J.C. Gibeling

Time-dependent deformation of metals

The basic characteristics of timedependent deformation of metals are described in terms of dislocation properties. At high temperatures, diffusion controlled climb of edge dislocations is the rate limiting process, whereas at low temperatures, other forms of recovery involving cross-slip of screw dislocations operate. A composite model of plastic flow is used to describe the coupling between these recovery processes. The model is patterned after the persistent slip band structures observed in cyclically deformed fcc single crystals. Screw dislocations are allowed to move in the cell interiors and to deposit edge dislocations into the adjoining walls. Cross-slip and climb lead to dislocation rearrangement and annihilation in the two regions. These processes are coupled not only through the dislocation microstructure, but also through the mechanics of the composite structure. The model is used to describe various deformation properties of metals, including stage II, stage III, and stage IV strain hardening and saturation of the flow stress. The coupling of cross-slip and climb controlled recovery processes leads to gradual transitions in strain hardening and gives a natural account of the transition from low temperature deformation to high temperature creep. The model also leads to polarized dislocation structures, internal stresses, and anelastic creep properties.

Principles of particle selection for dispersion-strengthened copper

Abstract A new fundamental approach to the design of high strength, high thermal conductivity dispersion-strengthened copper alloys for applications in actively cooled structures is developed. This concept is based on a consideration of the basic principles of thermodynamics, kinetics and mechanical properties. The design requirements for these materials include a uniform distribution of fine particles for creep and fatigue resistance, a high thermal conductivity, thermodynamic and chemical stability at temperatures up to 1300 K, a small difference in the coefficients of thermal expansion between the particle and matrix, and low particle coarsening rates at the processing and service temperatures. The theory for creep of dispersion-strengthened metals developed by Rosler and Arzt is used to predict the optimum particle size for a given service temperature and to illustrate the need for a high interfacial energy. Resistance to coarsening leads to a requirement for low diffusivity and solubility of particle constituent elements in the matrix. Based on the needs for a low difference in the coefficients of thermal expansion to minimize thermal-mechanical fatigue damage and low diffusivity and solubility of the constituent elements, several candidate ceramic phases are compared using a weighted property index scheme. The results of this quantitative comparison suggest that CeO2, MgO, CaO and possibly Y2O3 may be good candidates for the dispersed phase in a copper matrix.

CALCIUM BUFFERING IS REQUIRED TO MAINTAIN BONE STIFFNESS IN SALINE SOLUTION

This work determined whether mineral dissolution due to prolonged testing or storage of bone specimens in normal saline would alter their elastic modulus. In one experiment, small pieces of equine third metacarpal bone were soaked in normal saline supplemented with varying amounts of CaCl2. Changing Ca ion concentrations in the bath were monitored and the equilibrium concentration was determined. In a second experiment, the elastic moduli of twenty 4 x 10 x 100 mm equine third metacarpal beams were determined non-destructively in four-point bending. Half the beams were then soaked for 10 days in normal saline, and the other half in saline buffered to the bone mineral equilibrium point with Ca ions. Modulus measurements were repeated at 6 and 10 days. The equilibrium Ca ion concentration for bone specimens was found to be 57.5 mg l-1. The modulus of bone specimens soaked in normal saline significantly diminished 2.4%, whereas the modulus of those soaked in calcium-buffered saline did not change significantly.

Osteon pullout in the equine third metacarpal bone: effects of ex vivo fatigue

An important concept in bone mechanics is that osteons influence mechanical properties in several ways, including contributing to toughness and fatigue strength by debonding from the interstitial matrix so as to „bridge”︁ developing cracks. Observations of „pulled out„ osteons on fracture surfaces are thought to be indicative of such behavior. We tested the hypothesis that osteon pullout varies with mode of loading (fatigue vs. monotonic), cortical region, elastic modulus, and fatigue life. Mid‐diaphseal beams from the dorsal, medial, and lateral regions of the equine third metacarpal bone were fractured in four point bending by monotonic loading to failure under deflection control, with or without 105 cycles of previous fatigue loading producing 5000 microstrain (15–20% of the expected failure strain) on the first cycle; or sinusoidal fatigue loading to failure, under load or deflection control, with the initial cycle producing 10,000 microstrain (30–40% of the expected failure strain). Using scanning electron microscopy, percent fracture surface area exhibiting osteon pullout (%OP.Ar) was measured. Monotonically loaded specimens and the compression side of fatigue fracture surfaces exhibited no osteon pullout. In load‐controlled fatigue, pullout was present on the tension side of fracture surfaces, was regionally dependent (occurring to a greater amount dorsally), and was correlated negatively with elastic modulus and positively with fatigue life. Regional variation in %OP.Ar was also significant for the pooled (load and deflection controlled) fatigue specimens. %OP.Ar was nearly significantly greater in deflection controlled fatigue specimens than in load‐controlled specimens (p < 0.059). The data suggest that tensile fatigue loading of cortical bone eventually introduces damage that results in osteonal debonding and pullout, which is also associated with increased fatigue life via mechanisms that are not yet clear.

Residual strength of equine bone is not reduced by intense fatigue loading: Implications for stress fracture

Fatigue or stress fractures are an important clinical problem in humans as well as racehorses. An important question in this context is, when a bone experiences fatigue damage during extreme use, how much is it weakened compared to its original state? Since there are very limited data on this question and stress fractures are common in racehorses, we sought to determine the effect of fatigue loading on the monotonic strength of equine cortical bone. Beams were machined from the dorsal, medial and lateral cortices of the third metacarpal bones of six thoroughbred racehorses. Beams from left and right bones were assigned to control and fatigue groups, respectively (N = 18 each). The fatigue group was cyclically loaded in three-point bending at 2 Hz for 100,000 cycles at 0-5000 microstrain while submerged in saline at 37 degrees C. These beams, as well as those in the control group, were then monotonically loaded to failure in three-point bending. The monotonic load-deflection curves were analyzed for differences using three-factor (fatigue loading, anatomic region, and horse) analysis of variance. The mean failure load was 3% less in the fatigue group, but this reduction was only marginally significant. Neither elastic modulus nor yield strength was significantly affected by the fatigue loading. The principal effects of fatigue loading were on post-yield behavior (yield being based on a 0.02% offset criterion). The work done and the load increase between yield and failure were both significantly reduced. All the variables except post-yield deflection were significantly affected by anatomic region. In summary, loading equivalent to a lifetime of racing does not significantly weaken equine cortical bone ex vivo. The clinical implication of this may be that the biological repair of fatigue damage can actually contribute to stress fracture if pressed too far.

High temperature deformation of ultra-fine-grained oxide dispersion strengthened alloys

The high temperature deformation properties of two oxide dispersion strengthened (ODS) alloys, MA 754 and MA 6000, with initial grain sizes of 0.67 µm and 0.26 µm, respectively, have been studied. Tensile tests have been conducted at 1173, 1273, and 1373 K at strain rates ranging from 2 X 10−4 to 5 s−1. Tension creep tests were conducted on MA 6000 at 1273 K to extend the strain rate regime to 3 X 10−8 s−1. Microstructures of both undeformed and deformed samples have been characterized by transmission electron microscopy. MA 754 exhibits a maximum elongation of 200 pct and a maximum strain rate sensitivity of 0.30 at 1273 K. MA 6000 is superplastic, exhibiting a maximum elongation of over 300 pct and a maximum strain rate sensitivity of 0.47 at 1273 K. The microstructure of MA 754 is unstable during deformation, showing recrystallized grains and grains which have grown to 1 µm in diameter. No evidence for ordinary recrystallization is found for MA 6000, and grain growth is slight. For both alloys, strain rates less than about 1 s−1 alter the initial microstructure and prevent grain coarsening on subsequent annealing at higher temperature. Deformation of the fine-grained MA 6000 can be described as a combination of power law creep and diffusional (Coble) creep, with a threshold stress caused by the presence of γt’ particles existing only for the diffusional creep process. Structural instabilities do not permit a simple description of deformation of MA 754.

Creep Deformation of Dispersion-Strengthened Copper

The creep behavior of an internally oxidized, A12O3 dispersion-strengthened copper alloy, GlidCop Al-15, has been investigated in the temperature range of 745 to 994 K. The results exhibit a high apparent stress exponent (10 to 21) and a high apparent activation energy for creep (253.3 kJ/mole). To describe the creep behavior of this alloy, the Rösler-Arzt model for attractive particle/dislocation interaction is applied. The results are in good agreement with the model when account is taken of the effects of the fine elongated grains and heavily dislocated structures revealed through transmission electron microscopy. The analysis demonstrates that the dislocation/particle interaction is of moderate strength in this alloy, consistent with the observation that the particle/matrix interface is partially coherent. In addition, the analysis reveals that the choice of mechanism and corresponding activation energy for vacancy diffusion has only a small effect on the calculated model parameters. It is argued that the weak dependence of subgrain size on stress demonstrates that creep deformation is particle controlled, rather than subgrain size controlled. In addition, the poorly developed subgrain structure and high dislocation densities are attributed to the presence of the fine oxide particles. Finally, the dependence of rupture time on stress is shown to be consistent with a description of creep fracture based on diffusive cavity growth with continuous nucleation.

A new strain rate change technique for distinguishing between pure metal and alloy type creep behavior

Abstract A new technique involving strain rate changes has been developed for distinguishing between pure metal and alloy type creep behavior. Both positive and negative strain rate changes were performed on aluminum and Al-5.8 at.% Mg (pure metal and alloy type material respectively) using an Instron electromechanical testing machine. Tests were conducted at 573 K with initial total strain rates of either 4 × 10 −5 or4 × 10 −4 s −1 . Immediately following an order of magnitude change in total strain rate, the plastic strain rate was monitored as a function of stress. The observed transient response for both pure aluminum and Al-5.8 Mg was found to agree with predicted behavior, indicating that the strain rate change test can be used to distinguish between pure metal and alloy type creep behavior. The strain rate change test was also found to be a promising single specimen technique for studying constant structure deformation. The quality of the constant structure data obtained using this technique is shown to depend on the accuracy with which plastic strain rate can be determined. A procedure is described for determining the plastic strain rate with sufficient accuracy to allow the strain rate change test to be used in place of multiple stress reduction tests to study constant structure deformation.

Measurement of anelastic creep strains in Al-5.5 at.% Mg using a new technique: Implications for the mechanism of class I creep

Abstract The anelastic response of Al-5.5 at.% Mg has been studied in the Class I creep regime using stress change-strain transient experiments at 573 and 673 K. Conventional single drop experiments yield a nonlinear variation of total and back-extrapolated anelastic strain with stress reduction, while a linear response is found for stress increments. These results are inconsistent with the predictions of a new dislocation loop model for alloy type creep. An examination of the transient creep process using the loop model indicates that the anelastic backstrain and forward creep processes are not independent, and that the predicted anelastic response can be observed only for nearly complete unloadings. This analysis has led to the development of the constant reduced stress-dual drop experiment. The results of this new technique correlate well with the purely anelastic backstrain mechanism proposed in the loop model. This interpretation of the creep transient implies that measurements of internal stresses using the conventional single drop test are inaccurate, and suggests that the internal stress accounts for less than 10% of the applied stress for alloy type creep.

Cyclic deformation of dispersion-strengthened aluminum alloys

Abstract The cyclic deformation behavior of two dispersion-strengthened aluminum alloys produced by mechanical alloying is examined. The materials studied include an AlMg alloy (IN-9052) and a similar alloy containing an addition of lithium (IN-905XL). The results of plastic strain-controlled low cycle fatigue tests are compared with those obtained for a conventional AlMg alloy (AA5083-H321) and a conventional precipitation-strengthened alloy (AA7075-T6). The dispersion-strengthened materials exhibit a small amount of initial cyclic softening followed by moderate hardening to failure. These observations suggest that the residual stresses induced during processing may influence the initial cyclic response, but that the dispersoids are resistant to shear as expected. The dispersion-strengthened alloys also exhibit a substantial asymmetry in the tension and compression peak stresses due to the presence of the dispersoids. This result is similar to that for the AA7075-T6, but no such asymmetry was detected in the solid solution-strengthened alloy (AA5083-H321). The cyclic lifetime of IN-9052 is slightly greater than that of the other materials examined in this study. This result is attributed to the role of the dispersoid particles in promoting homogeneous deformation. Finally, the importance of incorporating a non-linear elastic strain calculation in low cycle fatigue testing of high-strength materials is discussed.