Study of mechanism of cleavage fracture at low temperature

In this investigation, a series of crack opening displacement (COD) tests were carried out at several low temperatures for C-Mn weld steel. Some of the specimens were loaded until fracture, and the mechanical properties and microscopic parameters on fracture surfaces were measured. Other specimens were unloaded before fracture at different applied loads. The distributions of the elongated cavities and the cleavage microcracks ahead of fatigue crack tips were observed in detail. Based on the experimental results, the combined criterion of a critical strainε _p ≥ ε_c) for initiating a crack nucleus, a critical stress triaxiality(σ _m/σ ≥ t_c) for preventing it from blunting, and a critical normal stress(σ _yy/σ_f) for the cleavage extension was proposed again, and the critical values of ε_p and σ_m/−σ for the C-Mn weld steel were measured. The reason why the minimum COD value could not be zero is explained. The mechanism of generation of the lower limit COD value on the lower shelf of the toughness transition curve is proposed.     

The strain dependence of postdynamic recrystallization in 304 H stainless steel

Using double-hit hot compression tests, the softening behavior of 304 H stainless steel was studied during unloading. The prestrains used were associated with the initiation of dynamic recrystallization (DRX) (ε _c), the peak strain (ε _p), 1/2 (ε _c+ε _p), the strain at maximum softening rate (ε _i), and the onset of steady state flow (ε _s). The following conditions of deformation were used: T=1000 °C, 1050 °C, and 1100 °C, =0.01 and 0.1 s⁻¹, and delay times of 0.3 to 1000 seconds. To define the above important strains, single-hit hot compression tests were performed over a wider range of deformation conditions than the double-hit ones—i.e., 900 °C to 1100 °C and =0.01 to 1 s⁻¹. The results show that a transition strain (ε*) separates the strain-dependent range of postdynamic softening from the strain-independent range. At strains between ε _c and ε*, both metadynamic and static recrystallization contribute to interhit softening. The value of ε* obtained in this work was ε*=4/3 ε _p. It was also found that the strain hardening rate was identical at all the critical strains (ε*) and took the value −22 MPa.     

Experimental investigation of mixing phenomena in a gas stirred liquid bath

Mixing phenomena in a room temperature water bath, agitated by injecting air through a straight circular nozzle fitted axially at the bottom of the vessel, were characterized by experimentally measuring mixing time(t _mix) by electrical conductivity technique. It was found thatt _mix defined at 99.5 pct homogenization did not depend on location and size of conductivity probe, location of tracer injection, and the amount of tracer injected. tpet decreased with increasing gas flow rate and bath height, but decreasing nozzle diameter. Visual observations of the two-phase plume and flow conditions in the bath revealed that the plume swirled above a certain gas flow rate which enhanced the mixing rates in the bath. The transitions in Int _mix vs In ε_b curves were found to correspond to onset of swirling; ε_b is the rate of buoyancy energy input per unit bath volume. Systematic analysis of experimental data revealed that a fraction of gas kinetic energy contributed to mixing in the bath. It was a function of bath height, being negligible at lower bath heights and almost 1 at larger bath heights. Further, it was experimentally found thatt _mix decreased with increasing bath height only up to a certain value, beyond which it started increasing. Visual observations of the bath revealed that the height at whicht _mix started increasing corresponded to a transition in which the bath was converted into a bubble column. The experimental data, for a particular bath height, were fitted into two separate straight lines of the formt _mix =cε ⁻ⁿ wherec andn are empirical constants and ε is the rate of energy input per unit bath volume. Formerly Graduate Student in the Department of Metallurgical Engineering at the Indian Institute of Technology, Kanpur, India     

Hydride formation and decomposition in electrolytically charged metastable austenitic stainless steels

An investigation of phase transformations in hydrogen-charged metastable austenitic stainless steels was carried out. Solution-annealed, high-purity, ultralow-carbon Fel8Crl2Ni (305) and laboratory-heat Fel8Cr9Ni (304) stainless steels were examined. The steels were cathodically charged with hydrogen at 1, 10, and 100 mA/cm², at room temperature for 5 minutes to 32 hours, in an lN H₂SO₄ solution with 0.25 g/L of NaAsO₂ added as a hydrogen recombination poison. Changes in microstructure and hydrogen damage that resulted from charging and subsequent room-temperature aging were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Hydrides from hydrogen charging (hep ε* in 305 SS and fcc γ* and hcp ε* in 304 SS) were observed. The evidence suggests the following mechanisms for hydride formation during charging: (1)γ → ε → ε* hydride and (2) γ → γ* hydride. These hydrides were found to be unstable and decomposed during room-temperature aging in air by the following suggested mechanisms: (1)ε* hydride (hcp) → expanded ε (hcp) phase →α′ (bcc) phase and (2) γ* hydride →γ phase. The transformation from ε* toα′, however, was incomplete, and a substantial fraction of ε was retained. A kinetics model for hydride decomposition and the accompanying phase transformation during aging is proposed.     

Further study on the mechanism of the ductile-to-brittle fracture transition in C-Mn base and weld steel

In the present study, the crack opening displacement (COD) tests of specimens of C-Mn base and weld steel were carried out in the ductile-brittle transition temperature region. The majority of the specimens were fractured and others were unloaded prior to fracture after ductile fracture initiated and extended. The cavities and cleavage microcracks located in the vicinities of tips of fibrous cracks of the unloaded specimens were observed in detail. The finite element method (FEM) calculations of the stress and strain distribution ahead of the tip of an extending fibrous crack were completed. The mechanism of the ductile-to-brittle fracture transition was further investigated. It was revealed that in the ductile-brittle transition temperature region, the ductile fracture process was independent of temperature. The ductile-to-brittle fracture transition was triggered by initiating a catastrophic extension of a cleavage crack ahead of the fibrous crack tip, which occurred in a condition satisfying a combined criterion composed of three items, i.e., ε _p ≥ ε _pc for initiating a crack nucleus; σ _m √σ ≥ T _c for preventing the crack nucleus from blunting; and σ _yy ≥ σ _f for propagating the crack nucleus. For a specimen in which a fibrous crack occurred and propagated, the critical event for initiating a brittle cleavage fracture was the propagation of a ferrite grain-sized crack into neighboring grains. With extension of a fibrous crack, the behavior of the ductile-to-brittle fracture transition could be analyzed by the effect of the size of an “active zone” on the initiation of the brittle cleavage fracture.     

Precipitation of heusler phase (Ni<Subscript>2</Subscript>TiAl) from B2-TiNi in Ni-Ti-Al and Ni-Ti-Al-X (X=Hf,Zr) alloys

The precipitation of Heusler phase (L2₁: Ni₂TiAl) from a supersaturated B2 (TiNi-based) matrix at 600°C and 800°C is studied using transmission electron microscopy (TEM), analytical electron microscopy (AEM), and three-dimensional atom-probe (3DAP) microscopy in Ni-Ti-Al and Ni-Ti-Al-X (X=Hf and Zr) alloys. The B2/L2₁ two-phase system, with ordered structures based on the bcc lattice, is chosen for its microstructural analogy to the classical γ/γ′ system with an fcc lattice. Knowledge of the temperature-dependent partitioning of alloying elements and their atomic volumes in the B2-TiNi and L2₁ phases is desired to support design of high-performance shape-memory alloys (SMAs) with controlled misfit strain and transformation temperatures. After aging at 600°C for up to 2000 hours, the L2₁ precipitates remain fully coherent at a particle diameter of ∼20 nm. The observed effects of a misfit strain of −1.9 pct on the microstructure of the B2/L2₁ system are similar to those theoretically predicted and experimentally observed for the γ/γ′ system. The similarities are demonstrated in terms of the precipitate shape, spatial distribution, and minimum distance of separation between L2₁ precipitates. However, all these effects disappear after aging the alloys at 800°C for 1000 hours, when the L2₁ precipitates become semicoherent at particle diameters above ∼400 nm. A simple analysis of the size evolution of L2₁ precipitates after an isochronal aging (1000 hours) experiment suggests that they follow coarsening kinetics at 600°C and growth kinetics at 800°C, consistent with the Langer-Schwartz theory of precipitation kinetics, which predicts that a high supersaturation suppresses the growth regime. Microanalysis using AEM and 3DAP microscopy define the TiNi-Ni₂TiAl phase boundaries at 800°C and 600°C. At 800°C, Hf and Zr partition to the B2-TiNi, while at 600°C, they partition slightly to the L2₁ phase, reducing the lattice misfit to −1.7 and −0.011 pct, respectively, and partition strongly to the metastable phase Ti₂Ni₃. To describe the composition dependence of the lattice parameter of multicomponent B2 and L2₁ phases, the atomic volumes of Al, Hf, Ni, Ti, and Zr in the B2-TiNi and L2₁ phases are determined. A simple model is proposed to predict the lattice parameters of these phases in multicomponent systems.     

Mechanical Properties of Uncoated and Aluminide-Coated René 80

Nickel-base superalloys such as René 80 are widely used in manufacturing aircraft turbine blades. They are usually coated in order to increase their wear, oxidation, erosion, and hot corrosion properties against environmental degradation. In this article, the mechanical behavior (tensile and low-cycle fatigue (LCF)) of uncoated and aluminide-coated (CODEP-B) René 80 has been studied at 871 °C and 982 °C. Experimental results show that the tensile properties of coated specimens are relatively lower than those of uncoated ones in the same conditions, but application of coating increases the LCF life of René 80 at T = 871 °C, 982 °C, R = (ε _min/ε _max) = 0, strain rate of 2 × 10⁻³ s⁻¹, and Δε _t  = 0.8 pct. Scanning electron microscopy (SEM) studies of coated specimens at N = Nf show that the nucleation of cracks occurs merely in substrate, but cracks start from the surfaces in uncoated specimens. Transmission electron microscopy (TEM) investigations have been performed on fractured uncoated specimens to evaluate the microstructures at different temperatures. The misfit dislocation, pair dislocations, and cutting of γ′ were observed at T = 871 °C and 982 °C. The TEM studies also showed that at 982 °C stacking fault was observed in γ′ particles.     

A study of the thermodynamics and phase relationships of the Fe−Co−S−O quaternary system

The oxygen potentials of several three- and two-phase equilibria in the Fe−Co−S−O quaternary system were measured atP _SO2=1, 0.1, and 0.01 atm over wide temperature ranges. The measurements were carried out using a solid oxide electrolyte emf technique. The equilibria measured are sp+ε+δ, s+ε+ξ, sp+ξ+η, and sp+δ. The symbols sp, ε, δ, ξ, and η denote the spinel, monoxide, monosulfide, metal sulfate, and Fe₂O₃ phases, respectively. Compositions for several of the equilibrated phases were measured using electron probe microanalysis. The present results and literature data for the constituent ternary systems were used to obtain thermochemical solution parameters for the sp, ε, δ and ξ solid solution phases. The calculated potential-composition stability diagrams for SO₂ pressures of 1, 0.1, and 0.01 atm at 973, 1023, and 1073 K, respectively, are in good agreement with the experimental results. OMRAN A. MUSBAH, formerly Research Associate at the University of Wisconsin-Madison     

Assessment of service induced microstructural damage and its rejuvenation in turbine blades

Correlations between service induced microstructural degradation and creep properties in investment cast IN738LC turbine blades are discussed. Microstructural degradation in the form of γ’ coarsen-ing, MC carbide degeneration, formation of continuous networks of grain boundary M₂₃C₆ carbides, and the disappearance of serrated grain boundaries are considered in some detail. Their influence on primary (t _p ,ε _p ), secondary (t _s , ε _s ,ε _m ) and tertiary (t_t, ε_t) creep behavior is analyzed through rela-tionships of the form:

Mixing models for gas stirred metallurgical reactors

The mixing of liquids in ladles, (0.5 ≦L/D ≦ 2.0), agitated by a centrally rising bubble plume, has been analyzed both theoretically and experimentally. An exhaustive review of previous metallurgical literature on mixing in ladles and furnaces demonstrates that the majority of previous investigators in the field consider mixing to be brought about primarily by turbulent diffusion phenomena. The present study clearly shows that mixing is a combination of both convection and eddy diffusion processes, neither of which can be disregarded for gas stirred systems. For predicting mixing times during such gas injection procedures, a simple empirical equation is proposed for axisymmetric systems:τ _mαε_m ^−1/3L⁻¹R^5/3. Hereτ _m is the 95 pct mixing time,ε _m is the specific energy input rate,R is the vessel radius, andL is the depth of liquid. On the basis of physical and mathematical modeling, the rate of liquid mixing in conventional gas injection ladle metallurgy operations is compared with those observed in C.A.S. (composition adjustment by sealed argon bubbling) systems. It was found that mixing in C.A.S. operations is relatively slow and highly insensitive to gas flowrates(i.e., specific energy input rates).     

Stable nonlinear relaxations in multiphase Al-Zn alloy

Experiment reveals the characteristics of stable damping in a multiphase Al-Zn eutectoid alloy as follows: (1) the whole damping (Q ⁻¹) has the same dependence on measuring frequency (f);i.e., Q ^-1 ∝f ⁻ⁿ , wheren is a parameter independent of temperature; (2) in a low-temperature (low-T) and low-strain-amplitude (low-A _ε) region,Q ⁻¹ = (B/f ⁿ) exp (-nH/kT) (whereB is a constant,H is the phase interface or interphase boundary atom diffusion activation energy,k is Boltzmann’s constant, andT is the absolute temperature);n andH are independent ofA _ε. The damping originates from an anelastic motion of phase interface; (3) in an intermediate region, including low-T and high-A _ε, middle-T and middle-A _ε, and high-T and low-A ^ε regions, we still have the equationQ ⁻¹ = (C/f ⁿ ) exp (-nH/kT), but the damping has a normal amplitude effect:C, n, andH all vary withA _ε. The damping results from a nonlinear relaxation of phase interface; and (4) in a high-T and high-A _ε region, there is no longer a linear relationship between InQ ⁻¹ and 1/T, whereas relationQ ⁻¹ ∝f ⁻ⁿ is still satisfied;n increases asA _ε increases; and the damping has a normal amplitude effect, but it is weaker than that in case (3). The damping may be attributed to another kind of nonlinear relaxation between phase interfaces.     

The effect of convection on the dendrite to eutectic transition

Vertical solidification experiments have been carried out in binary eutectic metal alloy systems using both upward and downward solidification directions with solutes heavier and lighter than the solvent metal. Hence, conditions of all four possible combinations of thermal and solutal Rayleigh numbers were studied: Ra _T /Ra _s = −/ −, −/+, +/+, +/ −. Compositions close to eutectic values were examined under conditions of gradient and velocity which produced cellular, dendritic, and eutectic structures. The experimental results indicate that solutal induced convection plays a dominant role in controlling both dendrite/liquid interface shape and the dendrite clustering effect at the dendrite/eutectic transition. In both the presence or absence of cluster formation, the transition is shown to occur by a gradual change in dendrite size, morphology, and number.     

Stress-state dependence of cavitation and flow behavior in superplastic aluminum alloys

A detailed and quantitative investigation of the stress-state dependence of superplastic cavitation in fine-grained aluminum alloys has been carried out to develop clear evidentiary support to build future models. Several stress states, such as uniaxial tension, plane-strain tension, plane-strain compression, shear, and equibiaxial tension have been examined. Tests were carried out to large strain in an interrupted manner under a constant effective strain rate ( _e ) in the range of 10⁻⁴ to 10⁻² s⁻¹. Measurements of volume fraction, population density, and size distribution of cavities, made by image analysis via optical microscopy, show continuous emergence of new cavities as well as growth of cavities during superplastic straining. The total cavity volume fraction (V) increases exponentially with strain. The cavity growth rate, represented by η (equal to d ln V/dε _e ), as well as the cavity population evolution rate with strain (dN _c /dε _e , where N _c is the cavity number/unit area) are found to increase with normalized mean hydrostatic tensile stress (σ _m / σ _e ). An empirical equation for the biaxial forming limit in terms of the principal surface strains (ε ₁ and ε ₂) has been defined for a fixed cavity volume, as given by ε ₁=a V ^b − α ε ₂, where a and b are constants determined from ε ₁ values for plane strain (ε ₂=0). The value of b is found to be 0.2 to 0.3, and α is 0.4 to 1.0.     

Effect of uniaxial stress on coarsening of precipitate clusters

The influence of moderate applied uniaxial stresses (σ_app/C ₄₄ ≈ 10⁻³) on the coarsening behavior of misfitting coherent precipitates in binary alloys has been studied. Three-dimensional (3-D) computer simulations of the coarsening have been performed for elastically homogeneous systems with tetragonal misfit strain and elastically heterogeneous systems with dilatational misfit strain. Precipitate shapes are restricted to spheres. Results depend on the sign of the misfit strain, the sign of the applied field, and the character of the elastic heterogeneity: precipitates softer than the matrix phase with positive (negative) misfit strain align along the direction of the applied stress for compressive (tensile) fields and arrange in planes perpendicular to it for tensile (compressive) fields. Precipitates harder than the matrix behave in the opposite way. This article is based on representation made during TMS/ASM Materials Week in the symposium entitled “ldAtomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

The application of a dislocation model to the strain and temperature dependence of the strain hardening exponentn in the Ludwik-Hollomon relation between stress and strain in mild steels

With the aid of a dislocation model for the stress-strain relationship of α-Fe, analytical expressions for the strain and temperature dependence of the exponentn in the relation, σ= K · ε ⁿ, are derived. These account quite accurately for experimental results obtained with several low alloy steels. It is shown thatn varies continuously with strain but that the theoretical and experimental log σ-log ε curve in most cases can be approximated by two straight lines in accordance with the well-known “double-n” behavior. The strain, ε₁ at which the two lines intersect is equal to the strain at which the theoretical n(ε) curve has an inflection point. With the model presented it is also possible to account for the temperature dependence ofn(ε) and of ε₁ within the temperature range −78° to 500°C.     

Kinetics of the growth of spinel,MgAl2O4, on alumina particulate in aluminum alloys containing magnesium

The growth of spinel in melts of alumina-reinforced composites not only consumes the magnesium required for age hardening, but also acts to increase the viscosity and thus adversely affects the castability of the material. The kinetics of this growth have been studied in molten Al-1 pct Mg alloys in the temperature range of 948 to 1073 K. The results are shown to fit an equation of the form ln(1−α = k(t₀ −t) describing deceleratory growth on fine particles, where α is a dimensionless reaction parameter. The rate constant,k, fits an Arrhenius equation giving an activation energy of 103 ± 7 kJ and a time constant of 50 s⁻¹. The incubation time,t ₀, which is about 2000 seconds forT < 1000 K, drops to below 500 seconds forT > 1000 K.     

The Normal spectral emittance of yttrium,lanthanum, cerium,praseodymium and neodymium above 1000 K

The normal spectral emittance (X = 0.645 μm) of yttrium, lanthanum, cerium, praseodymium and neodymium was determined at temperatures above 1000 K in an argon atmosphere at a pressure slightly above atmospheric. The measured normal spectral emittances are: yttrium, (solid), ε _nλ = 0.368 ± 0.005; (liquid), ε _nλ = 0.368 ± 0.005; lanthanum, (solid), ε _nλ = 0.409 ± 0.049; (liquid), ε _nλ = 0.257 ± 0.015; cerium, (liquid), ε _nλ = 0.309 ± 0.020; praseodymium, (solid), ε _nλ = 1.609 - 1.0964 x 10⁻³ T (K); (liquid), ε _nλ = 0.284 ± 0.005; neodymium, (solid), ε _nλ = 1.090 - 5.6 x 10⁻⁴ T (K); (liquid), ε _nλ = 0.394 ± 0.007. Formerly NSF Presidential Intern and PostDoctoral Fellow Formerly Undergraduate Research Helper, Prepared for the Energy Research and Development Administration under Contract No. W-7405-eng-82.     

Nitridation kinetics of niobium in the temperature range of 873 to 1273 K

The kinetics of Nb (Cb) nitridation and ε-NbN growth obey a parabolic relationship and their temperature dependence can be expressed ask _p = 5.19 × 10⁻⁷ exp (−125,500/RT) and (k _{p ξ} = 1.15 × 10⁻⁴ exp (−61,500/RT), respectively, with the activation energies in joules/mole. The nitrogen diffusion coefficient in niobium, obtained from microhardness traverses, is given byD = 1.02 × 10⁻⁵ exp (−77,000/RT). A diffusion model accounting for the partition of nitrogen between ε-NbN and Nb is proposed. The total nitrogen uptake calculated from the model is compared to that obtained experimentally.     

Thermodynamic properties of liquid copper-lead alloys

Activities of lead in liquid copper-lead alloys were measured in the temperature range 1000 to 1200 °C at intervals of 50 °C by the dew-point technique. Various partial and integral molar properties of the liquid alloys were evaluated from the data, and the boundaries of liquid immiscibility in the Cu-Pb phase diagram were calculated. The activity coefficients of lead and copper in dilute solutions are represented by: In γ^o _Pb = (346/T@#@) + 0.181, and In γ^o _cu = (3852/T) − 0.945. The temperature dependence of Gibbs energy self-interaction coefficients for lead and copper are given by: ε^Pb _Pb = − (7828/T) − 3.506, and ε^Cu _Cu = − (8804/T) + 3.140. Various coefficients have the following values at 1200 °C: γ^o _Pb = 12.58,ε^Pb _Pb = − 8.85.η^Pb _Pb = − 52,197 J/gfw, σ^Pb _Pb = 38.15 J/gfw-K, γ^o _Cu = 5.31, ε^Cu _Cu = −2.92,η^Cu _Cu = − 63,970 J/gfw, and σ^Cu _Cu = 19.19 J/gfw-K.     

Kinetics of pyrite oxidation in sodium carbonate solutions

The kinetics of pyrite oxidation in sodium carbonate solutions were investigated in a stirred vessel, under temperatures ranging from 50 °C to 85 °C, oxygen partial pressures from 0 to 1 atm, particle size fractions from −150 + 106 to −38 + 10 μm (−100 + 150 Mesh to −400 Mesh + 10 μm) and pH values of up to 12.5. The rate of the oxidation reaction is described by the following expression:−dN/dt = SbkpO ₂ ^0.5 [OH₋]^0.1 whereN represents moles of pyrite,S is the surface area of the solid particles,b is a stoichiometric factor,k is an apparent rate constant, pO```2`` is the oxygen partial pressure, and [OH⁻] is the hydroxyl ion concentration. The experimental data were fitted by a stochastic model for chemically controlled reactions, represented by the following fractional conversion(X) vs time (t) equation: (1−X)^−2/3−1 =k _STt The assumption behind this model,i.e., surface heterogeneity leading to preferential dissolution, is supported by the micrographs of reacted pyrite particles, showing pits created by localized dissolution beneath an oxide layer. In addition to the surface texture, the magnitude of the activation energy (60.9 kJ/mol or 14.6 ± 2.7 kcal/mol), the independence of rate on the stirring speed, the inverse relationship between the rate constant and the initial particle diameter, and the fractional reaction orders are also in agreement with a mechanism controlled by chemical reaction.