METALLURGICAL AND MATERIALS TRANSACTIONS A
ABSTRACTS
Volume 28A, No. 8, August 1997

This Month Featuring: Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Surface Treatment; Solidification. View August 1997 Contents.

ALLOY PHASES

Microstructure of Second-Phase Particles in Ti-5Al-4Sn-2Zr-1Mo-0.25Si-1Nd Alloy
G.P. LI, D. LI, Y.Y. LIU, S.X. GUAN, Q.J. WANG, D.H. PING, and Z.Q. HU
The microstructure of second-phase particles in the Ti-55 alloy (Ti-5Al-4Sn-2Zr-1Mo-0.25Si-1Nd) was studied by scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM) observations. The second-phase particles in the conventional ingot-cast Ti-55 alloy of 1 to 15 µm in diameter and uniform distribution in matrix were observed, where the majority of these particles are elliptical. The mean free path between the particles is about 46 µm, and the volume fraction (pct) is 2.35. The second-phase particles typically contain Nd, Sn, and O in substantial amounts, and the content of Nd is the largest in the three elements. The elements Ti, Al, Zr, Mo and Si are depleted in the particles. The second-phase particle consists of either a dark or bright matrix and some small dark blocks dispersed within the matrix. Dark blocks match SnO (orthorhombic, a=0.500 nm, b=0.572 nm, and c=1.120 nm), and the matrix consists of a nanocrystalline phase with a stoichiometric Nd3Sn structure having a space group of Pm3m and lattice parameter of a=0.344 nm. The grain size of the nanocrystalline Nd3Sn phase is about 3 to 15 nm. The melting range of the second-phase particle is estimated to be 1042°C to 1600°C. The microstructure of the second-phase particles in the quenched Ti-55 alloy was also studied. Fine and uniform dispersoids (6 to 15 nm in diameter) were observed in the as-quenched state. Some lenslike particles occur at the grain boundaries, other elliptical particles appear within the grains, and some particles within the grains form rows which are parallel to the advancing liquid-solid interface. After annealing at 980°C (1 to 10 hours), of the as-quenched Ti-55 alloy, coarse particles are 17 to 42 nm in average diameter, and the growth of the particles is very slow. The dispersoids in the as- annealed Ti-55 alloy are identified as nanocrystalline Nd5Sn (orthorhombic, Pnmn, a=0.814 nm, b=1.732 nm, and c=0.814 nm) intermetallic compound, and the interface between the Nd5Sn4 phase and the matrix is a typical high-angle grain boundary.

TRANSFORMATIONS

Effect of Long-Term Service Exposure at Elevated Temperature on Microstructural Changes of 5Cr-0.5Mo Steels
S. DAS and A. JOARDER
Effects of long-term service exposure at elevated temperature on microstructural changes have been studied for both virgin and service-exposed process heater tube pipes of 5Cr-0.5Mo steels used in oil refineries. Samples selected for this study had experienced a nominal temperature range of 450°C to 500°C for about 20 to 25 years. Two different initial virgin microstructures were taken and designated by steel A and steel B. The virgin microstructure of steel A exhibited fine platelets of fibrous or hairlike M2C carbides within the ferrite grains and occasionally irregularly shaped M23C6, both along the grain boundaries and at the grain interiors, and very few spheroidally shaped M3C, either along the grain boundaries or at the grain interiors. The size, shape, position, distribution, and type of carbides in virgin steel A changed significantly due to 220,000 hours of service exposure in the temperature range of 450°C to 500°C. Massive M23C6 carbides precipitated along the grain boundaries. In addition, regular geometrically shaped M23C6 carbides, such as hexagonal, square, and triangular type, were observed to form at the grain interiors. The virgin steel B microstructure exhibited predominantly M23C6 carbides, either along the grain boundaries or at the lath boundaries. Occasionally, fine platelets of M2C carbides were also observed within the laths. The position, shape, distribution, and type of carbides did not change significantly due to 172,000 hours of service exposure in the temperature range of 450°C to 500°C. The average interparticle spacings of the carbides increased from 0.35 to 1.2 µm due to 172,000 hours of exposure.

General Existence of Ledges on 1 Plates in Cu-Zn-Al Alloys
CHUN-MING LI, ZHI-GANG YANG, JIA-JUN WANG, YAN-KANG ZHENG, and HONG-SHENG FANG
A transmission electron microscopy (TEM) study was made of the interphase boundary structure between the matrix and 1 plates formed at 250°C, 300°C, and 350°C in three Cu-Zn-Al alloys. The experiments showed that the 1 plates formed at each temperature studied in all of the alloys are initially free of stacking faults. This observation, confirming the results of previous investigations, shows that these plates cannot have formed by a shear mechanism. Ledges were shown to exist on the broad faces and/or at the growth edges of the 1 plates. Indirect evidence implies that the ledges are mobile and that they can migrate along different directions on the broad faces.

The Transformation Phenomenon in Fe-Mo-C Alloys: A Solute Drag Approach
ZI-KUI LIU
A recently modified solute drag model, taking into account the interfacial segregation of alloy elements, is applied to the decomposition of austenite into ferrite in Fe-Mo-C alloys. The calculations are carried out with assessed thermodynamic and kinetic data stored in databases. The effect of the Gibbs energy of segregation is discussed in connection with published experimental observations of the bay in the time-temperature-transformation (TTT) curve for initiation of transformation. The solute draglike effect suggested in the literature is discussed in comparison with the present quantitative calculation.

TRANSPORT PHENOMENA

Oxidation of Low Carbon Steel in Multicomponent Gases: Part I. Reaction Mechanisms during Isothermal Oxidation
H.T. ABULUWEFA, R.I.L. GUTHRIE, and F. AJERSCH
The article describes rates of oxidation of low carbon steel in various nitrogen-based atmospheres of O2, CO2, and H2O in the temperature range 800°C to 1150°C. In characterizing the oxidation process, the weight gains of the samples per unit surface area vs time data were analyzed. Reaction rates during oxidation in the binary atmospheres of CO2-N2 and H2O-N2 followed a linear rate law and were found to be proportional to the partial pressures of CO2 or H2O. These rates were controlled by rate of reactions at the oxide surface and were highly dependent on oxidation temperature. The activation energies of the phase boundary reactions obtained were approximately 274 and 264 kJ/mole, for oxidation in CO2 and H2O atmospheres, respectively. Oxidation in gases containing free oxygen showed that the main oxidizing agent was the free oxygen and that additions of CO2 and H2O had little effect on the magnitude of the initial oxidation rates. Experiments for oxidation in multicomponent gases showed that the overall oxidation rates were the additions of rates resulting from oxidation with the individual gaseous species O2, CO2, and H2O. Oxidation in these atmospheres exhibited an initial linear rate law which gradually transformed into a parabolic. Examination of scale microstructure after 1 hour of oxidation showed that, for oxidation in carbon dioxide and water vapor atmospheres, only wustite was present, while in atmospheres containing free oxygen, all three iron oxides, wustite, magnetite, and hematite, were present.

Oxidation of Low Carbon Steel in Multicomponent Gases: Part II. Reaction Mechanisms during Reheating
H.T. ABULUWEFA, R.I.L. GUTHRIE, and F. AJERSCH
Oxidation behavior of low carbon steel during reheating in an industrial walking-beam steel reheat furnace was investigated. It was observed that scaling (oxidation) rates were reduced by reducing the input air/fuel ratio to the furnace, thereby lowering concentrations of free oxygen in the combustion products from about 3 to 1.5 pct. Laboratory experiments involving isothermal and nonisothermal oxidation were carried out in atmospheres consisting of oxygen, carbon dioxide, water vapor, and nitrogen. A general equation for the prediction of weight gains due to oxidation during reheating, using isothermal oxidation rate constants, was developed. The prediction of weight gains from nonisothermal oxidation conducted in the laboratory was poor, owing to a separation of the scale from the metal substrate which took place at about 900°C. The predicted weight gains during reheating in the industrial reheat furnace indicated that oxidation rates during reheating were intermediate between linear and parabolic, especially during reheating with high air/fuel ratio. However, the linear mechanism predominated. Laboratory isothermal experiments for oxidation in atmospheres containing free oxygen showed that the magnitude of the linear oxidation rates was determined by the oxygen concentration in the atmosphere. It was concluded that the observed reduction in scaling rates during reheating of low carbon steel in the industrial reheat furnace was a result of the lower free oxygen level in the furnace atmosphere.

MECHANICAL BEHAVIOR

A Numerical and Experimental Study of Deformation Characteristics of the Plane Strain Punch Stretching Test
YOUNGSUK KIM and CHUNDAL PARK
Recently, a simple new test method called the plane strain stretching (PSS) test has been developed to evaluate the stamping formability of sheet materials. The PSS test has been proven to have good reproducibility and show good correlation with press performance. In order to clarify the deformation characteristics of the PSS test and investigate the effect of material and process variables on the performance of the PSS test, three-dimensional finite element simulations for the PSS test were performed and the results compared with experiments.

High-Temperature Relaxations in Aluminum Studied by Isothermal Mechanical Spectrometry
ANDR;aaE RIVI;agERE and PASCAL GADAUD
Isothermal mechanical spectrometry experiments were carried out between 10-4 and 100 Hz on strain hardening aluminum samples, after annealings at various temperatures ranging between 300 and 700 K. For lower annealing temperatures (<480 K), internal friction spectra exhibit only a low frequency background. After annealings above 480 K, a relaxation peak superimposed to the background stands out. The peak height is, first, increasing for annealing temperatures below 580 K and then decreasing. The apparent relaxation parameters are found to be strongly dependent on the annealing temperature. The evolution of the internal friction spectra cannot be correlated with a grain boundary relaxation but with a reversible motion of dislocation segments inside dislocation networks developed during recovery annealings.

Effects of Microstructure on the Strength and Fatigue Behavior of a Silicon Carbide Fiber-Reinforced Titanium Matrix Composite and Its Constituents
W.O. SOBOYEJO, B.M. RABEEH, Y. LI, Y.C. CHU, A. LAVRENTENYEV, and S.I. ROKHLIN
The results of a systematic study of the effects of microstructure on the strength and fatigue behavior of a symmetric [0/90]2s Ti-15A1-3Cr-3A1-3Sn/SiC (SCS-6) composite are presented along with relevant information on failnure mechanisms in the composite constituents, i.e., the interface, fiber, and matrix materials. Damage micromechanisms are elucidated via optical microscopy, scanning electron microscopy (SEM), and nondestructive acoustic emission (AE) and ultrasonic techniques. Composite damage is shown to initiate early under cyclic loading conditions and is dominated by longitudinal and transverse interfacial cracking. Subsequent fatigue damage occurs by matrix slip band formation, matrix and fiber cracking, and crack coalescence, prior to the onset of catastrophic failure. However, the sequence of the damage is different in material annealed above or below the solvus of the Ti- 15-3 matrix material. Mechanistically based micromechanics models are applied to the prediction of the changes in modulus induced by fatigue damage. Idealized fracture mechanics models are also employed in the prediction of the fatigue lives of smooth specimens deformed to failure at room temperature. The article highlights the potential to develop mechanistically based predictive models based on simplified mechanics idealizations of experimental observations.

Further Study on the Mechanism of the Ductile-to-Brittle Fracture Transition in C-Mn Base and Weld Steel
G.Z. WANG, J.H. CHEN, and Z.H. LI
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., p pc for initiating a crack nucleus; m/ Tc for preventing the crack nucleus from blunting; and yy 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.

Communication: Room Temperature Superplasticity in a Zn-0.3 Wt Pct Al Alloy
TAE KWON HA, WON BEOM LEE, CHAN GYUNG PARK, AND YOUNG WON CHANG

SURFACE TREATMENT

Phase Transitions and Microstructure of a Laser-Induced Steel Surface Alloying
A. GLOZMAN and M. BAMBERGER
Alloying the surface of AISI 1045 steel with CrB by laser irradiation causes partial dissolution of the chromium boride in the melt and the formation of different borides of Fe and Cr in the treated layer. At a low laser scan velocity (0.01 m/s), the dissolution of CrB is almost complete, and the microstructure and properties of the top layer are uniform. At a higher scan velocity (0.05 m/s), a large number of CrB particles remain undissolved in the layer, and its properties are heterogeneous. The matrix consists of columns of iron boride, with up to 20 pct Cr dissolved in it, and between them a eutectic containing -Fe and chromium-boride with dissolved Fe. Iron boride grows on a transitional layer of Cr2B coating the surface of residual CrB particles, which cannot serve as nucleation sites because of the incompatibility of their crystal structure with that of (Fe,Cr)2B.

SOLIDIFICATION

In Situ Studies of Precipitate Formation in Al-Pb Monotectic Solidification by X-Ray Transmission Microscopy
WILLIAM F. KAUKLER, FRANZ ROSENBERGER, and PETER A. CURRERI
Al-1.5 wt pct Pb monotectic alloys were unidirectionally solidified. X-ray transmission microscope (XTM) observations, both during and after solidification, revealed various new morphological/compositional features in the melt and solid. In the melt, nonuniform lead-rich interfacial segregation layers and droplets were observed to form well ahead of the interface. In the solid, periodic striae formed at translation/solidification velocities as low as 6 x 10-6 m/s. The striae shape does not replicate that of the interface. The striae spacing decreases from 4 to 2 x 10-4 m with an increasing solidification rate between 6 and 16 x 10-6 m/s. High resolution postsolidification XTM examination reveals that these striae consist of Pb-rich particles of 2 to 3 x 10-6 m diameter. At translation/solidification velocities below 6 x 10-6 m/s, Pb incorporation into the solid occurs in the form of continuous fibers and strings of particles of about 5 x 10-6 m diameter. Bands, parallel to the interface, in which these fibers were aligned in the solidification direction, alternated with bands of poor fiber alignment. The width of these bands is comparable to the striae spacings obtained at the high solidification rates.
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