spectral emissivities of solid Cu-Ni alloys were determined using an ellipsometer combined with an electric furnace for a wavelength of 632.8 nm in the temperature ranges between 800 K and the respective solidus temperatures. The emissivities of the alloys had slightly positive temperature coefficients and seemed to be on quadratic functions of the atomic fraction of nickel. Thermal radiation of transition metals in the visible and near-infrared regions is generally considered to stem from the excitation-relaxation processes of free electrons and inner shell electrons, i.e., electrons at the d-like levels in the Cu-Ni system. Thus, the contribution from the former to the emissivities has been calculated on the basis of the free-electron model with damping. The difference between measured and calculated values corresponds to the contribution from inner shell electrons, which has been found to depend strongly upon neither temperature nor chemical composition in the range of the nickel atomic fraction, X _Ni>0.2. On the basis of these findings, an empirical prediction equation for the emissivity at 632.8 nm for solid Cu-Ni alloys has been proposed: this equation applies at temperatures higher than 800 K in the range of X _Ni>0.2 and can predict emissivity with an uncertainty less than 13 pct.     

Experimental studies on hydrogen solubility in liquid ternary iron-nickel-chromium alloys

The hydrogen solubilities in liquid ternary iron-nickel-chromium alloys have been experimentally determined by Sieverts’ method. The experimental data have been evaluated with the help of regression analysis (Gauß-Jordan) and using own published empirical equations (polynomials). With the help of experimentally determined hydrogen solubilities and the coefficients of the polynomials, the concentration dependencies of hydrogen solubilities, interaction coefficients, enthalpies and entropies of hydrogen solution have been determined. The hydrogen solubility increases with increasing temperature and with an increase in the nickel and chromium concentration. The results have been represented as isothermal planes. The hydrogen solubilities in liquid ternary iron-nickel-chromium alloys (x_Cr ≤ 50%) have been predicted with the help of “Central Atoms” model, assuming the concentration dependencies of the “model parameter λ” and selecting the value of the coordination number (Z’ = 10) for each of the “reference elements” iron or nickel. The comparison between the experimentally determined and the predicted hydrogen solubilities in iron-nickel-chromium alloys confirms that the prediction of hydrogen solubilities in liquid system iron-nickel-chromium can be qualitatively made over a wide range of alloy concentrations (x_Cr = 50%) with the aid of “Central Atoms” model and with the assumption of concentration dependencies of the “model parameter λ”.     

The dissolution behavior of metals from Ag/Cu and Ag/Au alloys in acidic and cyanide solutions

The dissolution behavior of silver, copper, and gold from silver/copper and silver/gold alloys in acid and cyanide solutions has been investigated using a rotating disc. Oxygen or ferric ion was used as an oxidant. The effect of composition of alloys, rotation speed, concentration of lixiviants, and temperature on the overall dissolution kinetics of these metals has been studied. The dissolution rate of silver was reduced due to galvanic effect, when it was leached from a silver/copper alloy disc in acidic media at relatively low oxidation potentials. On the other hand, when the oxidation potential was high, the galvanic effect of the dissolution rate became less pronounced. In the case of the silver/gold system in cyanide solutions, the effect of alloy composition on the dissolution rate of gold and silver was not as much as in the case of silver/ copper alloy systems. This may be attributed to the fact that both gold and silver are noble, and therefore, the electromotive forces of these two metals are not significantly different. Formerly with the South Dakota School of Mines and Technology Formerly Visiting Professor, South Dakota School of Mines and Technology     

Effect of specimen geometry, gage length, and width measurement locations on plastic strain ratio (R-value) in sheet metals

The effects of gage length, width measurement locations, and specimen geometry on plastic strain ratio have been investigated for the AA8011 aluminum alloy and interstitial-free (IF) steel sheets. The specimens were ASTM E 517 to 92a subsize, type A, and type A alternative with, respectively, 37.5, 76, and 57 mm reduced parallel sections and with different fillet radii. In each specimen type, there were differences in axial strains of gage marks and changes in width strain over the reduced parallel section, which depend on the applied axial strain, reduced parallel section legth, and fillet radius. Therefore, the magnitudes of the calculated R-values depend upon gage length, width measurement location, axial strain, and specimen geometry. These dependencies were more pronounced in the high R-value IF steel sheet relative to the low R-value AA8011 aluminum alloy sheet. The dependencies of R-value on gage length and width measurement location are negligible in all AA8011 specimens, while in IF specimens, these dependencies can be neglected only for type A specimens with 12.5-mm fillet radius. It is concluded that the observed differences in the measured R-values for specimens with different geometries can be attributed to the constraints imposed by the shoulders, which affect the width strain measurements and the resulting R-values.     

Discontinuity in normal spectral emissivity of solid and liquid copper at the melting point

Normal spectral emissivities for solid (ɛ_λ,solid) and liquid (ɛ_λ,liquid) copper at the melting point have been determined, and the discontinuity in ɛ_λ,liquid and ɛ_λ,solid at the melting point has been discussed. The spectral emissivity was determined by comparing the spectral radiation intensity of the sample to that of the fixed-point blackbody over the wavelength range 550 to 750 nm. The samples and fixed-point blackbody were heated in the continuous casting type cold crucible. Values of ɛ_λ,liquid and ɛ_λ,solid at 650 nm were 0.167 and 0.108, respectively. The emissivity ratio of ɛ_λ,liquid/ɛ_λ,solid increased from approximately 1.2 to 1.8 as the wavelength increased in the range 550 to 750 nm. The discontinuity in ɛ_λ,liquid and ɛ_λ,solid at the melting point has been qualitatively explained by the optical absorption mechanism due to the intraband transition.     

Determination of phosphorus in gold or silver brazing alloys

A spectrophotometric method has been devised for measuring microgram levels of phosphorus in brazing alloys of gold or silver that contain high nickel concentration. Phosphorus in such alloys is normally measured by solid mass spectrometry, but the high nickel concentration produces a double ionized nickel spectral interference. The described procedure is based upon the formation of molybdovanadophosphoric acid when a molybdate solution is added to an acidic solution containing orthophosphate and vanadate ions. The optimum acidity for forming the yellow colored product is 0.5 N hydrochloric acid. The working concentration range is from 0.1 to 1 ppm phosphorus using 100 mm cells and measuring the absorbance at 460 nm. The sample preparation procedure employsaqua regia to dissolve the alloy and oxidize the phosphorus to orthophosphate. Cation exchange chromatography is used to remove nickel ions and anion exchange chromatography to remove gold ions as the chloride complex. Excellent recoveries are obtained for standard phosphorus solutions run through the sample procedure. The procedure is applicable to a variety of gold or silver braze alloys requiring phosphorus analysis.     

Effect of threshold stress intensity on fracture mode transitions for hydrogen-assisted cracking in AISI 4340 steel

Fracture mode transition in hydrogen-assisted cracking (HAC) of AISI 4340 steel has been studied from an equilibrium aspect at room temperature with 8.6-mm-thick double cantilever beam (DCB) specimens. The threshold stress intensity,K _th , necessary for the occurrence of HAC and the corresponding fracture surface morphology have been determined as a function of hydrogen pressure and yield strength. The K _th increases with decrease in hydrogen pressure at a given yield strength and also with decrease in yield strength at a given hydrogen pressure. AsK _th increases, the corresponding HAC fracture mode changes from the intergranular (IG) and quasi-cleavage (QC) modes to the microvoid coalescence (MVC) mode. The experimental results indicate that the critical hydrogen concentration for crack extension in the IG mode is higher than that for crack extension in the MVC mode. The fracture mode transition with varying hydrogen pressure and yield strength is discussed by simultaneously considering the micromechanisms for HAC and the hydrogen pressure and yield strength dependencies ofK _th .     

Combined hardening of alloy Ti-6Al sheet workpieces during reversible hydrogen alloying

The effect of the initial hydrogen concentration, warm rolling, and vacuum annealing conditions on the formation of the phase composition, structure, and mechanical properties of rolled sheet workpieces made of a Ti-6Al α alloy is studied. When the initial hydrogen concentration increases to C _Hⁱⁿⁱ = 0.3–0.9%, the grain size decreases and the phase composition of the alloy is complicated. In the grain size range 27–5 μm, the yield strength of the alloy obeys the Hall-Petch relation with the lattice friction stress σ _i = 662 MPa. When the initial hydrogen concentration increases, the grain-boundary hardening intensity and the yield strength increase. At an average α grain size of 5 μm, the yield strength increases from 770 MPa in the alloy with C _Hⁱⁿⁱ = 0.004% to 970 MPa in the alloy with C _Hⁱⁿⁱ = 0.7%. The maximum yield strength (σ_y = 1064 MPa) is obtained for the alloy with C _Hⁱⁿⁱ= 0.5% after vacuum annealing at 650°C. The conditions and contributions of solid-solution hardening, grain-boundary hardening, precipitation hardening induced by the formation of the α₂ phase, and strain hardening to the total hardening of the alloy are considered.     

Quantitative analysis of the relative basicity of CaO and BaO by silver solubility in slags

The solubility of silver in molten CaO-B₂O₃ and BaO-B₂O₃ slags at high temperatures was measured to seek a new measure of the basicity of slags. The B₂O₃-bearing fluxes, which have wide range of liquids at the temperature of interest, have been applied to investigate the effects of flux composition on the solubility of silver from the saturation of acidic or basic components. The solubility of silver in slags has a minimum value as a function of flux composition, and it is suggested that silver behaves as an amphoteric substance and dissolves by different mechanisms in acidic and basic fluxes. The solubility of silver decreased with the increasing temperature and content of basic oxides in the acidic region, and vice versa in the highly basic region. In the acidic region, BaO is more basic than CaO by, at most, about 33 pct, based on the difference of the solubility of silver in both slags, indicating that BaO is not a much-stronger basic oxide than CaO in the acidic slags. In the highly basic region, BaO is about 5 times more basic than CaO. The solubility of silver in slags was compared to the nitride capacity, showing that the dissolutions of silver and nitrogen into the melts are similar to each other. The relationship between Ag solubility and theoretical optical basicity was also discussed.     

The normal spectral emissivity of liquid cerium-copper alloys

The normal spectral emissivities (⋋ = 0.645 nm) of liquid Ce-Cu alloys were measured over the complete range of composition by the integral blackbody comparison method. Composition analyses of the solidified and segmented samples confirmed their homogeneity. It was found that the observed values of the normal spectral emissivity for every composition was constant over the experimental temperature range studied. The composition dependence of the emissivity can be expressed by the equation∃ _nλ= 0.2920−0.0650X + 0.2242X ² + 1.1381X ³-1.4773X ⁴ where∃ _nλ is the normal spectral emissivity at λ = 0.645 Μ andX is the mole fraction of copper. Formerly Graduate Assistant, Department of Chemical Engineering, Iowa State University.     

Influence of the pH of the medium on the anodic behavior of scandium-doped Zn55Al alloy

The anodic behavior of the Zn55Al scandium-doped alloy is investigated. The extreme character of the dependence of the corrosion potential of this material on the content of Sc in it is shown and it is established that an increase in the concentration of chloride ions in the electrolyte promotes a decrease in this characteristics. The pitting-formation and repassivation potential displace to the positive region as the content of the doping elements in the alloys increases, while as the concentration of chloride ions decreases, they shift to the negative region over the entire pH range of the medium. The corrosion rate of the Zn55Al alloy decreases by a factor of 2–3 with the addition of 0.005–0.05 wt % Sc into it, which makes it possible to recommend an alloy of this composition as an anode coating for the anticorrosion protection of steel constructions, wares, and buildings.     

Nonequilibrium grain-boundary segregation and ductile-brittle-ductile transition in Fe-Mn-Ni-Ti age-hardening alloy

Nonequilibrium segregation kinetics of alloying elements and a ductile-brittle-ductile transition behavior have been investigated in an Fe-8.4Mn-7.4Ni-l.7Ti alloy. The alloy experienced a ductilebrittle-ductile (DBD) transition during isothermal aging. In the brittle region, the alloy showed a decrease in intergranular fracture strength and a subsequent increase with aging time. This is due to the segregation of titanium to the grain boundaries and its desegregation into the matrix. The intergranular fracture strength was higher on the zero tensile elongation finish curve than on the start curve. This is because the grain-boundary segregation level of titanium is relatively lower on the finish curve. The lowest intergranular fracture strength increased with increasing aging temperature, which was attributed to a lower grain-boundary segregation level of titanium at higher temperature. Manganese caused an overall reduction in intergranular fracture strength and, as a result, the delayed occurrence of the zero tensile elongation (ZTE) finish curve in a temperature and log-time plot. formerly Graduate Student, Department of Metallurgical Engineering, Seoul National University, Seoul, 151-742 Korea     

Use of Alloying to Effect an Equiaxed Microstructure in Additive Manufacturing and Subsequent Heat Treatment of High-Strength Titanium Alloys

This paper addresses the use of alloying additions to titanium alloys for additive manufacturing (AM) with the specific objective of producing equiaxed microstructures. The additions are among those that increase freezing ranges such that significant solutal undercooling results when combined with the rapid cooling rates associated with AM, and so be effective in inducing a columnar-to-equiaxed transition (CET). Firstly, computational thermodynamics has been used to provide a simple graphical means of predicting these additions; this method has been used to explore additions of Ni and Fe to the alloy Ti–6Al–4V (Ti64). Secondly, an experimental means of determining the minimum concentration of these alloying elements required to effect the CET has been developed involving gradient builds. Thirdly, it has been found that additions of Fe to Ti64 cause the alloy to change from an α/β Ti alloy to being a metastable β-Ti alloy, whereas additions of Ni do not produce the same result. This change in type of Ti alloy results in a marked difference in the development of microstructures of these compositionally modified alloys using heat treatments. Finally, hardness measurements have been used to provide a preliminary assessment of the mechanical response of these modified alloys.

     

Mechanical behavior of a cryomilled near-nanostructured Al-Mg-Sc alloy

In the present study, the mechanical behavior of a cryomilled Al-7.5 pct Mg-0.3 pct Sc alloy was investigated at temperatures in the range of 298 to 648 K. The grain size of the as-extruded alloy was determined to be approximately 200 nm by transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis. The data indicate that as a result of cryomilling, a supersaturated solid solution with high thermal stability was formed in the Al-Mg-Sc alloy. The high strength at room temperature was primarily attributed to three types of strengthening: grain size effect, solid solution hardening, and Orowan strengthening. The elevated temperature mechanical behavior of the Al-Mg-Sc alloy exhibits the following: (a) a strain-rate sensitivity, m, of less than 0.2; and (b) an activation energy, Q, that increases from 139 to 193 kJ/mol with increasing applied stress. An analysis of the experimental data at elevated temperatures shows that despite the fine-grained structure of the alloy, the deformation characteristics are not consistent with those arising from a superplastic deformation process that incorporates a threshold stress. On the other hand, the analysis suggests that the deformation characteristics agree with those associated with the transition in the creep behavior of Al-based solid solution alloys from that for the intermediate-stress region, where m=0.33 and Q=Q _D (Q _D is the activation energy for self-diffusion in Al), to that of the high-stress region, where m<0.2 and Q>Q _D .     

FeCrVTa0.4W0.4高熵合金氮化物薄膜的微观结构与性能

实验利用单靶射频磁控溅射技术,在单晶硅基底上,制备了两个系列FeCrVTa_0.4W_0.4高熵合金氮化物薄膜,即FeCrVTa_0.4W_0.4氮化物成分梯度多层薄膜和(FeCrVTa_0.4W_0.4)N_x单层薄膜,其中,多层薄膜用于太阳光谱选择性吸收薄膜。通过扫描电子显微镜(SEM)、X射线衍射仪(XRD)、纳米力学探针、原子力显微镜(AFM)、紫外?可见分光光度计、接触角测量仪和四探针测试台对FeCrVTa_0.4W_0.4高熵合金氮化物薄膜进行微观结构分析以及性能表征。结果表明:在不通入氮气时,薄膜为非晶结构,当氮气含量升高后,转变为面心立方固溶体结构;当表层氮气流量为15 mL·min?1时,FeCrVTa_0.4W_0.4氮化物多层薄膜及单层薄膜均具有最佳的力学性能,其中,多层薄膜的硬度为22.05 GPa,模量为287.4 GPa,单层薄膜的硬度为22.8 GPa,模量为280.7 GPa,随着表层氮气含量的继续增加,力学性能下降;FeCrVTa_0.4W_0.4氮化物成分梯度多层薄膜在300～800 nm波长范围内均具有太阳光谱选择吸收性,当氮化物薄膜层数较少时具有较好的疏水性;(FeCrVTa_0.4W_0.4)N_x单层薄膜随着氮气含量的增加,薄膜方块电阻增加。      

High resolution study of ordering reactions in gold-chromium alloys

The ordering reactions in Au₄Cr and Au₃Cr alloys have been investigated using high resolution dark field and lattice imaging techniques. In all cases the ordered structures can be described in terms of compositional modulations as occur during spinodal decomposition. In alloys quenched from above the critical temperature sro microdomains of size 10 to 15? have been resolved with structures represented by modulations in composition along <420>. After aging the Au₄Cr alloy belowT _c the Dla lro phase appears as ordered domains of size 50 to 100? showing composition modulations with wavelength of 5d<531>. These domains grow in size up to 500? becoming finally the D1a fully ordered structure. A new observation has been mdde in the Au₃Cr alloy,viz that the lro structure is nonstoichiometricL1₀ withλ _[001] = 4?. The transformation from sro in this alloy occurs with the formation of a transitional ordered phase, (similar toDO ₂₂, which can be described in terms of composition modulations along [001], with wavelength equal to thec parameter (8?) of theDO ₂₂ structure. Lattice imaging of the two variants of superlattice planes in ordered Au₄Cr enabled the atomic arrangement in the area of translational and rotational antiphase boundaries to be determined. J. Dutkiewicz is on one year leave from the Institute of Metallurgy, Polish Academy of Sciences, 30-059 Krackow, ul. Reymonta 25, Poland.     

Effect of composition on the formation and thermal stability of Ni72(Mo,Co)8B20 metallic glass

The effect of the addition of Co and Mo on the formation and stability of a Ni-B amorphous alloy has been investigated. Metal ribbons were prepared by melt-spinning in vacuum and characterised by X-ray diffraction and isochronal differential scanning calorimetry. When no Mo is present, only crystalline material containing the phases Co₃B and Ni is produced. As the content of Mo is increased, the distance between metal-metal nearest neighbours increases, the glass transition and the crystallisation temperatures increase, the enthalpy of crystallisation decreases, and there is a marked increase in the activation energy associated with crystallisation. The explanation for this pattern of glass formation and the increase in thermal stability induced by Mo has been explored by reference to similar systems and to a semi-empirical kinetic model of the thermal stability of amorphous alloys. This model predicts the dependence of crystallisation temperature on composition in this quaternary alloy from the enthalpies of hole formation of its constituent elements with good success.     

The effect of iron and manganese on the recrystallization behavior of hotrolled and solution-heat-treated aluminum alloy 6013

The influence of manganese and iron additions on the recrystallization behavior of a hot-rolled Al-Mg-Si-Cu alloy has been investigated. In the as-cast ingot, some manganese remains in supersaturated solid solution and, during the preheating step, precipitates as fine (0.1 to 0.3 μm) dispersoidsα- Al₁₂Mn₃Si and α(AlFeMnSi), depending on the local iron content. The presence of the dispersoids increases the alloy’s resistance to recrystallization. The remaining manganese along with iron forms coarse (30 to 100 μm) α(AlFeMnSi) phases in the interdendritic channels upon solidification. During rolling, these coarse phases are broken up and the matrix surrounding the phases is highly strained. The highly strained regions around the precipitates are preferential sites for the nucleation and growth of new, strain-free, recrystallized grains. Decreasing the iron content decreases the amount of coarse α(AlFeMnSi) constituent phase and increases the recrystallization resistance of the alloy for a given concentration of manganese. For iron-free alloys, coarse α-Al₁₂Mn₃Si phase forms during solidification. The increased recrystallization resistance is attributed to the lower probability for particle-stimulated recrystallization. Increasing the manganese content beyond what is normally found in 6013 increases the volume fraction of both the dispersoids and the coarse constituent phases. However, the increased volume fraction of the dispersoids in the iron-containing alloys more than compensates for the increase in volume fraction of coarse constituent particles, resulting in an increase in the recrystallization resistance with increased manganese content. In this research, alloys containing varying amounts of manganese and iron with levels of magnesium, silicon, and copper consistent with the nominal composition of 6013 were hot deformed and solution heat treated to produce microstructures ranging from fully recrystallized to unrecrystallized.