K.S. Ravichandran

Evolution of microstructure and phases in in situ processed Ti–TiB composites containing high volume fractions of TiB whiskers

A series of titanium composites, with varying volume fractions of titanium monoboride (TiB) whiskers, were made by mixing various proportions of titanium (Ti) and titanium diboride (TiB 2 ) powders followed by hot pressing. The phases present were identified by x-ray diffraction. Microstructural examination revealed three different types of TiB whisker morphologies: (i) long and needle-shaped TiB whiskers that are isolated and randomly oriented in the Ti matrix at relatively low volume fractions (0.3), (ii) colonies of refined and densely packed TiB whiskers from intermediatevolume (0.55) to high volume (0.73 and 0.86) fractions, and (iii) coarse and elongated TiB particles with a few needle-shaped whiskers at the highest volume fraction (0.92). In all the composites, TiB was found to be the predominant reinforcement. However, in Ti–TiB composites with 0.86 and 0.92 volume fractions of TiB, a significant amount of TiB 2 was also present. The relative volume fractions of Ti, TiB, and TiB 2 phases were estimated from the integrated intensities of diffraction peaks by the direct comparison method employing the calculated structure factors and Lorentz polarization factors. The composite microstructure, as well as the evolution of different morphologies, of TiB whiskers is discussed.

Elastic properties of in-situ processed Ti-TiB composites measured by impulse excitation of vibration

The elastic moduli, shear moduli and the Poisson ratios of four different in-situ processed titanium (Ti) composites consisting of titanium monoboride (TiB) whiskers were measured using a dynamic method based on impulse excitation of vibration. For comparison, elastic modulus measurements were also made using tensile specimens with strain gages bonded to the specimen. The Ti–TiB composites consisted of 30, 54, 69, and 83 vol.% of TiB whiskers. The 83 vol.% TiB composite also consisted of 7 vol.% TiB2 phase. The study also included a TiB–TiB2 composite containing 16 vol.% TiB2 phase. The elastic and shear moduli of all the composites were found to increase with an increase in the volume fraction of TiB. The Poisson ratio decreased with increasing vol.% of TiB. The dynamic elastic modulus data were found to be in reasonable agreement with the tensile elastic modulus data. The variations in elastic properties with the vol.% of TiB whiskers are assessed in the light of theories on effective elastic response of two phase composites. The probable values of the elastic modulus, the shear modulus, and the Poisson’s ratio of the TiB phase were also estimated using these theories.

A survey of toughness in ductile phase composites

The available literature data on toughness in ductile phase composites are overviewed, and the known theoretical toughness models are assessed for their usefulness for predicting fracture toughness in ductile phase composites. It is shown that the predictions of simple equations incorporating ductile phase properties agree with the measured fracture toughness of ductile phase composites. It was found that the fracture toughness of ductile phase composites is governed by the modulus of the composite, the flow stress, and the plastic rupture displacement of the ductile phase. It is suggested that these equations should be useful for the alloy design of high-temperature materials consisting of ductile phases. 32 refs.

Microstructural effects and crack closure during near

Near threshold fatigue crack growth behavior of a high strength steel under different tempered conditions was investigated. The important aspect of the study is to compare the crack growth behavior in terms of the closure-free component of the threshold stress intensity range, ΔKth,eff While a systematic variation in the absolute threshold stress intensity range with yield strength was observed, the trend in the intrinsic ΔKth or ΔKth,eff exhibited a contrasting behavior. This has been explained as due to the difference in fracture modes during near threshold crack growth at different temper levels. It is shown that in a high strength and high strain hardening microstructure, yielding along crystallographic slip planes is difficult and hence it exhibited a flat transgranular fracture. In a steel with low strain hardening characteristics and relatively low strength, a tendency to crystallographic planar slip is observed consequently resulting in high ΔKth. Occurrence of a predominantly intergranular fracture is shown to reduce intrinsic ΔKth drastically and increase crack growth rates. Also shown is that crack closure can occur in high strength steels under certain fracture morphologies. A ‘transgranular planar slip’ during the inception of a ‘microstructure sensitive’ crack growth is essential to promote intergranular and faceted fracture. The occurrence of a maximum in the fraction of intergranular fracture during threshold crack growth corresponds to the ΔK value at which the cyclic plastic zone size becomes equal to the prior austenitic grain size.

Fracture mechanical character of small cracks in polycrystalline materials: concept and numerical K calculations

Abstract The impact of local variations in elastic modulus on the fracture mechanical character of “microstructurally small” cracks commonly encountered in early stages of fatigue in polycrystalline materials, is assessed. In particular, the stress intensity factor (SIF), K , variations along the crack front, influenced by the random crystallographic orientations of grains were numerically calculated for random variations of elastic moduli of grains. A conceptual framework for calculating the K variations due to local anisotropy was established on the basis of the weight function technique for three-dimensional cracks. As a model, an elliptical crack having sizes: three, five and twenty times the grain size, in polycrystalline Ni, Fe and Ti, was considered. Using patterns of random crystallographic orientations of grains, local inhomogeneous stresses that existed over the crack faces were determined on the basis of isostrain condition. These inhomogeneous stresses were used in the weight function-based calculations, to evaluate the effect of these stresses on the K variation along the crack front. It is shown that the effect of local anisotropy on the K variation is significant at small crack sizes and that the fracture mechanical character of microstructurally small cracks is quite different from that of large cracks in the isotropic environment. It is suggested that the local deviations in shapes of microstructurally small cracks, experimentally observed in many materials, are connected to the local K variations and their dependence on crack size, relative to the grain size.

Strength and ductility of microscale brass-steel multilayer composites

Multilayer composites or laminates show potential for structural applications due to a good combination of strength and toughness. For the first time, the authors have successfully fabricated metallic multilayer sheets with layer thicknesses in the nanometer range, while maintaining good layer discreteness, using brass and steel as components. This paves the way for the economic manufacture of laminates with very high specific strength levels. Depending on layer thickness and components, high strength in the laminates can arise from composite strengthening, Hall-Petch type boundary-induced strengthening as well as strengthening due to dislocation density and texture. However, there has been no study of strengthening in microscale laminates made by rolling. In the present work, strength and ductility of rolled microlaminates having alternating layers of brass and steel were investigated. Strength levels are correlated to layer thicknesses and the mechanisms of strengthening are discussed.

Microstructure and crack-shape effects on the growth behavior of small fatigue cracks in Ti24Al11Nb

Abstract The effects of four different microstructures in the titanium aluminide alloy Ti24Al11Nb (at.%) on the fatigue crack growth behavior of small surface cracks and large cracks have been investigated. The four microstructures were a Widmanstatten basketweave, a Widmanstatten aligned colony, an equiaxed primary α2 in a Widmanstatten matrix and a completely equiaxed α2 structure. Small cracks were found to develop arbitrary shapes owing to the effects of microstructure. The crack shapes (aspect ratios) were measured using a laser interferometric and photomicroscopic system, and these measurements allowed accurate calculation of crack growth rates at the surface position as well as at the depth position for the surface cracks. After accounting for the continuous variation in crack shape in crack growth rate calculations, the trends in small-crack growth rates agreed reasonably well with the corresponding large-crack growth rates. While the crack growth rates at depth positions for small cracks correlated well with large-crack data in the basketweave microstructure, crack growth rates at surface positions correlated well with the corresponding large-crack data in the other microstructures. The microstructural factors that may be responsible for this behavior are discussed.

Mechanisms of cleavage during fatigue crack growth in Ti6Al4V alloy

Beta processed microstructures of titanium alloys consisting of Widmanstatten structures offer attractive combinations of strength, fracture toughness and resistance to fatigue crack growth (FCG) for damage-tolerance applications [1-3]. This is generally attributed to large prior beta grain sizes incorporating coarse lamellar colonies across the boundaries of which cracks are deflected or deviated [4]. FCG resistance of these microstructures is far superior that of other microstructures owing to increased levels of crack deflection or deviation causing an increase in the intrinsic crack growth resistance,

Deformation behavior of interpenetrating-phase composites

\Delta K_e_f_f_,_t_h

Creep of Cr2Hf + Cr in situ intermetallic composites

, as well as crack closure [5]. However, the fatigue crack initiation resistance is lowfor these structures due to the formation of large intense planar slip bands. Growth of fatigue cracks along these bands normal to