METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume 26A, No. 5, May 1995 This Month Featuring: Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Environment; Welding and Joining; Surface Treatment. View May 1995 Contents.

THE 1994 DISTINGUISHED LECTURE IN MATERIALS AND SOCIETY

ALLOY PHASES

Phase Stability and Microstructure of Al-Ti-Fe near Al₃Ti
T.Y. YANG and EDWARD GOO
The phase stability and microstructure of Al-Ti-Fe near Al₃Ti at 800°C are studied by using high resolution electron microscopy and backscattered electron imaging. One of the most significant changes resulting from the addition of small amounts of iron in this system is to convert tetragonal Al₃Ti or Al₂Ti into cubic Ll₂ structures. In addition, the added iron can change the stacking structure of Al₂Ti and the domain size of the long-period one-dimensional antiphase domain structures. A partial ternary phase diagram in this region is also determined.

TRANSFORMATIONS

Effect of Alumina Fibers on the Aging Characteristics of 2024-Based Metal-Matrix Composites

Homogeneous Formation of Epsilon Carbides within the Austenite during the Isothermal Transformation of a Ductile Iron at 410°C
I. GUTIERREZ, J. ARANZABAL, F. CASTRO, and J.J. URCOLA
The transformation of a ductile iron at 410°C for different times, after austenitization for 30 minutes at 900°C, is analyzed in detail. Upper bainite and a high volume fraction of austenite are formed for intermediate annealing times. A certain amount of martensite is observed after quenching not only for short transformation times but also for intermediate times. The formation of the martensite on cooling after intermediate transformation times is due to the decrease in carbon concentration of the retained austenite because of the homogeneous precipitation of epsilon carbides within. This homogeneous precipitation of epsilon carbide inside austenite is unambiguously observed. The epsilon carbide, pre-precipitated in austenite, which transforms to martensite on cooling, continues growing in the martensite after transformation. For long times of austempering at 410°C, some complex large carbides or silicocarbides are formed, probably from the epsilon carbide, which result in the total decomposition of austenite.

Two-Dimensional Grain Growth in Rapidly Solidified Succinonitrile Films
M. PALMER, K. RAJAN, M. GLICKSMAN, V. FRADKOV, and J. NORDBERG
The kinetics and topological mechanisms of normal grain growth have been examined through in situ dynamic studies on rapidly solidified succinonitrile (SCN). The in situ studies allowed for continuous monitoring of the evolution of individual grains during growth. We have assessed the Mullins-Von Neumann topological grain growth law and the Burke-Turnbull parabolic rate law and have determined rate constants that describe grain growth. This work demonstrates that both laws are both obeyed globally and consistently. These in situ studies permit one to follow the unit operations associated with grain growth kinetics. This article demonstrates the usefulness of succinonitrile as a model analog system for studying grain growth.

Communications: Amorphization Promoted by Mechanical Alloying of Aluminum-Rich Al-Ti-Fe Mixed Powders
SHIGEOKI SAJI, YUTAKA NEISHI, HIDEKI ARAKI, YORITOSHI MINAMINO, and TOSHIMA YAMANE

TRANSPORT PHENOMENA

MECHANICAL BEHAVIOR

Cracking of Duplex Stainless Steel Due to Dissolved Hydrogen
J.-H. HUANG and C.J. ALTSTETTER
Ferallium 255 duplex stainless steel was cathodically precharged with hydrogen at 265°C in a molten salt electrolyte. Sustained load tests were carried out in air at 0°C, 25°C and 50°C with average hydrogen contents from 3 to 15 wt ppm. The DC potential drop method was calibrated with optical measurements to continuously monitor the crack position and allow calculation of crack velocity and stress intensity. The crack velocity vs stress intensity (K) curves generally rose gradually over a large range in K and had definite thresholds for subcritical crack growth. Second and third stages were not always clearly delineated. Threshold stress intensities decreased as hydrogen content increased. An identifiable stage 11 occurred most often for alloys containing about 10 wt ppm dissolved hydrogen. The crack growth velocities generally increased with increasing temperature or hydrogen content. As the dissolved hydrogen increased, the fracture mode changed from microvoid coalescence (MVC) to microcrack coalescence (MCC) with some tearing ridges. At high hydrogen content, both ferrite and austenite phases showed brittle morphology. which was identical to the fracture surface of the uncharged specimens tested in hydrogen gas at 108 kPa pressure. Comparing the embrittling effect of internal hydrogen with that of external hydrogen it is found that the threshold stress intensity in hydrogen gas at 1 atm is lower than that at the highest internal hydrogen concentration (15 wt ppm). In the case of external hydrogen. the hydrogen source is at the crack tip, in this region, even when the bulk hydrogen content is fairly high.

Powder Metallurgy by Bidimensional Compression
S.M. JOSLIN and K.H.G. ASHBEE
By subjecting encapsulated aluminum and aluminum alloy powders to hot bidimensional compression, the time needed to fabricate monolithic material can be reduced to seconds. The high shear stresses generated by bidimensional compression lead to high frictional forces between neighboring powder particles. These forces foster particle surface abrasion and cleaning and particle joining by time-independent mechanisms that rely on the generation locally of high temperature, frictional heat. Porosity and mechanical property measurements show that the compacts can be of near-theoretical density and of sound tensile strength and impact resistance.

Transformation Strengthening by Thermomechanical Treatments in C-Mn-Ni-Nb Steels
SUNGHAK LEE, DONGIL KWON, YOUNG KOOK LEE, and OHJOON KWON
The purpose of this study is to clarify the correlation between microstructural factors and mechanical properties of ultrafine steels processed by thermomechanical controlled treatments. Three steels deformed at high strain rates in a pilot plant rolling mill showed very fine ferritic microstructure, whose grains became more equiaxed and finer with increasing fraction of alloying elements, and had good tensile and fracture properties, although they contained only about 0.01 pct carbon. Especially in the Ni-added steel, tensile properties were greatly improved because of the high dislocation density and the fineness of the ferritic substructure, readily satisfying the requirements for commercial-grade high-strength, high-toughness steels. The formation of ultrafine equiaxed grains in the steels might be explained by a possible strain-induced dynamic transformation mechanism associated with the austenite ferrite transformation caused by heavy deformation in the austenite range.

Study on the Relationship between Intermediate-Temperature Embrittlement and Triaxial Stress Induced by the Ellipsoidal Graphite in Hot-Rolled Ferritic Spheroidal Graphite Cast Iron
Y.F. LIN, T.S. LUI, and L.H. CHEN
The intermediate-temperature embrittlement of a hot-rolled ferritic spheroidal graphite cast iron was studied with the consideration that biaxial stress is induced by the ellipsoidal graphite particles of three unequal axial radii. The graphite shape was changed by various rolling reductions, and the tensile tests were performed at 673K. The results show that the elongation and flow stress are independent of rolling reduction, and intergranular fracture occurs in all specimens. In the plasticity analysis, the triaxiality ratio (_m/_eq) at a point in the ferrite matrix center can be expressed in terms of graphite shape ratio (bd) and graphite interparticle spacing (2a_d) as

_m/eq = 1/3 + a_d/(2b_d)

where _m is the hydrostatic tensile stress and _eq) is the equivalent stress. Accordingly, the average triaxiality ratio in the matrix center region is independent of rolling reduction and greater than one, a result that is consistent with the fact that the elongation is about constant, and all specimens undergo intergranular fracture. Moreover, the rolling reduction independent flow-stress behavior can be rationalized by the analytical result that the average _m/_eqis unchanged with rolling reduction, where _zis the internal stress along the tensile direction.

Deformation of Two C36 Laves Phases by Microhardness Indentation at Room Temperature
YAPING LIU, SAMUEL M. ALLEN, and JAMES D. LIVINGSTON
Microhardness indentation and electron microscopy were used to study the room-temperature deformation behavior of two Laves phases, ZrFe₂ and MgNi₂, in two-phase alloys. Evidence of extensive plastic deformation was found around the indentations. Activation of basal and nonbasal slip systems forms a distinctive cell structure in both Laves phases. Slip on planes nearly normal to the basal plane occurs in the lamellar form of ZrFe₂. Burgers vectors of the dislocations and resulting shear displacements are studied using high-resolution transmission electron microscopy (TEM). Curved or zigzag nonbasal slip surfaces in atomic scale were observed. Each slip plane can be considered to be composed of a series of small facets of high atomic density. A common Burgers vector and displacement vector component of 1/4 [0001] was found on many different slip planes. The "zonal glide" concept for slip on prismatic planes is discussed.

High-Temperature Low-Cycle Fatigue and Lifetime Prediction of Ti-24AI-11 Nb Alloy
G. MALAKONDAIAH and T. NICHOLAS
The influence of hold time on low-cycle fatigue (LCF) of Ti-24AI-11Nb was studied at 650°C. At 0.167 Hz, the alloy exhibits cyclic hardening at all strain levels studied and obeys the well known Manson-Coffin behavior. A 100-second hold at peak tensile or compressive strain at ±0.6 pct strain has no observable effect on cycles to failure. For hold times at ±0.5 pct strain, however, the fatigue lives are nearly halved and specimens show secondary cracking normal to the stress axis. The increase in inelastic strain as a result of hold time appears to be primarily, responsible for the observed loss in fatigue life. A linear life fraction model, which considers both fatigue and creep damage, is found to provide good correlation of measured lives with predictions. For the range of test conditions employed, the total and the tensile hysteretic energy, per unit volume, absorbed until fracture, remain nearly constant. The tensile hysteretic energy appears to be a more useful measure of fatigue damage for life prediction.

Crack Detection by Resonant Frequency Measurements
SHAUN M. McGUIRE, MORRIS E. FINE, and JAN D. ACHENBACH
The resonant frequency of 304 stainless steel specimens with a center-drilled hole has been measured as a function of fatigue cycles during crack initiation and propagation. Simultaneous measurements of crack lengths by scanning electron microscopy yield the resonant frequency vs crack length. The change of resonant frequency is equivalent to the change of an effective elastic modulus. Analytical results for a "spring~ model to predict the change in effective modulus due to the presence of cracks have been compared with results derived from resonant tests.In the model, the load transfer across the plane of the crack is represented by a distribution of springs of stiffness that depends on the crack length. Good agreement is observed between theory and experiment for cracks up to 500-µm long. The model may be used to obtain the crack length from resonance measurements.

Fiber Fracture during Processing of Continuous Fiber, Metal-Matrix Composites Using the Foil / Fiber/ Foil Technique
P.D. NICOLAOU. H.R. PIEHLER, and S.L. SEMIATIN
The fracture of continuous fibers during processing of foil/fiber/foil (F/F/F) metal-matrix composites (MMCs) has been investigated both experimentally and theoretically. Experimental observations revealed that fiber fracture occurs during the heat-up portion of the consolidation cycle primarily in a bending mode in regions where cross-weave wires are present. Based on these observations. a general model that describes fiber fracture as a function of the processing stress and fiber mat geometry was developed. Model results showed that fiber stresses and, hence, the propensity for fracture are very sensitive to the distance between cross-weave wires in adjacent fiber mats; analytical expressions that allow the definition of a critical distance between such cross-weave wires were derived. The model relations demonstrated that fiber fracture is more likely in areas of a composite in which the fibers are arranged in a rectangular, rather than a triangular, pattern. The experimental and theoretical results were used to develop guidelines for the design of F/F/F lay ups to avoid fiber fracture during processing.

Fracture Characteristics of Ti-6AI-4V and Ti-5AI-2.5Fe with Refined Microstructure Using Hydrogen
M. NIINOMI, B. GONG, T. KOBAYASHI, Y. OHYABU, and O. TORIYAMA
The hydrogenation behavior of Ti-6A14V, with the starting microstructures of coarse equiaxed and coarse Widmanstätte respectively, was investigated under a hydrogen pressure of 0.1 MPa at temperatures between 843 and 1123 K. The hydrogen content was determined as a function of hydrogenation time, hydrogenation temperature, and hydrogen flow rate. The phases presented in the alloy of after hydrogenation were determined with X-ray and electron diffraction analysis in order to define the effect of Thermochemical Processing (TCP) on the microstructure of the alloy. Mechanical properties and fracture toughness of Ti-6AI-4V and Ti-5AI-2.5Fe subjected to the various TCP were then investigated. Hydrogenation of Ti-6AI-4V with the starting microstructure of coarse equiaxed at 1023 K, just below hydrogen saturated (denoted (H)) transus temperature, produces a microstructure of orthohombic martensite (denoted " (H)) and (H). Hydrogenation at 1123 K, above (H) transus, results in a microstructure of " (H)) and (H). Microstructure refinement during TCP results mainly from decomposition of " (H)) and (H).into a fine mixture of + during dehydrogenation. An alternative TCP method is below (H) transus hydrogenation (BTH), consisting of hydrogenation of the alloy below the hydrogenated (H) transus temperature, air cooling to room temperature, and dehydrogenation at a lower temperature, which is found to improve mechanical properties significantly over a conventional TCP treatment. Compared with the untreated material, the BTH treatment increases the yield strength and increases the ultimate tensile strength significantly without decreasing the tensile elongation in the starting microstructure of coarse equiaxed a or with a little decrease in the tensile elongation in the starting microstructure of coarse Widmanstätten although the conventional TCP treatment results in a large decrease in elongation over the unprocessed material in Ti-6AI-4V. In Ti-5Al-2.5 Fe, both conventional TCP and BTH result in a increase in yield strength, ultimate tensile strength, and elongation; however, the BTH gives the best balance between strength and elongation. The TCP-treated Ti-6AI-4V shows smaller fracture toughness compared with the unprocessed material, while TCP-treated Ti-5AI-2.5Fe shows greater fracture toughness compared with the unprocessed material. The BTH treatment results in a improvement in fatigue strength in both Ti-6AI-4V and Ti-5Al-2.5Fe.

Characterization and Analysis of Low-Temperature Superplasticity in 8090 Al-Li Alloys
H.P. PU, F.C. LIU, and J.C. HUANG
The 8090 Al-Li alloys, after a special thermomechanical process (TMP), exhibited low-temperature superplasticity (LTSP) from 350°C to 450°C and behaved differently from the conventional high-temperature superplasticity (HTSP). The LTSP sheets after ~700 pct elongation at 350°C and 8 x 10^-4 S^-1 still possessed fine "(sub)grains" 3.7 µm in size and narrow surface Lidepletion zones 11 µm in width, resulting in a post-SP T6 strength of ~500 MPa, significantly higher than that of the 8090 alloys tested at normal superplastic temperature of 525°C or above. Examination from the movement of surface marker lines in LTSP samples confirmed the role of grain boundary sliding (GBS), coupled with grain rotation and migration. During the initial stage (<150 pct), GBS along certain higher-angled boundaries was proceeded along a plane ±45 deg with respect to the sample surface. With increasing straining, sliding between individual grains or grain groups was observed on other planes, forming a zigzag morphology. Transmission electron microscopy (TEM) observations revealed appreciable dislocation activities, suggesting the involvement of dislocation creep. The tensile behavior and deformation mechanisms of the HTSP and LTSP sheets were investigated and analyzed over the strain rates range 10^-5 to 10^-2 S^-l. The strain-rate sensitivity (m value) for the LTSP and HTSP materials was found to be ~0.33 and 0.50, respectively. The activation energy was extracted to be 92 kJ/mole for the LTSP sheets and to be 141 kJ/mole for the HTSP sheets. Based upon these results, the primary deformation and accommodation mechanisms for the HTSP and LTSP sheets are GBS and dislocation creep, respectively.

Thermal Fatigue of a SiC/Ti-15 Mo-2.7Nb-3AI-0.2Si Composite
W.C. REVELOS, J.W. JONES, and E.J. DOLLEY
The influence of thermal cycling and isothermal exposures in air on the residual ambient temperature strength of SCS-6/Ti-lSMo-2.7Nb-3AI-0.2Si (weight percent) metal-matrix composites comprised of [0]₄ and [0/90]_s laminates has been determined. A maximum temperature of 815°C was used in thermal cycling and isothermal exposure. Temperature range, cycle count, maximum/minimum temperature, environment, and hold time at temperature were systematically varied. Postexposure ambient-temperature tension testing, scanning electron and optical~microscopy, and fractography were performed on selected specimens to determine the degree.of damage. A reduced residual strength was noted in thermal fatigue with increasing cycle count, maximum temperature, and hold time for all specimens tested in air. Isothermal exposures at 815°C also substantially reduced residual ambient-temperature strength. Considerably less reduction in strength occurred in inert environment than in air. Damage processes included matrix cracking, fiber/matrix interface damage, matrix embrittlement by interstitials, and oxide scale formation at specimen surfaces and, in some cases, at matrix/fiber interfaces. Fiber orientations which allowed rapid ingress of oxygen lead to greater matrix embrittlement and resulted in more pronounced reductions in strength.

Effect of Cooling Rate after Hot Rolling and of Multistage Strain Aging on the Drawability of Low-Carbon-Steel Wire Rod
A. KARIMI TAHERI, T.M. MACCAGNO, and J.J. JONAS
Tensile testing was used to simulate the multistage strain aging occurring in low-C steel during. the relatively short intervals between dies in a multiple-die wire-drawing machine. The effects were examined of three simulated post-hot-rolling cooling rates and three thermal treatments on the strain-aging susceptibility of a high- and a low-N steel. This was measured by applying a 6 pct tensile strain, followed by aging at either 65 or 100°C for 20 seconds, and then pulling the specimen to failure at room temperature. Increases in flow stress and decreases in the elongation to fracture both indicated high susceptibility to strain aging. It was found that the nitrogen content, the cooling rate from the hot-rolling temperature to about 300°C, as well as the cooling rate below 300°C, all have dramatic effects on the strain-aging behavior. Moreover, multistage strain aging is more severe than single-stage strain aging. The implications of these observations on increasing the drawability of low-carbon-steel wire are discussed.

Effects of Be and Fe Additions on the Microstructure and Mechanical Properties of A357.0 Alloys
YEN-HUNG TAN, SHENG-LONG LEE, and YU-LOM LIN
A357 hypoeutectic alloy is a heat-treatable Al-Si-Mg system with a nominal composition of Al-7 pct Si and about 0.6 pct Mg have widespreaded applications, especially in the aerospace and automotive industries. The purpose of this study was to determine the influences of Be and Fe content on the microstructure and mechanical properties of A357.0 alloys. Distinct morphologies were discerned between Be-containing and Be-free alloys. The Be-free alloys contain larger amount of iron-bearing phases with Mg than in Be-containing alloys. The addition of Be can change the plateletlike structure of iron-bearing phases to a comparatively harmless round nodular form. Also, the amounts of iron-rich phases are significantly lower and the silicon particles are smaller and more spherical in the Be-containing alloys. Small amounts of Be in A357.0 caused significant increases in the precipitation kinetics of Mg₂Si. It was found that the addition of Be lowers the ternary and binary eutectic melting point. The amount of Mg available to form the major strengthening phase Mg₂Si is increased promoting the tensile strength of A357.0 casting. The tensile properties were improved with decreasing Fe content and the addition of Be. The effect is more apparent in the higher Fe alloys than that in the lower Fe alloys.

A Comparative Evaluation of Low-Cycle Fatigue Behavior of Type 316LN Base Metal, 316 Weld Metal, and 316LN/316 Weld Joint
M. VALSAN, D. SUNDARARAMAN, K. BHANU, SANKARA RAO, and S.L. MANNAN
A comparative evaluation of the low-cycle fatigue (LCF) behavior of type 316LN base metal 316 weld metal and 316LN/316 weld joints was carried out at 773 and 873 K. Total straincontrolled LCF tests were conducted at a constant strain rate of 3 x 10^-3 s^-1 with strain amplitudes in the range ±0.20 to ±1.0 pct. Weld pads with single V and double V configuration were prepared by the shielded metal-arc welding (SMAW) process using 316 electrodes for weld-metal and weldjoint specimens. Optical microscopy scanning electron microscopy (SEM), and transmission electron microscopy (TEM) of the untested and tested samples were carried out to elucidate the deformation and the fracture behavior. The cyclic stress response of the base metal shows a very rapid hardening to a maximum stress followed by a saturated stress response. Weld metal undergoes a relatively short initial hardening followed by a gradual softening regime. Weld joints exhibit an initial hardening and a subsequent softening regime at all strain amplitudes except at low strain amplitudes where a saturation regime is noticed. The initial hardening observed in base metal has been attributed to interaction between dislocations and solute atoms/complexes and cyclic saturation to saturation in the number density of slip bands. From TEM, the cyclic softening in weld metal was ascribed to the annihilation of dislocations during LCF. Type 316LN base metal exhibits better fatigue resistance than weld metal at 773 K whereas the reverse holds true at 873 K. The weld joint shows the lowest life at both temperatures. The better fatigue resistance of weld metal is related to the bottle transformed delta ferrite structure and the high density of dislocations at the interface which inhibits the growth rate of cracks by deflecting the crack path. The lower fatigue endurance of the weld joint was ascribed to the shortening of the crack initiation phase caused by surface intergranular crack initiation and to the poor crack propagation resistance of the coarse-grained region in the heat-affected zone.

Classification of Fatigue Crack Growth Behavior
A.K. VASUDÉVAN and K. SADANANDA
A self-consistent theory has been developed to account for the variation in fatigue crack growth rates with load ratio, R. without reference to crack closure concepts. The theory states that (a) for an unambiguous description of cyclic damage two loading parameters are required; (b) consequently, there are two thresholds corresponding to each parameter that must be satisfied for a crack to grow (c) these two thresholds are intrinsic and are independent of specimen geometry (d) a fundamental threshold curve can be developed that is independent of test methods defining these two thresholds from the asymptotic values and last; (e) the two thresholds vary with the degree of slip planarity. microstructure and environment. Based on these new concepts we have classified the entire fatigue crack growth behavior into five different classes using the experimental K_th-R data. The characteristic feature of each class is discussed and the supporting examples of materials behavior are provided. This classification could provide a basis for understanding the synergistic effects of mechanical and chemical driving forces and microstructure contributing to fatigue crack growth.

Microstructure and Mechanical Properties of Spray-Deposited Al-17Si-4.5Cu-0.6Mg Wrought Alloy
YUE WU, WILLIAM A. CASSADA, and ENRIQUE J. LAVERNIA
High Si content in Al-Si alloys usually leads to the formation of coarse, brittle Si phase under slow solidification conditions. In the present study, an Al-17Si-4.5Cu-0.6Mg (referred to here-after as AS17) was synthesized using spray deposition to modify the Si phase. In the spray deposition process, the master alloy of AS17 was atomized using N₂ gas, and was deposited on a collecting substrate directly into a three-dimensional material. The microstructure and mechanical behavior of the spray-deposited AS17 were studied using optical microscopy (OM) scanning electron microscopy (SEM), transmission electron microscopy (TEM). X-ray diffraction, and tensile tests. The present results indicate that in the spray-deposited AS 17. the eutectic Si phase was modified from a "flakelike" morphology, characteristic of ingot metallurgy (IM) materials, into a "particulate" morphology. The formation of the coarse primary Si blocks was suppressed. Moreover, the size and morphology of Si particulates were found to have signficant influences on the deformation behavior. During plastic deformation, extensive fracture of Si occurred. The percentage of fractured Si increased with the increasing amount of plastic deformation and the size of Si particulates. Finally, the room-temperature mechanical properties of the spray-deposited AS17 were compared with its IM counterpart A390 (an IM alloy with identical composition as AS 17). The strength and ductility of AS17 were improved over those of A390. In the T6 condition, the yield strength and tensile elongation of AS 17 were 503 Mpa rand 3.0 pct, respectively. whereas those of A390 were 374 MPa and 1.3 pct. respectively.

Stable Nonlinear Relaxations in Multiphase Al-Zn Alloy
XIANFANG ZHU and LIDE ZHANG
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, where n is a parameter independent of temperature; (2) in a low-temperature (low-T) and low-strain-amplitude (low-A) region, Q^-1 = (B/fⁿ) exp (-nH/kT) (where B is a constant, H is the phase interface or interphase boundary atom diffusion activation energy, k is Boltzmann's constant, and T is the absolute temperature); n and H are independent of A. The damping originates from an anelastic motion of phase interface; (3) in an intermediate region, including low-T and high-A, middle-T and middle-A, and high-T and low-A regions, we still have the equation Q^-1 = (C/fⁿ) exp (-nH/kT), but the damping has a normal amplitude effect. C, n, and H all vary with A. The damping results from a nonlinear relaxation of phase interface; and (4) in a high-T and high-A region, there is no longer a linear relationship between In Q and l/T, whereas relation Q^-1 f^-n, is still satisfied; n increases as A 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.

Communications: Discussion of "Interstitial Precipitation in Fe-Cr-Al Alloys"
IGOR S. GOLOVIN

Communications: Authors' Reply
W.S. SPEAR and D.H. POLONIS

ENVIRONMENT

Communications: SCC Path in Forged AerMet 100 Steal
E.U. LEE

WELDING AND JOINTS

The Relaxation of Residual Stresses with Postweld Heat Treatment in High-Performance Weld Measured with Neutron Diffraction
R.A. WINHOLTZ and A.D. KRAWITZ
The residual stresses in a cylindrical weldment of HP-9-4-30 steel were measured with neutron diffraction in the as-welded (AW) state and after postweld heat treatment (PWHT). Large residual stresses are present in the interior of the material in the as-welded condition. The maximum principal stresses measured were found around the edges of the cap-pass heat-affected zone and reached up to 1045 MPa (76 pct of the base metal yield strength) in the as-welded condition. The principal stress directions for the residual stress tensors do not in general follow the hoop. axial, and radial axes of the weld and change from position to position within the weld. although the highest values are generally in the hoop direction. The postweld heat treatment relaxed the largest residual stresses, with the maximum value being 30 pct of the base metal yield stress. The need for position-dependent stress-free standards and the implications of stress gradients over the measurement volumes are discussed.

SURFACE TREATMENT