Microstructure and creep behavior of an orthorhombic Ti-25Al-17Nb-1Mo alloy

Microstructural evolution during three heat-treatment schedules and the terminal microstructures in an orthorhombic alloy of Ti-25Al-17Nb-1Mo were observed and analyzed with optical microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The creep behavior of the alloy with three different microstructures (a coarse-lath, fine-lath, and fine equiaxed microstructure) was studied over a temperature range of 600 °C to 750 °C and over a stress range of 150 to 400 MPa in air. The steady-state creep rates, apparent stress exponents, and apparent creep activation energies of the various samples have been determined. The results show that creep behaviors in the alloy are strongly influenced by microstructure. The effect on creep by some of the microstructural features, such as the multivariants within the coarse laths and the interfaces of the laths and the equiaxed grains, is also discussed.     

A finite element model of the effects of primary creep in an Al-SiC metal matrix composite

A two dimensional axisymmetric finite element model has been developed to study the creep behavior of a high-temperature aluminum alloy matrix (alloy 8009) reinforced with 11 vol pct silicon carbide paniculate. Because primary creep represents a significant portion of the total creep strain for this matrix alloy, the emphasis of the present investigation is on the influence of primary creep on the high-temperature behavior of the composite. The base alloy and composite are prepared by rapid solidification processing, resulting in a very fine grain size and the absence of precipitates that may complicate modeling of the composite. Because the matrix microstructure is unaffected by the presence of the SiC paniculate, this material is particularly well suited to continuum finite element modeling. Stress contours, strain contours, and creep curves are presented for the model. While the final distribution of stresses and strains is unaffected by the inclusion of primary creep, the overall creep response of the model reveals a significant primary strain transient. The effects of true primary creep are more significant than the primary-like transient introduced by the redistribution of stresses after loading. Examination of the stress contours indicates that the matrix axial and shear components become less uniform while the effective stress becomes more homogeneous as creep progresses and that the distribution of stresses do not change significantly with time after the strain rate reaches a steady state. These results also confirm that load transfer from the matrix to reinforcement occurs primarily through the shear stress. It is concluded that inclusion of matrix primary creep is essential to obtaining accurate representations of the creep response of metal matrix composites. formerly Graduate Research Assistant, University of California-Davis This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the joint TMS-SMD/ ASM-MSD Composite Materials Committee.     

Mechanical properties of an ultrafine-grained Al-7.5 Pct Mg alloy

In the present study, the relationships between the structure and properties of a cryomilled Al-7.5 pct Mg alloy were investigated. The microstructure of the cryomilled Al-7.5 pct Mg alloy consisted of equiaxed grains with an approximate size of 300 nm. Thermal treatment had only a minor effect on microstructure, as evidenced by X-ray diffraction (XRD) and transmission electron microscopy (TEM) results. The tensile behavior was characterized by high strength, high ductility, and low-strain-hardening. The tensile deformation was relatively uniform, with limited necking deformation, and fracture surfaces were characterized by microdimples. The variation of strain rates from 4 · 10⁻⁴ to 4 · 10⁻² s⁻¹ had an insignificant effect on tensile behavior. Comparison of compressive and tensile behavior revealed similar moduli and yield strengths, although the postyield behavior was markedly asymmetric. The present results indicate that grain-size effects, solid-solution strengthening, Orowan strengthening, and dislocation strengthening contribute significantly to the properties of a cryomilled Al-7.5 pct Mg alloy.     

A comparison between creep of laminated aluminum and the transverse creep behavior of a unidirectional boron-aluminum composite

The transverse creep behavior of a unidirectional 30 vol pct boron/1145-0 aluminum composite material was investigated over the temperature range 573 to 773 K. The creep curve of the composite exhibited primary, steady state, and tertiary stages of creep, as did the unreinforced laminated matrix; however, the primary stage of creep was consid-erably less pronounced in the composite than in the matrix. The minimum (or steady state) creep rate of the composite was less than that of the laminated matrix alone below a transition stress Σ = 1.74 × 10⁻⁴ E whereE is Young's Modulus of aluminum. Above this transition stress, the minimum creep rate of the composite exceeds that of the unreinforced matrix; further, the strain to failure of the composite generally decreased when the applied stress was above the transition stress. The temperature dependence of the minimum creep rate was the same for both the composite and the laminated matrix. Failure in the composite was initiated by debonding of the filament-matrix interface and it is suggested that debonding of the interface contributes in an additive way to the creep of the composite and to a greater extent at high stresses, leading to the transition stress observed.     

A comparison between creep of laminated aluminum and the transverse creep behavior of a unidirectional boron-aluminum composite

The transverse creep behavior of a unidirectional 30 vol pct boron/1145-0 aluminum composite material was investigated over the temperature range 573 to 773 K. The creep curve of the composite exhibited primary, steady state, and tertiary stages of creep, as did the unreinforced laminated matrix; however, the primary stage of creep was consid-erably less pronounced in the composite than in the matrix. The minimum (or steady state) creep rate of the composite was less than that of the laminated matrix alone below a transition stress Σ = 1.74 × 10⁻⁴ E whereE is Young's Modulus of aluminum. Above this transition stress, the minimum creep rate of the composite exceeds that of the unreinforced matrix; further, the strain to failure of the composite generally decreased when the applied stress was above the transition stress. The temperature dependence of the minimum creep rate was the same for both the composite and the laminated matrix. Failure in the composite was initiated by debonding of the filament-matrix interface and it is suggested that debonding of the interface contributes in an additive way to the creep of the composite and to a greater extent at high stresses, leading to the transition stress observed. Formerly graduate student     

High-temperature creep in an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification

The creep behavior of an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification is investigated at three temperatures ranging from 623 to 723 K. The measured minimum creep strain rates cover seven orders of magnitude. The creep behavior is associated with the true threshold stress, decreasing with increasing temperature more strongly than the shear modulus of aluminum. The minimum creep strain rate is controlled by the lattice diffusion in the alloy matrix, and the true stress exponent is close to 5. The apparent activation energy of creep depends strongly on both applied stress and temperature and is generally much higher than the activation enthalpy of lattice self-diffusion in aluminum. Also, the apparent stress exponent of minimum creep strain rate depends on applied stress as well as on temperature and is generally much higher than the true stress exponent. This behavior of both the apparent activation energy and apparent stress exponent is accounted for by the strong temperature dependence of the threshold stress-to-shear modulus ratio. The true threshold creep behavior of the alloy is interpreted in terms of athermal detachment of dislocations from fine incoherent Al₁₂(Fe, V)₃Si phase particles, admitting a temperature dependence of the relaxation factor characterizing the strength of the attractive dislocation/particle interaction.     

Effect of a solid solution on the steady-state creep behavior of an aluminum matrix composite

The effect of an alloying element, 4 wt pct Mg, on the steady-state creep behavior of an Al-10 vol pct SiC_p composite has been studied. The Al-4 wt pct Mg-10 vol pct SiC_p composite has been tested under compression creep in the temperature range 573 to 673 K. The steady-state creep data of the composite show a transition in the creep behavior (regions I and II) depending on the applied stress at 623 and 673 K. The low stress range data (region I) exhibit a stress exponent of about 7 and an activation energy of 76.5 kJ mol^-1. These values conform to the dislocation-climb-controlled creep model with pipe diffusion as a rate-controlling mechanism. The intermediate stress range data (region II) exhibit high and variable apparent stress exponents, 18 to 48, and activation energy, 266 kJ mol^-1, at a constant stress, σ = 50 MPa, for creep of this composite. This behavior can be rationalized using a substructure-invariant model with a stress exponent of 8 and an activation energy close to the lattice self-diffusion of aluminum together with a threshold stress. The creep data of the Al-Mg-A1₂O_3f composite reported by Dragone and Nix also conform to the substructure-invariant model. The threshold stress and the creep strength of the Al-Mg-SiC_p, composite are compared with those of the Al-Mg-Al₂O_3f and 6061 Al-SiC_p.w, composites and discussed in terms of the load-transfer mechanism. Magnesium has been found to be very effective in improving the creep resistance of the Al-SiC_p composite.     

Mechanical properties and fracture behavior of an ultrafine-grained Al-20 wt pct Si alloy

The effect of powder particle size on the microstructure, mechanical properties, and fracture behavior of Al-20 wt pct Si alloy powders was studied in both the gas-atomized and extruded conditions. The microstructure of the as-atomized powders consisted of fine Si particles and that of the extruded bars showed a homogeneous distribution of fine eutectic Si and primary Si particles embedded in the Al matrix. The grain size of fcc-Al varied from 150 to 600 nm and the size of the eutectic Si and primary Si was about 100 to 200 nm in the extruded bars. The room-temperature tensile strength of the alloy with a powder size <26 μm was 322 MPa, while for the coarser powder (45 to 106 μm), it was 230 MPa. The tensile strength of the extruded bar from the fine powder (<26 μm) was also higher than that of the Al-20 wt pct Si-3 wt pct Fe (powder size: 60 to 120 μm) alloys. With decreasing powder size from 45 to 106 μm to <26 μm, the specific wear of all the alloys decreased significantly at all sliding speeds due to the higher strength achieved by ultrafine-grained constituent phases. The thickness of the deformed layer of the alloy from the coarse powder (10 μm at 3.5 m/s) was larger on the worn surface in comparison to the bars from the fine powders (5 μm at 3.5 m/s), attributed to the lower strength of the bars with coarse powders.     

Mechanical properties and fracture behavior of an ultrafine-grained Al-20 wt pct Si alloy

The effect of powder particle size on the microstructure, mechanical properties, and fracture behavior of Al-20 wt pct Si alloy powders was studied in both the gas-atomized and extruded conditions. The microstructure of the as-atomized powders consisted of fine Si particles and that of the extruded bars showed a homogeneous distribution of fine eutectic Si and primary Si particles embedded in the Al matrix. The grain size of fcc-Al varied from 150 to 600 nm and the size of the eutectic Si and primary Si was about 100 to 200 nm in the extruded bars. The room-temperature tensile strength of the alloy with a powder size <26 μm was 322 MPa, while for the coarser powder (45 to 106 μm), it was 230 MPa. The tensile strength of the extruded bar from the fine powder (<26 μm) was also higher than that of the Al-20 wt pct Si-3 wt pet Fe (powder size: 60 to 120 μm) alloys. With decreasing powder size from 45 to 106 μm to <26 μm, the specific wear of all the alloys decreased significantly at all sliding speeds due to the higher strength achieved by ultrafine-grained constituent phases. The thickness of the deformed layer of the alloy from the coarse powder (10 μm at 3.5 m/s) was larger on the worm surface in comparison to the bars from the fine powders (5 μm at 3.5 m/s), attributed to the lower strength of the bars with coarse powders.     

Influence of calcium on the microstructure and properties of an Al-7Si-0.3Mg-xFe alloy

The effect of Ca addition on the microstructure, physical characteristics (density/porosity), and mechanical properties (tensile and impact strength) has been investigated in an Al-7Si-0.3Mg-xFe (x=0.2, 0.4, and 0.7) alloy. The size of Al-Fe intermetallic platelets (β-Al₅FeSi) increased with increasing Fe content. The addition of Ca modified the eutectic microstructure and also reduced the size of intermetallic Fe-platelets, causing improved elongation and impact strengths. A low level of Ca addition (39 ppm) reduced the proosity of the alloys. The tensile strength was decreased marginally with Ca addition. However, Ca addition improved the ductility of the alloy by 18.3, 16.7, and 44 pct and the impact strength by 44, 48, and 15.8 pct for Fe contents of 0.2, 0.4, and 0.7 pct, respectively.     

Steady state and transient creep properties of an aluminum alloy reinforced with alumina fibers

Steady state and transient creep experiments have been conducted on a metal matrix composite consisting of an aluminum-5 wt% magnesium alloy matrix reinforced with 26 vol.% alumina fibers. The composite exhibits high steady state stress exponents which range from 12.2 at 200°C to 15.5 at 400°C. The apparent activation energy for creep over this temperature range is found to be 225 kJ/mol. After unloading, large anelastic strains are observed. The anelastic recovery process is found to vary non-linearly with stress and temperature and exhibits an activation energy of 151 kJ/mol. A finite element model of the composite microstructure has been developed by treating the alumina fibers as an interconnected network of elastic beam elements. This model accurately simulates the transient response of the material to changes in loading. The model suggests that the high stress exponents, high activation energy, and large recoverable strains observed for this material can be explained by a balance between load transfer to and damage of the fiber network.     

Grain refining of Al-4.5Cu alloy by adding an Al-30TiC master alloy

A particulate Al-30 wt pct TiC composite was employed as a grain refiner for the Al-4.5 wt pct Cu alloy. The composite contains submicron TiC particles. The addition of the TiC grain refiner to the metal alloy in the amount of 0.1 Ti wt pct effected a remarkable reduction in the average grain size in Al-4.5 wt pct Cu alloy castings. With the content of over 0.2 Ti wt pct, the grain refiner maintained its refining effectiveness even after a 3600-second holding time at 973 K. The TiC particles in the resulting castings were free of interfacial phases. It is concluded that the TiC are the nucleating agents and that they are resistant to the “fading effect” encountered with most grain refiners.     

Thermal-Mechanical fatigue of Ti-48Al-2V alloy and its composite

Thermal-mechanical fatigue (TMF) and isothermal fatigue (IF) of a Ti-48Al-2V alloy and its composite, reinforced with TiB₂ particles, were studied. In-phase TMF testing was conducted under the condition of a minimum temperatureT _min = 100 °C and a maximum temperatureT _max, which ranged from 750 °C to 1400 °C. The applied cyclic stress ranges were 2.8 to 28 MPa and 4.2 to 42 MPa. The IF tests were carried out at aT _max. The TMF and IF lives are longer for lowerT _max and for smaller stress ranges in both the matrix alloy and its composite. The IF life at a givenT _max is shorter than the TMF life in the matrix alloy at all temperatures employed and in the composite only at higher temperatures. At lower temperatures, the TMF and IF lives are essentially the same in the composite. The resistance to TMF is similar in the matrix alloy and the composite, but the IF resistance is greater in the composite than in the matrix alloy. The proposed TMF mechanism is nucleation and growth of voids on interlamellar plate, twin, and grain boundaries; their interlamellar, translamellar, and intergranular linkage; intergranular separation; and disintegration of lamellar structure.     

Tribological properties of aluminum alloy matrix TiB2 composite prepared by in situ processing

An investigation of the wear behavior, in lubricated sliding and rolling of in situ prepared TiB₂ particle-reinforced 2024 T4 Al alloy matrix composites against 52100 steel and hardened pearlitic nodular cast iron, respectively, was undertaken. In sliding contact, the 10 vol pct 0.3-μm TiB₂-metal matrix composite (MMC) showed slightly less wear than the 10 vol pct 1.3-μm TiB₂-MMC. Transmission electron microscopy of cross sections, taken normal to the wear track and parallel to the sliding direction, revealed that the TiB₂ particles on the wear track were polished and particle pullout was largely absent. This was attributed to the strong interfacial bonding between the Al-alloy matrix and the TiB₂ reinforcing phase. The TiB₂ particles on the wear track inhibited spalling. Subsurface damage of the MMC did not occur. The wear of the steel mating surfaces worn against the TiB₂-MMCs was minor and caused by the cutting action of the TiB₂ particles that resided on the MMC wear track. In rolling contact, the 0.3-μm-size TiB₂-MMC showed 5 times higher weight loss than the 1.3-μm TiB₂-MMC for the same content of reinforcement, but the weight loss of the cast iron mating surface was less for the former. For the smaller particle size, the wear of 5 and 10 vol pct TiB₂-MMCs was the same. A high density of surface cracks was present on the wear track of the 0.3-μm TiB₂-MMC but not on the 1.3-μm MMC. The significance of strong particle/matrix interfacial bonding and particle size effect on the wear behavior of ceramic particulate-reinforced MMCs in lubricated sliding and rolling wear is discussed.     

Creep and rupture properties of an austenitic Fe-30Mn-9Al-1C alloy

The creep deformation behavior and rupture properties of as-quenched austenitic Fe-30Mn-9Al-1C alloy have been studied at 923, 948, and 973 K under applied stresses ranging from 50 to 350 MPa. The creep curves of the alloy exhibited an extended tertiary stage prior to failure. The stress and temperature dependencies of the minimum creep rate indicated two regimes of creep deformation as well as a transition from creep to power-law breakdown. These two regimes of creep deformation were identified as a low-stress creep regime having an activation energy of 140 kJ/mol and a stress exponent of about 1, and a power-law creep regime having an activation energy of 350 kJ/mol and a stress exponent of about 6. Transmission electron microscope (TEM) observations of the deformed specimens revealed that a low density of dislocations, coarse dislocation networks, and profuse slip bands were developed in the low stress, power law, and power-law breakdown regimes, respectively. Optical microscope and scanning electron microscope (SEM) observations of the ruptured specimens showed that creep cavitation shifted from round-type in the low-stress creep regime to wedge-type in the power-law breakdown regime. The observed creep and rupture characteristics of the alloy are interpreted in terms of creep mechanisms, which involve the Coble creep and dislocation climb creep.     

Solidification of binary hypoeutectic alloy matrix composite castings

We consider a binary hypoeutectic alloy casting which solidifies in dendritic form in an unreinforced engineering casting and seek to predict its microstructure in a metal matrix composite. We focus on the case where the reinforcement is fixed in space and fairly homogeneously distributed. We assume that the reinforcement does not catalyze heterogeneous nucleation of the solid. We show that the reinforcement can cause several microstructural transitions in the matrix alloy, depending on the matrix cooling rate, the width, A, of interstices left between reinforcing elements, and the initial velocityV of the solidification front. These transitions comprise the following: (1) coalescence of dendrite arms before solidification is complete, causing solidification to proceed in the later stages of solidification with a nondendritic primary phase mapping the geometry of interstices delineated by reinforcement elements; (2) sharp reduction or elimination of microsegregation in the matrix by diffusion in the primary solid matrix phase; and (3) a transition from dendrite to cell formation, these cells featuring significant undercoolings or a nearly plane front configuration when reinforcing elements are sufficiently fine. Quantitative criteria are derived for these transitions, based on previous work on composite solidification, observations from directional solidification experiments, and current solidification theory. Theory is compared with experimental data for aluminum-copper alloys reinforced with alumina fibers and for the dendrite to cell transition using data from directional succinonitrile-acetone solidification experiments. Theory and experiment show good agreement in both systems. This article is based on a presentation made at the “Analysis and Modeling of Solidification” symposium as part of the 1994 Fall meeting of TMS in Rosemont, Illinois, October 2–6, 1994, under the auspices of the TMS Solidification Committee.     

Effect of hydrogen on the low-temperature creep of a submicrocrystalline Ti-6Al-4V alloy

The effect of alloying with 0.002–0.24 wt % H on the creep of the Ti-6Al-4V alloy at a temperature of ∼0.15T _m (T _m is the melting temperature) is studied. The formation of a submicrocrystalline structure in the alloy is found to increase the stress-rupture strength and the hydrogen embrittlement resistance. Possible causes of the increase in the deformation localization resistance of the alloy in the presence of hydrogen in a solid solution are discussed.