Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 1970"

The effect of carbide and nitride additions on the heterogeneous nucleation behavior of liquid iron

Abstract: A systematic study of carbide and nitride additions on the heterogeneous nucleation behavior of supercooled liquid iron was undertaken. It was found that titanium nitride and titanium carbide were very effective in promoting heterogeneous nucleation. These compounds were followed by silicon carbide, zirconium nitride, zirconium carbide, and tungsten carbide in decreasing order of effectiveness. The degree of potency of the nucleation catalysts is explained on the basis of the disregistry between the lattice parameters of the substrate and the nucleating phase. Through the inclusion of planar terms the Turnbull-Vonnegut “linear” disregistry equation was modified to more accurately describe the crystallographic relationship at the interface during heterogeneous nucleation.

The relation between polycrystal deformation and single-crystal deformation

Abstract: A phenomenological description of crystallographic slip and pencil glide in single crystals is outlined, with emphasis on the behavior under prescribed strains Theoretical relations are established between these single-crystal properties and the behavior of quasi-homogeneous, quasi-isotropic polycrystals deforming uniformly on a macroscopic scale, at subdiffusive temperatures Experimental comparisons between single crystals and polycrystals are reviewed, considering flow stress, work hardening, temperature and strain rate effects, and various effects of grain size

Interdendritic fluid flow and macrosegregation; influence of gravity

Abstract: An analysis is given of interdendritic flow behavior during solidification of castings and ingots, assuming resistance to flow is as in other types of porous media. Driving forces for the flow are solidification contractions and gravity acting on a fluid of variable density. Detailed flow calculations are given for horizontal, unidirectional, steady-state solidification, using aluminum-copper alloys as examples. Conditions are quantitatively described under which gravity induced convection becomes an important contributory cause of macrosegregation. A critical condition of flow is shown to produce local melting with resulting formation of “channel-type” segregates. Qualitative examples are given of application of the ideas presented to interpretation of macrosegregation in commercial ingots, with specific reference to centerline segregation and “channel-type” segregation, including “V” segregates, “A” segregates and “freckles”.

The role of dislocations in the flow stress grain size relationships

Abstract: Calculations involving pile ups of dislocations, both analytical and numerical, using either discrete dislocations or continuous distribution of dislocations of infinitesimal Burgers vectors, are reviewed in the light of their effects on the relation between yield or flow stress and grain size. The limitations of the pileup models are discussed and some nonpileup theories of yielding are critically reviewed also. More critical experiments are still needed to reveal the fundamental mechanicm of yielding.

The origin of freckles in unidirectionally solidified castings

Abstract: The origin of freckles during unidirectional solidification is studied in a transparent, low melting model system, 30 wt pct NH4C1-H2O. In 30NH4Cl-H2O, freckles are caused by upward flowing liquid jets in the mushy zone. The jets erode the mushy zone causing localized segregation and start new grains by producing dendritic debris. It is shown that the jets observed in 30NH4C1-H2O are free convection resulting from a density inversion in the mushy zone. A comparison of driving force, thermal transport effects and solute transport effects in 30NH4C1-H2O and metallic systems shows that jets are possible in metallic alloys where light elements segregate normally or heavy elements segregate inversely. It is concluded that freckles in unidirectionally solidified castings and vacuum consumable-electrode ingots are caused by convective jets. It is shown that the tendency to freckle is greatest in alloys with a large density inversion, high thermal diffusivity, low solute diffusivity, and low viscosity. For a given alloy, the driving force for freckling is proportional to the inverse square of the thermal gradient. Erosion by the jets is decreased by increasing the thermal gradient and growth rate. The location of freckles is influenced by mushy zone curvature.

Aluminum films deposited by rf sputtering

Abstract: The feasibility of depositing aluminum thin films by means of rf sputtering has been investigated. Film characteristics compatible with the requirements of conductive stripes in integrated circuits have been obtained. The rate of film deposition has been related to rf power dissipation, argon pressure, geometry, and magnetic field. Film characteristics such as electrical resistivity, stress, grain size, and orientation have been studied and compared to the characteristics of films deposited by means of evaporation. The effects of annealing on some of these characteristics have been determined.

The mechanism of creep in gamma prime precipitation-hardened nickel-base alloys at intermediate temperatures

Abstract: The creep deformation of Mar-M200 single crystals of various orientations has been studied at a temperature of 1400°F. It was found by a combination of transmission microscopy and analysis of lattice rotations that shear of the γ−γ′ structure occurs by the glide of loosely coupled intrinsic/extrinsic fault pairs with a net Burgers vector ofa . The orientation dependencies of both the rate and extent of primary creep are correlated with the Schmid factors and multiplicity of slip for {111} systems. It is also shown that strain hardening due to intersecting slip is necessary to obtain the transition from primary to steady-state creep. In addition, it was found that the deformation mode is a function of strain rate at 1400°F. In contrast to the observed glide mechanism during creep, tensile deformation occurs by the shear of the γ and γ′ phases bya/2 superlattice pairs.

The influence of polycrystal grain size on several mechanical properties of materials

Abstract: The ductile-brittle transition, hardness, fatigue, and creep behavior of polycrystalline materials are known to be influenced under certain conditions by the polycrystal grain size. These properties have been correlated, historically, with the material stress-strain behavior. The (Hall-Petch) stress-grain size relations are useful for describing the complete stress-strain behavior for polycrystals and, therefore, these relations provide a reference for understanding the manner in which these other properties should also depend on the grain size. In some cases, the grain size dependence of a particular property follows directly from this connection.

The Growth of Gamma Prime Precipitates in Aged Ni-Ti Alloys

Abstract: The kinetics of growth of the γ′ precipitate in a Ni-8.74 wt pct Ti alloy were studied by magnetic analysis and transmission electron microscopy. The variation of the titanium content of the nickel-rich matrix as a function of aging time was studied by measuring the ferromagnetic Curie temperature of alloys aged at 692°, 593°, and 525°C. The kinetics of this process accurately obeyed the predictions of the Lifshitz-Wagner theory of diffusion controlled coarsening after relatively short aging times at all aging temperatures. Dark-field transmission electron microscopy was used to measure the particle-size distributions and the average particle sizes of samples aged for various times at 692°C. The kinetics of particle growth also obeyed the time law predicted by the Lifshitz-Wagner theory within the limits of experimental error. Additional analysis of the data provided a value of approximately 21 erg per sq cm for the interfacial free energy of the γ′-matrix interface, and a value for the diffusion coefficient of titanium in nickel which is in very good agreement with an independently determined value. The distribution of γ′ particle sizes was found to be significantly broader than the theoretical distribution of the Lifshitz-Wagner theory. It is suggested that this is due to the relatively large lattice parameter mismatch between γ′ and the Ni−Ti matrix. The results and conclusions of this study are critically compared with those of other investigations.

A study of the deformation of patented steel wire

Abstract: Comparisons of transmission electron micrographs of transverse sections of heavily drawn patented steel wire with existing metallographic and strength data were made with the aid of a computer. Both fragmentation of the cementite and the local deformation mode within the wire,i.e., plane strain elongation, an effect of the wire texture of the ferrite, were taken into account in order to obtain a model for large-strain deformation of pearlitic or bainitic microstructures which is consistent with the observed microstructural changes and strain hardening rate. The functional dependence of this model on true strain and original substructural spacing is similar to the equation of Embury and Fisher for the strain hardening of drawn pearlite because their assumptions (no fragmentation of the cementite and homogeneous, axially symmetric elongation) produced offsetting errors. The present model allows for additional sensitivity of the strain hardening rate of drawn patented steel wire to metallurgical and processing variables over and above the simple dependence on original substructural scale predicted by the model of Embury and Fisher.

The effect of solidification rate on structure and high-temperature strength of the eutectic NiAl-Cr

Abstract: The eutectic NiAl-Cr, consisting of chromium rods in a NiAl matrix, was directionally solidified at rates varying from 1/4 to 30 in. per hr. The inter-rod spacing and elevated temperature tensile properties were measured and the structure determined for each solidification rate. The spacing, λ, obeyed the relation λ2V= Constant, whereV is the solidification rate. AtV greater than 1 in. per hr, a cell or colony structure formed and the cell size decreased with increasing solidification rate. At 600°C, the tensile strength increased substantially with decreasing inter-rod spacing, reaching 100,000 psi at the highestV. At 800° and 1000°C, the strength first increased then decreased at the highest solidification rates reaching maximum strengths of 63,000 psi and 47,000 psi, respectively. At 1200°C, the strength decreased slightly with increasing solidification rate. Fracture occurred by shear along grain boundaries and cell walls. The decreased strength at the highest solidification rates may be related to the increased cell boundary area.

Cross terms in the thermodynamic diffusion equations for multicomponent alloys

Abstract: The kinetic equations for the atom fluxes in a multicomponent alloy are derived in terms of atomic driving forces for a random alloy model. In this model, the contribution to each atom flux from the vacancy wind effect is directly proportional to the vacancy flux. The kinetic equations when written in the same form as the thermodynamic diffusion equations allow theL ik coefficients to be identified. The cross terms relating the flux of speciesi to the chemical potential gradients of other speciesk are found to be nonzero. These cross terms can have an appreciable effect on the atom fluxes and can lead to negative intrinsic diffusion coefficients even in ideal systems. Neglecting the cross terms can lead to serious inaccuracies. TheL ik in ann-component system depend on then tracer diffusion coeficients. In this respect, it still is possible to express then atom fluxes in terms of onlyn coefficients.

A systematic investigation of elastic moduli of Wc-Co alloys

Abstract: Measurements of Young’s modulus, shear modulus, compression modulus, and Poisson’s ratio on WC-(1 ~ 30) wt pct Co alloys were carried out by dynamic resonance method. The effects of volume fraction of the WC phase, the carbon content, and the WC particle-size on the elastic moduli were investigated in some detail. The result shows that the various elastic moduli of the alloys depend solely on volume fractions of the constituent phases and they invariably fall within Hashin and Shtrikman’s bounds. Moreover, the moduli are found to be approximated by Paul’s “strength of materials” type formulas. It is concluded that elastic behavior of the alloys can be essentially predicted on the basis of a simplified geometrical model of WC particles embedded in continuous cobalt matrix.

Thermodynamics of austenitic Fe-C alloys

Abstract: New data on the activity of carbon in austenite have been obtained by CO2/CO equilibration of Fe-C alloys in the temperature range 900° to 1400°C. Equations for the thermodynamic properties of carbon and iron in austenite are obtained from the data combined with selected data from the literature. A slightly modified phase diagram is presented. The stability of cementite is also determined from data published earlier and the results of the present study. Parameters of the various models of the behavior of carbon in austenite taken from the literature are also calculated from the data.

Electrically active point defects in cadmium telluride

Abstract: Hall coefficient and conductivity measurements were made on single crystals of CdTe at temperatures up to 950°C while controlling the partial pressure of either cadmium or tellurium. Measurements made with a cadmium reservoir showed the material to ben-type due to the presence of a doubly ionized native donor, in agreement with the results of Whelan and Shaw. The apparent enthalpy of formation of the donor and its concentration along the cadmium-rich solidus were obtained. Measurements made with a tellurium reservoir close to tellurium saturation are in disagreement with the usual model, which attributes formation ofp-type material under these conditions to the presence, of a native acceptor defect. The results are consistent with a hole concentration originating from an excess of acceptor impurities rather than from a native acceptor.

Grain size effects in the strain hardening of polycrystals

Abstract: The degree to which the flow stress of a polycrystal is sensitive to grain size is discussed in terms of the distribution of slip and dislocation structure that develops in the vicinity of grain boundaries as deformation proceeds. The point of view is taken that the two principal classes of grain boundary hardening models, namely, those based on dislocation pile-ups and those based on dislocation density concepts respectively represent special cases of a single rationale developed in this paper. Grain boundary strengthening is intimately related to strain hardening which is affected by slip mode,i.e., the number of slip systems and the ability to cross slip. The effects of substitutional solute elements on grain boundary strengthenings is considered to be a consequence of their influence on slip modes rather than on their interaction with dislocation sources.

Internal stresses and structures developed during creep

Abstract: Specimens of 304 stainless steel subjected to different thermomechanical histories develop different internal stresses, σi, and different substructures. Creep rate is uniquely related not to the applied stress, σA, but to the effective stress, σ*=(σA−σi). Values of σ* are determined from experimental results and σi calculated from σi=(σA−σ*). Results show σi increases with the applied stress according to σi∝σA1.7. Transmission electron microscopic observations show that the density of dislocations within subgrains, ϱD, and the subgrain diameter,D, vary with applied stress according to: ϱD∝σAK,D ∝ σA−0.8, whereK=1.4 to 2.0. Subgrain misorientation is independent of creep stress, strain, or temperature. The contributions of these structural variables to the internal stress are discussed.

Macrosegregation in ternary alloys

Abstract: General expressions are given to describe macrosegregation in ternary alloys resulting from mass flow of interdendritic liquid during solidification. Basic parameters determining whether a given alloy element segregates positively or negatively are given, and it is shown that alloy elements which form a second phase,e.g., an inclusion, can often be expected to segregate positively where other alloy elements segregate negatively. Numerical examples are given for alloys from the aluminum rich cornee of the Al−Cu−Ni system and qualitative examples are given for the Fe−Si−O system. Experimental measurements of macrosegregation in the Al−Cu−Ni system are in agreement with theory.

The use of young’s modulus for predicting the plastic-strain ratio of low-carbon steel sheets

Abstract: The variation in Young’s modulus,E, and plastic-strain ratio,R, with angle to the rolling direction, o, was measured for 35 samples of cold-rolled and annealed low carbon steel sheets. Functions ofE andR were fitted to an empirical expression and correlations were made from which it was possible to predict not only the average plastic-strain ratio, −R, but also the planar anisotropy, ΔR, and the dependence ofR on o Calculations were made of the orientation dependence ofE and the quantity R/(R + 1) for single crystals of α-Fe. Although these calculations show that the correlations made for polycrystalline samples are reasonable, they offer little guidance as to the probable limits of applicability of the correlations. The limiting drawing ratio (LDR) was determined_for 12 sheet samples by Swift-cup testing. For these limited results, the average modulus, −E, and the average strain ratio, −R, correlate equally well with the LDR. Use of E(o), to predict deep drawability and earing, should reduce the effort required to characterize the formability of. low-carbon steel sheets.

Dislocation structures in single-crystal tungsten and tungsten alloys

Abstract: Deformation of tungsten single crystals as a function of strain, temperature, and alloying was studied by transmission electron microscopy. Single crystals oriented for (−101)[lll] slip were grown by electron beam zone refining. Compression specimens of tungsten, W-l and 3 pct Re and W-l and 3 pct Ta were deformed to 2 pct strain at 150°, 300°, and 590°K (0.04, 0.08, and 0.16T m). Specimens were also strained to 0.5 and 5.0 pct strain at 300°K. Transmission microscopy revealed that the dislocation substructures in single-crystal tungsten are similar to substructures in other refractory metals when compared on a homologous temperature basis. At temperatures greater than 0.1T m, the substructure is characterized primarily by edge dipoles. At temperatures less than 0.1T m, long screw dislocations lying parallel to the primary [111] slip direction characterize the substructure. Rhenium additions to tungsten promote formation of edge dipoles at temperatures of 300° and 150°K and increase dislocation density at all three temperatures. In addition, dislocations consistent with (1−12)[−111] slip were observed in the W-Re single crystals after deformation at 150°K. Tantalum additions had a lesser effect on the dislocation substructure compared to rhenium additions. The W-l and 3 pct Ta alloys exhibited higher dislocation densities than unalloyed tungsten after similar strains and, at 150°K, W-3 pct Ta contained a few dislocations consistent with (1−12)[−111] slip. It is concluded that the reduction in ductile-brittle transition temperature of poly crystalline tungsten containing dilute rhenium additions, 1 to 5 pct, can be attributed to an increase in dislocation mobility at temperatures less than 0.1 Tm.

Work strengthening by a deformation-induced phase transformation in 'MP Alloys'

Abstract: Work strengthening and microstructure were investigated for a class of alloys, designated “MP Alloys”, containing 20 pct Cr, 10 pct Mo, and the remainder cobalt and nickel in proportions ranging from 60Co∶10Ni to 30Co∶40Ni. These alloys, in the fully annealed, homogenized condition, have a fcc structure with yield strengths ranging from about 45 to 60 ksi. Deformation at room temperature rapidly increases the yield strength of the alloys to about 250 ksi. Structural analyses by X-ray and electron diffraction techniques indicate that this marked increase in strength is associated with a deformation-induced martensitic transformation forming a network of extremely thin hcp platelets within the fcc grains. The nature of this martensitic transformation was studied as a function of alloy composition, deformation temperature, and structural variables, such as the platelet size, thec/a ratio of the hcp phase, and twinning.

The effect of natural convection on the shape and movement of the melt-solid interface in the controlled solidification of lead

Abstract: Experiments are reported on the study of the effect of natural convection on the controlled solidification of lead under conditions of nearly unidirectional heat flow within the system. The experimentally found shape and position of the solid-melt interface was reasonably well predicted by the consideration of heat transfer by natural convection from the melt to the solid surface. Transient runs were also carried out and here the rate of movement of the melt-solid interface was readily predicted from the (numerical) solution of the appropriate unsteady state conduction equation.

Enhanced oxidation of molybdenum disilicide under tensile stress: relation to pest mechanisms.

Abstract: The mechanical disintegration of molybdenum disilicide in air or oxygen at temperatures of 300° to 600°C, known in the literature as “pest”, is ascribed here to stress enhanced oxidation at the tips of Griffith flaws, eventually leading to brittle fracture. Stress-free single crystals of molybdenum disilicide, which are not ordinarily subject to pest, were shown to exhibit delayed failure in four-point bend tests under the conditions that lead to pest in polycrystals. The delayed failure times showed the same temperature and oxygen pressure dependence as the times to onset of pest disintegration in polycrystals. Pest in polycrystals occurs even in the absence of an applied stress, due to the high residual stresses which are introduced upon cooling this highly anisotropic material from the melt during fabrication. Nonetheless, the superposition of an applied stress, over and above the residual stress, was shown to decrease the time to pest failure compared to unstressed specimens. The absence of pest above 600°C, and the longer times required for pest disintegration as temperature is increased above about 500°C, is explained by the presence of plastic deformation in the matrix, which tends to reduce stress concentration at the tips of flaws and to counteract the flaw sharpening effect of stress enhanced oxidation. Slip was readily observed at 600°C. The delayed failure results were consistent with the general Charles-Hillig model for the mechanical failure of brittle materials that react chemically with their environment.

The development of macrostructure in ingots of increasing size

Abstract: A series of Al-Cu ingots ranging in volume from approximately 50 to 500 and to 5000 cc has been conventionally prepared in two ways, by normal chill casting or by using a magnetic field to control the convective currents. Comparisons allow us to differentiate among the various mechanisms proposed for the origin of equiaxed grains and for the columnar-to-equiaxed transition in castings: i) In normal casting, with the natural convection associated with standard superheats, the majority of grains is provided by the mechanism suggested by Chalmers (and also known as the Big Bang), or by crystal settling from a free surface as suggested by Southin. ii) If new grains, no matter how they may form, are inhibited from mixing with the rest of the liquid, no significant equiaxed structure appears. iii) Unless some special or preselected conditions prevail, other mechanisms such as those involving constitutional supercooling or dendrite remelting, do not seem operative in the formation of a central equiaxed zone.

Thermodynamic properties of the Cu−Sn and Cu−Au systems by mass spectrometry

Abstract: The activities and partial molar heats of mixing have been determined in the liquid Cu−Sn system at 1320°C and the liquid Cu−Au system at 1460°C. The experimental technique consisted of the analysis of Knudsen cell effusates with a T.O.F. mass spectrometer. The ion current ratio for the alloy components was measured for each system over a range of temperature and composition and the thermodynamic values calculated by a modified Gibbs-Duhem equation. Both systems exhibited negative deviations from ideal behavior. The results can be partially represented by the equations $$\begin{gathered} \log \gamma _{Cu} = - 0.0175x^2 _{Sn} - 0.302 (0 \leqslant x_{Cu} \leqslant 0.20) \hfill \\ log \gamma _{Sn} = - 0.342x^2 _{Cu} + 1.084(0 \leqslant x_{Sn} \leqslant 0.20) \hfill \\ \end{gathered} $$ for the Cu−Sn system at 1320°C and by $$\begin{gathered} \log \gamma _{Cu} = - 0.703x^2 _{Au} - 0.083(0 \leqslant x_{Cu} \leqslant 0.52) \hfill \\ \log \gamma _{Au} = - 1.057x^2 _{Cu} + 0.098(0 \leqslant x_{Au} \leqslant 0.47) \hfill \\ \end{gathered} $$ for the Cu−Au system at 1460°C.

Ni 8 Ta in nickel-rich Ni−Ta alloys

Abstract: An ordering reaction has been observed in the nickel-rich binary alloys of the Ni−Ta system that are aged below 570°C following quenching from elevated temperatures. This reaction has been investigated by employing electron diffraction microscopy, X-ray diffraction, and electrical resistivity techniques. The ordered phase has been identified as Ni8Ta and its structure is identical to that of the Ni8Nb (Cb) reported earlier. In the stoichiometric Ni8Ta alloy (Ni-11.1 at. pct Ta) three variants of the Ni8Ta phase are nucleated apparently “homogeneously” throughout the matrix. The precipitate or domain morphology has been identified as a cuboid elongated in one direction such that the cube faces are parallel to the {100} planes of the matrix. The cuboid morphology of the precipitates tends to disappear as the precipitates coalesce and the particle size approaches 1000A.

Correlation of uniaxial yielding and substructure in aluminum-stainless steel composites

Abstract: Uniaxial yielding correlation with substructure in Al-stainless steel metal matrix composites, using compressive loading test