Effect of albite particles on the coefficient of thermal expansion behavior of the Al6061 alloy composites

An investigation has been made for the change in coefficient of thermal expansion (CTE) of albite reinforced 6061 aluminum composites. The composites were prepared by the liquid metallurgical technique for varying percentages of albite reinforcement in steps of 0, 2, 4, and 6 pct by weight. This article tries to bring an overall view of fundamentals of various approaches made to measure the CTE of composites including experimental and theoretical methods such as the Turner model, Kerner’s model, Schapery’s model, and the Eshelby model. The result shows the CTE significantly increased with increasing temperature but decreased with increasing reinforcement. The CTE was expected to vary with relative residual strains, which in turn is dependent on the percentage of reinforcement when cooled from 500 °C to room temperature. The CTE values were found to be comparable with theoretical results. The Eshelby model (showed conformance with experimental results) was well suited with the experimental results. The observed behavior of these particulate composites are described on the basis of the thermal residual stresses developed as a result of the difference in the CTE between matrix and reinforcement. This residual stress relaxation is more difficult in the higher weight (above 6 pct) percentage composites at high temperatures, but upon cooling, the residual stresses are found to be relaxed.     

NiTi and NiTi-TiC composites: Part IV. Neutron diffraction study of twinning and shape-memory recovery

Neutron diffraction measurements of internal elastic strains and crystallographic orientation were performed during compressive deformation of martensitic NiTi containing 0 vol pct and 20 vol pct TiC particles. For bulk NiTi, some twinning takes place upon initial loading below the apparent yield stress, resulting in a low apparent Young's modulus; for reinforced NiTi, the elastic mismatch from the stiff particles enhances this effect. However, elastic load transfer between matrix and reinforcement takes place above and below the composite apparent yield stress, in good agreement with continuum mechanics predictions. Macroscopic plastic deformation occurs by matrix twinning, whereby (1 0 0) planes tend to align perpendicular to the stress axis. The elastic TiC particles do not alter the overall twinning behavior, indicating that the mismatch stresses associated with NiTi plastic deformation are fully relaxed by localized twinning at the interface between the matrix and the reinforcement. For both bulk and reinforced NiTi, partial reverse twinning takes place upon unloading, as indicated by a Bauschinger effect followed by rubberlike behavior, resulting in very low residual stresses in the unloaded condition. Shape-memory heat treatment leads to further recovery of the preferred orientation and very low residual stresses, as a result of self-accommodation during the phase transformations. It is concluded that, except for elastic load transfer, the thermal, transformation, and plastic mismatches resulting from the TiC particles are efficiently canceled by matrix twinning, in contrast to metal matrix composites deforming by slip.     

Elastoplastic finite element analysis of short-fiber-reinforced SiC/Al composites: effects of thermal treatment

Elastoplastic finite element analyses of realistic models of short-fiber-reinforced composites were extended to include the effects of prior thermal treatments on predictions of subsequent mechanical properties. Two three-dimensional models were used, one in which the fiber ends were transversely aligned and another in which they were staggered. Both models were found to be necessary for accurate predictions of the behavior of higher volume fraction composites. The temperature dependence of the yield stress of the matrix material was explicitly included in the analysis. The spatial and temporal history of calculated. The room temperature residual stresses were also predicted. Both the plastic deformation and the residual stresses in the matrix were spatially non-uniform and varied rapidly from the regions near the ends of the fiber to those near the midpoint. Predictions of subsequent tensile stress-strain properties were in good quantitative agreement with experiments. The presence of residual stresses and locally deformed regions caused the tensile behavior to differ from the compressive behavior. These differences were complex and depended on the volume fraction and aspect ratio of the reinforcement. The analyses provide detailed insight into the deformation mechanisms of these composites.     

Fatigue behavior of SiC reinforced Ti(6AI-4V) at 650 °C

Axial, low cycle fatigue properties of 25 and 44 fiber vol pct SiC/Ti(6Al-4V) composites, measured at 650 °C, were compared with the fatigue properties of unreinforced Ti(6Al-4V) at the same temperature. A prior study of the fatigue behavior of this composite system at room temperature indicated that the SiC fiber reinforcement did not provide the anticipated improvement of fatigue resistance of this alloy. At 650 °C, the composite fatigue properties degraded somewhat from those at room temperature. However, these properties degraded more for the unreinforced matrix at 650 °C with the result that the composite fatigue strength was two to three times the fatigue strength of the matrix alloy. The reasons for this reversal are discussed in terms of crack initiation at broken fibers and residual matrix stresses.     

Measurement of residual strain in An AlSiCw composite using convergent-beam electron diffraction

Residual strains were introduced into an AlSiC_w composite by in situ cooling a thin foil from room temperature to -160°C. A detailed analysis was conducted using convergent-beam electron diffraction (CBED) to quantify the elastic residual stresses and strains in the matrix near the end and side of a SiC whisker. Large hydrostatic and effective stresses were measured in the matrix near the side of the whisker; the maximum stresses were located near the Al/SiC_w interface and decreased to zero approximately 1 μm (∼2 whisker diameters) from the interface. Residual strains were also observed in the matrix near the whisker end, but these strains could not be measured due to the complexity of the strain field. At the whisker end, the largest residual strains were located near the Al/SiC_w interface and decreased to zero approximately 0.5 μm (∼1 whisker diameter) from the interface. Finite element techniques were used to predict the residual strains in the composite material and these results were compared to experimental measurements.     

Hardness,Microstructure, and Residual Stresses in Low Carbon Steel Welding with Post-weld Heat Treatment and Temper Bead Welding

This paper investigates the effects of post-weld heat treatment (PWHT) and temper bead welding (TBW) on hardness, microstructure and residual stresses in multi-layer welding on low carbon steel specimens made with two different weld geometries, viz. (1) smooth-contoured and (2) U-shaped. It was found that the PWHT technique gave overall lower hardness than the TBW technique, but the hardness values in both techniques were acceptable. Microscopy analysis showed that the TBW technique was more effective in tempering the heat affected zone as the grain size decreased slightly at the fusion line in spite of the higher temperature at the fusion line. Residual stresses measured using the hole-drilling method showed that the residual stress is not reduced below yield stress near the last bead solidified in TBW. Only PWHT gives low residual stress results in this area. High tensile residual stresses may result in sensitivity to fatigue loading.     

Three-dimensional modelling of thermal residual stresses and mechanical behavior of cast SiC/A1 particulate composites

Thermal residual stresses developed during casting of SiC/aluminum particulate-reinforced composites were investigated as a function of cooling rate and volume fraction of particles using thermo-elastoplastic finite element analysis. The phase change of the matrix during solidification and the temperature-dependent material properties as the composite is cooled from the liquidus temperature to room temperature were taken into account in the model. Further, the effect of thermal residual stresses on the mechanical behavior of the composites was also studied. Based on the study, it was found that the matrix undergoes significant plastic deformation during cool down and has higher residual stress distribution as the cooling rate increases. The model which does not include the solidification of the matrix tends to overestimate the residual stresses in the matrix and underestimate the tensile modulus of elasticity of the composites. In addition, the presence of thermally induced residual stresses tends to decrease the apparent modulus of elasticity and increase the yield strength of the composites compared to those without residual stresses.     

Microstructure and properties of spray-deposited 2014+15 vol pct SiC particulate-reinforced metal matrix composite

A metal matrix composite (MMC) of 2014 aluminum alloy reinforced with 15 vol pct SiC particulate was produced by the spray-forming-deposition process. The as-deposited preform revealed a high density and a homogeneous reinforcement distribution. Reactive products were not found on interfaces between the reinforcement and the matrix. Compared to the control alloy, the composite showed accelerated aging after solutionizing at 502 °C, while aging was retarded after solutionizing at 475 °C. Analysis indicated that the activation energy was almost the same for the aging process after different solutionizing treatments. This suggested that while the thermal barrier for the aging process was the same, other factors affecting the aging process should be considered. For example, the effective concentration of the precipitate forming elements possibly decreased after incompletely solutionizing at 475 °C. After heat treatment, the composite showed a tensile strength similar to the control alloy. The wear resistance of the composite improved considerably. The aging behavior of the composite was also studied using the nanoindentation technique. Steep gradient distribution of elastic modulus and hardness around the reinforcement SiC particulate was observed. Theoretical analysis showed that this could be attributed to the gradient distribution of precipitates, resulting from a gradient distribution of dislocation density around the SiC particulates caused by residual thermal misfit stresses.     

Phase-stress partition during uniaxial tensile loading of a TiC-particulate-reinforced Al composite

Using neutron diffraction, we measured during in situ loading the lattice elastic mean phase (LEMP) strains in the matrix and reinforcement of a 15 vol pct TiC-particulate-reinforced 2219 Al composite. From the strain components longitudinal to and transverse to loading, the in situ normal phase stresses (average normal stresses in the constituent phases) were obtained through Hooke’s law. The internal stress partition between the matrix and reinforcement, i.e., load sharing, can then be inferred. Internal stress development was also modeled using the finite-element method (FEM), showing good agreement with the experimental results. Both indicate that the relationship between the LEMP strains/phase stresses and the applied load noticeably deviates from linearity during composite microyielding, long before the nominal 0.2 pct proof stress is reached. The nonlinearity arises (despite the linear elastic relationship between phase stresses and LEMP strains) because the applied traction is not synonymous with the phase stresses, and the ratio of phase stresses may vary during loading. Notably, the morphology of the LEMP strain development with applied load differs in the directions parallel to or perpendicular to the load. The differences are explained by considering the evolution of local matrix plasticity. Thermal residual stresses and inelastic stress relaxation, driven by interfacial diffusion, are also discussed.     

On determining temperature dependent interfacial shear properties and bulk residual stresses in fibrous composites

The model of the preceding paper is applied to the analysis of interfacial sliding during thermal cycling in three titanium or titanium aluminide alloys reinforced by SiC fibers. The model is found to give an excellent account of the experimental measurements. By fitting the model to the data, values are obtained in all cases for the critical interfacial shear stress, τ₀ at room temperature. In two cases, values are also obtained for the bulk, axial residual stresses at room temperature, and the average of dτ₀/dT over the interval of temperature T between room temperature and the maximum temperature attained in the thermal cycling. The residual stresses are in good agreement with other measurements. In the third case, the residual stresses cannot be determined; but, if values for them are taken from other experiments, then the same average of dτ₀/dT can be determined. The values of τ₀ and the implied coefficient of friction are consistent in all cases with frictional sliding in graphitic layers in the carbon rich coatings on the SiC fibers.     

Computational modeling of metal matrix composite materials—II. Isothermal stress-strain behavior

The mechanical behavior of particulate reinforced metal matrix composites, in particular an SiC reinforced Al-3 wt% Cu model system, was analyzed numerically using the computational micromechanics approach. In this, the second in a series of four articles, the isothermal overall stress-strain behavior and its relation to microstructural deformation is examined in detail. The macroscopic strengthening effect of the reinforcement is quantified in terms of a hardness increment. As seen in the first article for microscale deformation, inhomogeneous and localized stress patterns develop in the microstructures. These are predominantly controlled by the positions of the reinforcing particles. Within the particles stress levels are high, indicating a load transfer from matrix to reinforcement. The higher straining that develops in the matrix grains, relative to the unreinforced polycrystal, causes matrix hardness advancement. Hydrostatic stress levels in the composite are enhanced by constraints on plastic flow imposed by the particles. Constrained plastic flow and matrix hardness advancement are seen as major composite strengthening mechanisms. The latter is sensitive to the strain hardening nature in the matrix alloy. To assess the effects of constraint more fully, simulations using external confining loads were performed. Both strengthening mechanisms depend strongly on reinforcement volume fraction and morphology. In addition, texture development and grain interaction influence the overall composite behavior. Failure mechanisms can be inferred from the microscale deformation and stress patterns. Intense strain localization and development of high stresses within particles and in the matrix close to the particle vertices indicate possible sites for fracture.     

Effect of elevated temperature exposure on the structure,stability, and mechanical behavior of aluminum-stainless steel composites

The effect of elevated temperature exposure on subsequent ambient temperature tensile behavior of aluminum-stainless steel composities (V _f= 6.5 pct) has been studied. In particular, ambient temperature tensile yielding, flow, and fracture were correlated with the associated interface microstructures, matrix substructure, and fracture morphology in the as-pressed condition and following elevated-temperature exposure at 550°C (823 K) or 625°C (898 K) for 24 h (86.4 ks). Compared to the as-pressed condition, exposure at either temperature results in a small increase (?4 pet) in initial modulus, and a decrease in the level of residual stress (tensile) in the matrix; tensile stress-strain behavior in stage II (matrix plastic, reinforcement elastic) is essentially unaffected. Lower strength levels in stage III (matrix and reinforcement plastic) after exposure are due to premature cracking in the interface reaction zone, primarily a ternary (Fe, Cr) Al intermetallic, with associated notch effects on the wire reinforcement. Changes in fracture surface morphology of the composites confirm the degradation. Wires extracted from composites after hot pressing or following exposure at 550°C (823 K) possess a unique strength. Exposure at 625°C (898 K) leads to a bimodal distribution in the strength of extracted wires. In each condition, a matrix dislocation cell structure develops in stage III; the invariant form and size of the cell structure withV _f and distance from the matrix wire interface confirm isostrain conditions.     

Thermally and mechanically induced residual strains in Al-SiC composites

Neutron diffraction experiments were conducted on 15vol.% whisker and 20vol.% particulate reinforced aluminum/silicon carbide composites subjected to a rapid quench followed by various deformation histories. Corresponding numerical simulations were carried out using an axisymmetric unit cell model, with a phenomenological, isotropic hardening descriotion of matrix plasticity. Thermal expansion and the temperature dependence of material properties were accounted for. For the whisker reinforced matrix, quantitative agreement was generally found between the measured and calculated residual elastic strains. For the particulate reinforced matrix, the calculations tended to overestimate the magnitude of the residual strains parallel to the deformation axis, but very good agreement was obtained transverse to the deformation axis. For the silicon carbide reinforcement, both whisker and particulate, the variation of predicted residual elastic strains along the deformation axis was qualitatively consistent with the measurements, although quantitative agreement was often lacking. Measured and predicted residual strains perpendicular to the deformation axis for the silicon carbide typically were not in agreement. Parametric studies were carried out to ascertain the dependence of calculated flow strengths and residual strains on cell and reinforcement aspect ratio, and on reinforcement spacing and shape.     

Finite-element analysis of effects of the laser-processed bimaterial component size on stresses and distortion

Processing of bimaterial parts via a moving laser beam has been investigated using three-dimensional (3D) finite-element modeling. Effects of the size of parts on the temperature distribution, thermal transient stresses, residual stresses, and distortion have been evaluated. The result indicates that the size of the part to be processed has strong influence on the transient temperature, transient stresses, residual stresses, and distortion of the part. Distortion is small when the size of the part is small and can be predicted from the thermal-expansion mismatch between the two materials. However, when the size of the part becomes large, both distortion and residual stresses increase. Furthermore, both distortion and residual stresses, in this case, cannot be predicted based on the thermal-expansion mismatch alone. The distortion, in this case, is mainly determined by the asymmetrical plastic deformation driven by transient thermal stresses, while residual stresses are dictated by the thermal-expansion mismatch and the temperature gradient of the part before and during cooling.     

On the toughening of intermetallics with ductile fibers: Role of interfaces

The role of fiber debonding and sliding on the toughness of intermetallic composites reinforced with ductile fibers is examined. The toughness is shown to be a function of the matrix/fiber interface properties, residual stresses and the volume fraction, size and flow behavior of the fibers. Mechanical testing and in situ microstructural observations were carried out on a Ti-25at.%Ta-50at.%Al intermetallic matrix reinforced with W-3Re fibers. The fibers were coated with a thin oxide layer in order to induce debonding and prevent interdiffusion between the fiber and the matrix. The ductility, high strength and debond characteristics of coated tungsten-rhenium fibers promote a large increase in toughness. However, the mismatch in thermal expansion coefficients is the source of large residual tensile stresses in the matrix that induces spontaneous matrix cracking. Matrix cracking and composite toughness are examined as a function of the interfacial properties, residual stresses and properties of the fiber.     

Interfacial sliding near a free surface in a fibrous or layered composite during thermal cycling

This paper presents a simple shear lag model of interfacial sliding at a free surface in a layered or continuous fiber composite. The interface is characterized by a critical interfacial shear stress, τ₀, which might represent the critical stress for frictional sliding at a weakly bonded interface, or the shear flow stress of a thin, ductile interface layer at a well bonded interface. We calculate the history during heating and cooling of the relative normal displacement of the reinforcing inclusions and the matrix on a free surface cut normal to the inclusions. The calculated history is shown to depend on both the absolute value and the temperature dependence of τ₀, as well as on the magnitudes of the bulk residual stresses. Analytical results are obtained for the first few heating and cooling cycles and the equilibrium hysteresis loop under thermal cycling of uniform amplitude. The variety of possible displacement histories suggests that they are a rich source of information about τ₀ and the residual stresses. A discussion of feasible experiments and some results for continuous fiber titanium and titanium aluminide composites are presented in a companion paper.     

Optimization of the range of elastic behavior of unidirectional composites by prestraining

The effect of tensile prestrain on the stage I tensile yield stress has been studied both analytically and experimentally for composites whose stress-strain curves obey the rule of mixtures. The mathematical analysis provides a means for calculating the optimum amount of prestrain, the residual stresses (in the direction of the fibers) in the matrix and fiber materials after unloading from the prestraining, and the stage I yield stress in the composite after the prestrain treatment. It is shown that the improvement in stage I yield stress by prestraining is due to the development of negative residual stresses in the matrix. The stage I yield stress in composites with negligible residual stresses in the as-fabricated condition can usually be improved by a factor of two by prestraining; the amount of improvement is even greater if the as-fabricated composites have the usual state of residual stress,i.e., tension in the matrix. Experimental studies on 2024 aluminum-tungsten composites (filament-wound; hot-pressed) having tungsten fiber volume fractions between 0.08 and 0.40 verified the mathematical analysis. The stage I yield stresses measured in these composites after a prestrain of 4.2 × 10⁻³ were in good agreement with predicted values. Improvements of up to a factor of six were found in the stage I yield stress as a result of prestraining.     

Pretension-Dependent Residual Stress of Alumina Fiber-Reinforced Composite Wire

The relationship between pretension and residual stress of an aluminum wire reinforced with 45 vol pct continuous Nextel? 610 alumina fibers is investigated. It is shown that as pretension stress increases, the matrix residual stress decreases. A transition in matrix residual stress from tension to compression occurs at a pretension stress of about 80 MPa. The initial rapidly decreased residual stress caused by pretension at relatively low pretension stresses is a result of matrix elastic compressive deformation; while the later gradually decreased residual stress at higher pretension stresses comes from matrix plastic compressive deformation. As the matrix yield stress and hardening exponent increase, the decrease in matrix residual stress with pretension stress is more rapid and the absolute value of matrix residual stress increases. An analytical model suitable for fiber-reinforced metal matrix composites (MMCs) with strong interfacial bonding is developed to describe the relationship between pretension and matrix residual stress and is shown to be in good agreement with the experimental and finite-element calculated results. The pretension-dependent matrix residual stress phenomenon suggests that the mechanical properties of fiber-reinforced MMCs associated with matrix residual stress may be effectively improved by applying tensile loads.     

The influence of matrix microstructure and particle reinforcement on the creep behavior of 2219 aluminum

The influence of matrix microstructure and reinforcement with 15 vol pct of TiC particles on the creep behavior of 2219 aluminum has been examined in the temperature range of 150 °C to 250 °C. At 150 °C, reinforcement led to an improvement in creep resistance, while at 250 °C, both materials exhibited essentially identical creep behavior. Precipitate spacing in the matrix exerted the predominant influence on minimum creep rate in both the unreinforced and the reinforced materials over the temperature range studied. This behavior and the high-stress dependence of minimum creep rate are explained using existing constant structure models where, in the present study, precipitate spacing is identified as the pertinent substructure dimension. A modest microstructure-independent strengthening from particle reinforcement was observed at 150 °C and was accurately modeled by existing continuum mechanical models. The absence of reinforcement creep strengthening at 250 °C can be attributed to diffusional relaxation processes at the higher temperature.     

The influence of matrix microstructure and particle reinforcement on the creep behavior of 2219 aluminum

The influence of matrix microstructure and reinforcement with 15 vol pct of TiC particles on the creep behavior of 2219 aluminum has been examined in the temperature range of 150 ‡C to 250 ‡C. At 150 ‡C, reinforcement led to an improvement in creep resistance, while at 250 ‡C, both materials exhibited essentially identical creep behavior. Precipitate spacing in the matrix exerted the predominant influence on minimum creep rate in both the unreinforced and the reinforced materials over the temperature range studied. This behavior and the high-stress dependence of minimum creep rate are explained using existing constant structure models where, in the present study, precipitate spacing is identified as the pertinent substructure dimension. A modest microstructure-independent strengthening from particle reinforcement was observed at 150 ‡C and was accurately modeled by existing continuum mechanical models. The absence of reinforcement creep strengthening at 250 ‡C can be attributed to diffusional relaxation processes at the higher temperature.