METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume26A, No. 11, November 1995 This Month Featuring: Symposium on Mechanics and Mechanisms of Material Damping; Transformations; Transport Phenomena; Mechanical Behavior; Physical Chemistry; Environment; Solidification; Materials Processing; Composite Materials. View November 1995 Table of Contents.

SYMPOSIUM ON MECHANICS AND MECHANISMS OF MATERIAL DAMPING

All of the papers present aspects of an effort to identify structural materials with high damping. This is a difficult task as it has been known since Zener's time that the linear damping is at its largest when the elastic energy density is at its smallest. Therefore, it is clear that high damping structural materials must be composites designed such that the mechanical interaction between the soft and hard components is optimized for high damping. Alternatively, high nonlinear damping can be utilized if a mechanical amplification mechanism can be built into the composite. The papers reflect this overall strategy covering seemingly diverse topics which, however are all related to the desire to understand more completely known or identify new damping mechanisms so that they may be combined into a high damping matrix to be incorporated into a composite.

Analysis of Elastothermodynamic Damping in Particle-Reinforced Metal-Matrix Composites
JOSEPH E. BISHOP and VIKRAM K. KINRA
When a composite material is subjected to a homogeneous or inhomogeneous stress field, different phases undergo different temperature fluctuations due to the well-known thermoelastic effect. As a result, irreversible heat conduction occurs and entropy is produced. This entropy production is the genesis of elastothermodynamic damping. Recently, taking the second law of thermodynamics as a starting point, a general methodology for calculating the elastothermodynamic damping was presented by Kinra and Milligan. Using this method, we calculate the elastothermodynamic damping for two canonical problems concerning particle-reinforced metal-matrix composites: (1) a single spherical inclusion in an unbounded matrix and (2) an N layer finite concentric composite sphere. In both cases, a uniform radial time-harmonic loading is considered.

An Investigation of the Mechanical Damping of Ductile Iron
S.H. CARPENTER, T.E. STUCH, and R. SALZBRENNER
Ductile iron has been suggested as a candidate material for a number of practical applications, including turbine castings, automotive components, and transportation and storage casks for hazardous and radioactive materials. The applications require the enhanced ductility resulting from the presence of spherical graphite nodules in the ductile ferrite iron matrix. Proper design of such components requires a knowledge of the mechanical properties, including how energy is absorbed and dissipated (mechanical damping) by the test material. This article is a study of the mechanical damping of a series of well-characterized ductile iron materials (four separate materials) as a function of strain amplitude, temperature over the range of -100 °C to + 100 °C, and magnetic field. The major sources of damping were found to be dislocation motion in the graphite phase and magnetomechanical damping in the ferrite phase. The magnitude of the magnetomechanical damping was much larger than that due to dislocation motion. An additional goal of the investigation was to determine if any correlation existed between the measured mechanical damping and the fracture toughness of the ductile iron materials; no correlation was found.

Effect of Intrinsic Damping on Vibration Transmissibility of Nickel-Titanium Shape Memory Alloy Springs
EDWARD J. GRAESSER
A research study was undertaken to measure the transmissibility of nickel-titanium (Ni-Ti) shape memory alloy (SMA) springs and to compare the results to corresponding data on steel and INCONEL springs. It was motivated by interest in an effective metal alternative to rubber-based machinery isolation mounts, with possible active control features. Ni-Ti was used due to its well-known properties of shape memory and high intrinsic damping. Acceleration transmissibility was measured on a spring-mass system. Due to the distributed mass in the spring coils, standing waves occurred at high frequencies. However, due to the high intrinsic damping in Ni-Ti, the standing wave resonance peaks were as much as 20 dB lower than corresponding peaks in steel and INCONEL springs. Thus, the capability of Ni-Ti springs tor high frequency acoustic isolation is significantly better than that of steel or INCONEL. Also, it is judged that the Ni-Ti material could be used in a variety of other isolation mount designs with a high likelihood for further improvement in passive isolation properties. In addition, it may be possible to use the shape memory effect (SME) in active control concepts.

Magnetomechanical Damping in Giant Magnetostriction Alloys
K.B. HATHAWAY, A.E. CLARK, and J.P. TETER
Magnetomechanical damping in the giant magnetostriction material Terfenol-D has been investigated by quasi-static stress-strain measurements and modeled theoretically. The damping capacity is a strong function of applied stress amplitude, increasing to a maximum at some finite stress and then decreasing slowly at still larger applied stresses. Maximum damping capacities greater than 1.0 were observed for the lowest magnetic bias fields and prestresses at applied stress amplitudes of 4 MPa. Both the qualitative behavior of the damping and its magnitude are successfully described by a model of abrupt magnetization jumps within individual domains driven by the applied stress.

Damping Behavior of Discontinuously Reinforced Al Alloy Metal-Matrix Composites
E.J. LAVERNIA. R.J. PEREZ, and J. ZHANG
High damping materials allow undesirable mechanical vibration and wave propagation to be passively suppressed. This proves valuable in the control of noise and the enhancement of vehicle and instrument stability. Accordingly, the scientific community is continually working toward the development of high damping metals (hidamets) and high damping metal-matrix composites (MMCs). The MMCs are particularly attractive in weight-critical applications when the matrix and reinforcement phases are combined to provide desirable property combinations, such as high damping and low density. Inspection of the available scientific literature, however, reveals that an understanding of the precise correlation between the presence of secondary phases (either reinforcements or precipitates) and material damping has eluded investigators, partly as a result of the superposition of multiple mechanisms. As a step toward the clarification of damping phenomena in discontinuously reinforced MMCs, this article describes the damping behavior and mechanisms that are present in discontinuously reinforced MMCs, with particular emphasis on particulate-reinforced Al alloy MMCs processed using spray atomization and deposition. The operative damping mechanisms in the particulate-reinforced MMCs are discussed in light of the data obtained from microstructural studies and damping capacity measurements.

Resonance Apparatus for Damping Measurements
GILBERT F. LEE
The resonance apparatus was developed to characterize the damping properties of materials. The apparatus has been used extensively to determine Young's modulus and loss factor of polymers in the kHz frequency range over a temperature range of -60 °C to 70 °C. However, very little work has been done in using the apparatus to characterize metallic materials. To demonstrate the limits of the instrument, measurements were made on a high damping material, an epoxy polymer, and a low damping material, aluminum. The loss factor for the epoxy polymer is 2.4, which is the highest value measured in this apparatus. The loss factor for the aluminum is 0.003, which is the lowest value measured in this apparatus. Based on the aluminum results, the accuracy and precision were found to be 2 pct for Young's modulus. However, the accuracy and precision for loss factor were not as good as for the modulus.

Damping and Acoustic Harmonics in Cracked Laminated Composites
K.R. THUMMA and D.N. BESHERS
This article reports on a novel experimental technique conceived and used to study damping and generation of acoustic harmonics in a cracked laminated composite. The cracked jaws of a double cantilever specimen with an interlaminar crack were vibrated symmetrically at 22.5 kHz; damping in the specimen and acoustic harmonics in the vicinity of the crack tip were measured as functions of amplitude of vibration. At low amplitudes, the damping was linear (i.e., independent of amplitude) with no detectable acoustic harmonics. At high amplitudes, the damping was nonlinear with generation of acoustic harmonics up to the fifth. Among known models for harmonic generation, the best choice to explain these data seems to be smooth motion of dislocations in graphite vibrating within a potential well of small asymmetry.

TRANSFORMATIONS

Communication: Microstructural Changes during Aging of a Binary Al-0.1 Wt Pct Be Alloy
A.K. MUKHAPADHYAY, VIJAYA SINGH, and A.K. SINGH

TRANSPORT PHENONEMA

MECHANICAL BEHAVIOR

Hydrogen-Enhanced Localization of Plasticity in an Austenitic Stainless Steel
DANIEL P. ABRAHAM and CARL J. ALTSTETTER
Microscopic observations and the results of static strain aging, stress relaxation, and strain rate change tests on 310s stainless steel foils, with and without hydrogen, have been presented to complement the stress-strain curves in a previous article. The hydrogen-free specimens showed minute yield points during static strain aging, while the hydrogen-containing specimens demonstrated "preyield microstrain". Thermal activation analysis of the strain rate change and stress relaxation plots led to the conclusion that the activation area for dislocation motion is decreased by hydrogen. Microstructural examination with the scanning electron microscope (SEM) revealed extensive strain localization, while transmission electron microscopy (TEM) studies showed microtwinning and austenite faulting in hydrogenated specimens tested at room temperature. The relation of hydrogen-induced changes in plastic deformation to hydrogen embrittlement is discussed.

Temperature and Strain Rate Dependence of the Portevin-Le Chatelier Effect in a Rapidly Solidified Al Alloy
D.M. Ll and A. BAKKER
The Portevin-Le Chatelier (PL) effect was studied for a rapidly solidified aluminum alloy in a temperature range of 193 to 423 K and a strain rate range of 10^-5 to 10^-1 s^-1. Emphasis was put on the dependence of the critical strain (_c) for the PL effect on temperature (T) and strain rate (). It is found that the overall _c -T- relation is grouped into two categories: _c increases with increasing but with decreasing T in the low T/high regime and _c increases with increasing T but with decreasing in the high T/low regime. The underlying dynamic strain aging (DSA) mechanism is discussed to account for the two different yet unified categories by calculating activation energies and by introducing the transition conditions.

The Effect of Grain Size on the High-Strain, High-Strain-Rate Behavior of Copper
MARC A. MEYERS, UMBERTO R. ANDRADE, and ATUL II. CHOKSHI
Copper with four widely differing grain sizes was subjected to high-strain-rate plastic deformation in a special experimental arrangement in which high shear strains of approximately 2 to 7 were generated. The adiabatic plastic deformation produced temperature rises in excess of 300 K, creating conditions favorable for dynamic recrystallization, with an attendant change in the mechanical response. Preshocking of the specimens to an amplitude of 50 GPa generated a high dislocation density; twinning was highly dependent on grain size, being profuse for the 117- and 315-µm grain-size specimens and virtually absent for the 9.5-µm grain-size specimens. This has a profound effect on the subsequent mechanical response of the specimens, with the smaller grain-size material undergoing considerably more hardening than the larger grain-size material. A rationale is proposed which leads to a prediction of the shock threshold stress for twinning as a function of grain size. The strain required for localization of plastic deformation was dependent on the combined grain size/shock induced microstructure, with the large grain-size specimens localizing more readily. The experimental results obtained are rationalized in terms of dynamic recrystallization, and a constitutive equation is applied to the experimental results; it correctly predicts the earlier onset of localization for the large grain-size specimens. It is suggested that the grain-size dependence of shock response can significantly affect the performance of shaped charges.

Mechanical Behavior of Aluminum Deformed under Hot Working Conditions
E.S. PUCHI and M.H. STAIA
The stress-strain behavior of aluminum 3-9 purity deformed at elevated tempcratures has been analyzed on a rational basis. Emphasis has been given to the analysis of the curves corresponding to typical deformation conditions of interest for hot rolling of commercial aluminum alloys. The strain hardening behavior has been modeled assuming the validity of the typical saturation exponential equation earlier proposed by Voce. The temperature and strain dependence of the flow stress parameters involved in such an equation has been introduced by means of a model based on the power law relationship, where the stress-sensitivity exponent of the strain rate is considered to be temperature dependent. The strong temperature dependence of this parameter precluded the use of the exponential relationship expressed in terms of the Zener Hollomon parameter. Therefore, a different temperature-compensated strain rate parameter similar to the MacGregor-Fisher parameter has been employed following the earlier developments put forward by Kocks. Thus, a satisfactory correlation of the flow stress parameters with the deformation conditions has been obtained. The final constitutive equation derived provides a satisfactory reproduction of the experimental values of the flow stress and follows quite closely the strain-hardening behavior. The mean activation energy determined by the different models confirmed the predominance of both climb of edge dislocation segments and motion of jogged screw dislocations as the rate-controlling mechanisms during deformation of this material under hot-working conditions. The use of a constitutive equation which expresses the flow stress of the material in terms of the applied strain, rate of straining, and deformation temperature to calculate the power dissipation efficiency of the material () deformed under hot-rolling conditions has shown that it could be strongly strain dependent, particularly toward the end of the rolling schedule. Hence, it has been concluded that the calculation of both the power co-content as defined in dynamic material modeling (DMM) and its maximum value, taking into consideration the constitutive equation previously developed, represents a more plausible and soundly based approach toward the determination of .

The Effects of Reinforcement Additions and Heat Treatment on the Evolution of the Poisson Ratio during Straining of Discontinuously Reinforced Aluminum Alloys
PREET M. SINGH and JOIIN J. LEWANDOWSKI
The effects of reinforcement additions and heat treatment on the evolution of the Poisson ratio were determined for a 7xxx aluminum alloy reinforced with 15 vol pct SiC_p, a 2xxx alloy with 20 pct SiC_p, and a 2014 alloy with 15 pct Al₂O₃. The Poisson ratio of the monolithic alloy was 0.31 to 0.32 in the elastic regime. At the onset of the plastic regime, the Poisson ratio of the monolithic materials rose rapidly to about 0.45 and then gradually increased to 0.47 by 3.5 pct strain. For discontinuously reinforced aluminum (DRA) materials, the Poisson ratio in the elastic regime was considerably lower than that exhibited by the matrix alloy, while the magnitude of the difference was dependent upon the type, volume fraction, and elastic properties of the reinforcement. In addition, the evolution of the Poisson ratio for DRA material depends upon heat treatment and level of strain due to damage evolution (e.g., SiC_p cracking, matrix failure, etc.) which accompanies straining in these materials. Both the magnitude and extent of change in the Poisson ratio with increasing strain in the composite is rationalized by the accumulation of damage which accompanies increasing strain.

Experimental Study on the Thermoelastic Martensitic Transformation in Shape Memory Alloy Polycrystal Induced by Combined External Forces
PETR SITTNER, YASUHIRO HARA, and MASATAKA TOKUDA
Combined tension and torsion experiments with thin wall specimens of Cu-Al-Zn-Mn polycrystalline shape memory alloy (SMA) were performed at temperature T = A_f. + 25 K. The general stress-strain behaviors due to the thermoelastic martensitic transformation, induced by a combination of external forces of axial load and torque, were studied. It is shown that the progress of martensitic transformation (MT) at general stress conditions can be well considered as triggered and controlled by the supplied mechanical work (a kind of equivalent stress) in the first approximation. Pseudoelastic strains in proportional as well as nonproportional combined tension-torsion loadings were found fully reversible, provided that uniaxial strains were reversible. The axial strain can be controlled by the change of torque and vice versa due to the coupling among tension and torsion under stress, not only in forward transformation, but also in reverse transformation on unloading. The pseudoelastic strains of SMA polycrystal are path dependent but well reproducible along the same stress path. The evolution of macroscopic strain response of SMA polycrystal, subjected to the nonproportional pseudoelastic loading cycles with imposed stress path, was systematically investigated. The results bring qualitatively new information about the progress of the MT in SMA polycrystal, subjected to the general variations of external stress.

Effects of Be and Fe Content on Plane Strain Fracture Toughness in A357 Alloys
YEN-HUNG TAN, SHENG-LONG LEE, and YU-LOM LIN
The effect of Be and Fe content on the plane strain fracture toughness K_IC of aluminum-based A357 alloys is investigated. The fracture behavior of A357 alloys has been evaluated as a function of both the magnitude and morphology of iron-bearing compounds and silicon particles. Addition of Be is beneficial for tensile properties and fracture toughness in the case of alloys containing interrnediate (0.07 pct) and higher (0.15 pct) Fe levels. On the other hand, Be added to alloys containing the lower Fe (0.01 pct) level appears detrimental to tensile strength, but the quality index, notch-yield ratio (NYR), and plane strain fracture toughness were improved. Fractographic analysis reveals that crack extension of A357 alloys occurs mainly in an intergranular fracture mode. The fracture processes are initiated by void nucleation at iron-bearing compounds or irregularly shaped eutectic silicon particles as a result of their cracking and decohesion from the matrix. Then, void growth and coalescence result in growth of the main crack by shear-linkage-induced breakdown of submicron strengthening particles. The effect of Be on increasing K_IC is more apparent in the higher Fe alloys than in the lower Fe alloys. Superior toughness obtained by microstructural control has also been achieved in the intermediate and higher Fe levels of Be-containing alloys, with values equal to those obtained in alloys of lower Fe content.

Creep Deformation in Near- TiAl: Part I. The Influence of Microstructure on Creep Deformation in Ti-49Al-1V
BRIAN D. WORTH, J. WAYNE JONES, and JOHN E. ALLISON
The influence of microstructure on creep deformation was examined in the near- TiAl alloy Ti 49Al-1V. Specifically, microstructures with varying volume fractions of lamellar constituent were produced through thermomechanical processing. Creep studies were conducted on these various microstructures under constant load in air at temperatures bctween 760 °C and 870 °C and at stresses ranging from 50 to 200 MPa. Microstructure significantly influences the creep behavior of this alloy, with a fully lamellar microstructure yielding the highest creep resistance of the microstructures examined. Creep resistance is dependent on the volume fraction of lamellar constituent, with the lowest creep resistance observed at intermediate lamellar volume fractions. Examination of the creep deformation structure revealed planar slip of dislocations in the equiaxed microstructure, while subboundary formation was observed in the duplex microstructure. The decrease in creep resistance of the duplex microstructure, compared with the equiaxed microstructure, is attributed to an increase in dislocation mobility within the equiaxed constituent, that results from partitioning of oxygen from the phase to the 2 phase. Dislocation motion in the fully lamellar microstructure was confined to the individual lamellae, with no evidence of shearing of / or /2 interfaces. This suggests that the high creep resistance of the fully lamellar microstructure is a result of the fine spacing of the lamellar structure, which results in a decreased effective slip length for dislocation motion over that found in the duplex and equiaxed microstructures.

Creep Deformation in Near- TiAI: Il. Influence of Carbon on Creep Deformation in Ti-48Al-1V-0.3C
BRIAN D. WORTH, J. WAYNE JONES, and JOHN E. ALLISON
The influence of interstitial strengthening and microstructure on creep deformation has been examined in the near- TiAl alloy Ti-48Al-1V-0.3C. Creep studies were conducted under constant load in air at 815 °C in the stress range of 50 to 200 MPa. Significant improvement in creep resistance was observed in this alloy compared with a similar alloy (Ti-49Al-1V) containing low levels of carbon (0.07 at. pct). The degree of strengthening resulting from the addition of carbon was found to be dependent on microstructure. At 815 °C and 150 MPa, the addition of carbon reduced the minimum creep rate by a factor of approximately 20 in the equiaxed and duplex microstructures and by a factor of 3 in the fully lamellar microstructures. Carbide precipitation occurred in this alloy when aged in the temperature range of 700 °C to 950 °C. The addition of carbon leads to a decrease in the stress exponent from 4 to 3 in the duplex and equiaxed microstructures and the inhibition of subboundary formation in the duplex microstructure. This suggests that solute/dislocation interaction mechanisms, rather than a direct effect of carbide precipitates, are responsible for the significant increase in creep resistance observed in this alloy.

Near-Threshold Fatigue Crack Growth in 8090 Al-Li Alloy
X.J. WU, W. WALLACE, A.K. KOUL, and M.D. RAIZENNE
Near-threshold fatigue crack growth was studied in 8090-T8771 Al-Li alloy tested in moist laboratory air. The testing was conducted using (1) the ASTM E-647 load-shedding procedure, (2) a power-law load-shedding procedure, and (3) a constant-amplitude (CA) loading procedure. Crack closure in the three procedures was analyzed. In reconciling fatigue crack growth rates (FCGRs) with different crack closure levels under identical testing parameters, the conventional K_eff (=K_max - K_op) fails to correlate the test data and the modified K_eff (=K_max - K_op, where is the shielding factor, defined by an energy approach) is proven to be the true crack driving force. A parallel slip-rupture model is proposed to describe the mechanism of near-threshold fatigue crack growth in this alloy. The model explains the mode transition from crystallographic slip band cracking (SBC) to subgrain boundary cracking (SGC)/brittle fracture (BF) in terms of a microstructure-environment synergy. The transition is related to the material's short-transverse grain size.

PHYSICAL CHEMISTRY

ENVIRONMENT

SOLIDIFICATION

MATERIALS PROCESSING

Combustion Synthesis in the Ti-C-Ni-Mo System: Part II. Analysis
J.C. LaSALVlA and M.A. MEYERS
Combustion-wave arrest experiments provide the means of greater understanding of the physical phenomena which occur during the propagation of a combustion wave within a Ti-C-Ni-Mo powder mixture. The apparent activation cnergy for the process (E ; 120 ± 40 kJ/mol) and the observations reported in the companion article indicate that the rate-limiting step in the reaction between Ti and C is the dissolution of C into the Ti-Ni-C melt. Temperature profile analysis indicates that the 2 µm C flakes are completely consumed within approximately 0.2 seconds. The formation of TiC_x spherules and their subsequent detachment is explained in terms of compressive stresses established in the growing TiC_x layer on the C particle. The compressive stresses are estimated to exceed 1 GPa, and an energy balance analysis predicts the formation of spherules for layer thicknesses on the order of 1 µm, consistent with the experimental results.

COMPOSITE MATERIALS

The Effect of Particle Size and Volume Fraction on the Aging Behavior of a Liquid-Phase Sintered SiC/Aluminum Composite
G.M. JANOWSKI and B.J. PLETKA
The aging response of a SiC particulate reinforced powder metallurgy aluminum composite was examined as a function of particle size and volume fraction. The addition of SiC particles ranging in size from 24 to 142 µm at 9 vol pct had no effect on the aging kinetics of the composites. Acceleration of the aging behavior or inhibition of the initial stage of the age-hardening process was observed at 18 and 27 vol pct. The accelerated aging kinetics were consistent with smaller particles creating larger thermal misfit dislocation densities. In addition, it was shown that different combinations of ceramic particle size and volume fraction lead to similar effects on the aging behavior. Loss of the initial hardening response was attributed to the suppression of Guinier-Preston (GP) zone formation due to the annihilation of excess vacancies at the thermal misfit dislocations.

Microstructure of the Ti3Al(Nb)/TiB Composite Produced by Combustion Synthesis
W.Y. YANG, H.C. YI, and A. PETRIC
The combustion synthesis technique was used to produce the intermetallic composite ₂-Ti₃Al(Nb) reinforced with TiB fibers. The microstructure was examined by scanning electron microscopy (SEM), X-ray diffraction analysis, and transmission electron microscopy (TEM). Neither the nor the ₀ phase was found in the intermetallic matrix. The TiB phase with B27 structure existed in the form of polycrystalline faceted whiskers growing in the (010) direction of the unit cell. No defined crystallographic relationship between the TiB whiskers and the matrix was found in this investigation.