With an ageing population, war, and sports related injuries there is an ever-expanding requirement for hard tissue replacement such as bone. Engineered artificial scaffold biomaterials with appropriate mechanical properties, surface chemistry and surface topography are in a great demand for enhancing cell attachment, cell growth and tissue formation at such defect sites. Most of these engineering techniques are aimed at mimicking the natural organization of the bone tissues and thereby create a conducive environment for bone regeneration. As the interaction between the cells and tissues with biomaterials at the tissue–implant interface is a surface phenomenon, surface properties play a major role in determining both the biological response to implants and the material response to the physiological condition. Hence surface engineering of biomaterials is aimed at modifying the material and biological responses through changes in surface properties while still maintaining the bulk mechanical properties of the implant. Therefore, there has been a great thrust towards development of Ca–P-based surface coatings on various metallic and nonmetallic substrates for load bearing implant applications such as hip joint prosthesis, knee joint prosthesis and dental implants. Typical coating methodologies like ion beam assisted deposition, plasma spray deposition, pulsed laser physical vapor deposition, magnetron sputtering, sol–gel derived coatings, electrodeposition, micro-arc oxidation and laser deposition are extensively studied at laboratory scale. In the present article, attempts are made to give an overview of the basic principles behind the coating techniques as well as advantageous features such as bioactivity and biocompatibility associated with these coatings. A strong emphasis will be given on laser-induced textured and bioactive coatings obtained by the author's research group [A. Kurella, N.B. Dahotre, Journal of Biomedical Applications 20 (2005) 5–50; A. Kurella, N.B. Dahotre, Acta Biomaterialia 2 (2006) 677–688; A. Kurella, N.B. Dahotre, Journal of Minerals, Metals and Materials Society (JOM) 58 (2006) 64–66; A. Kurella, N.B. Dahotre, Journal of Materials Science: Materials in Medicine 17 (2006) 565–572; P.G. Engleman, A. Kurella, A. Samant, C.A. Blue, N.B. Dahotre, Journal of Minerals, Metals and Materials Society (JOM) 57 (2005) 46–50; R. Singh, A. Kurella, N.B. Dahotre, Journal of Biomaterials Applications 21 (2006) 46–72; S.R. Paital, N.B. Dahotre, Biomedical Materials 2 (2007) 274–281; S.R. Paital, N.B. Dahotre, 2009, Acta Biomaterialia, doi:10.1016/j.actbio.2009.03.004 ; R. Singh, N.B. Dahotre, Journal of Materials Science: Materials in Medicine 18 (2007) 725–751.]. Since cells are sensitive to topographical features ranging from mesoscale to nanoscale, formation of these features by both pulsed and continuous wave Nd:YAG laser system will be highlighted. This can also be regarded as advancement towards third generation biomaterials which are bioinert, bioactive and which once implanted will stimulate cell adhesion, proliferation and growth at the interface. Further, an overview of various bio-implants and bio-devices and materials used for these kinds of devices, performance factors such as mechanical and corrosion behavior and surface science associated with these materials are also explained. As the present article is aimed at describing the multidisciplinary nature of this exciting field it also provides a common platform to understand this subject in a simple way for students, researchers, teachers and engineers in the fields ranging from medicine, dentistry, biology, materials science, biomedicine, biomechanics to physics.

Calcium phosphate coatings for bio-implant applications: Materials, performance factors, and methodologies

Lightweight materials for aircraft applications

Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks. In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.

Influence of grain size on the mechanical behaviour of some high strength materials

An investigation of fatigue failures of turbine blades in a gas turbine engine by mechanical analysis

Deformation and dynamic recrystallization behavior of a high Nb containing TiAl alloy

A study was carried out on the microstructure and creep properties of aero engine first-stage turbine blades made from Alloy 713C nickel-base superalloy. Results are reported for new blades, blades in two service-exposed conditions, and service-exposed blades subjected to one of three rejuvenation treatments: a recoating heat treatment, a hot isostatic pressing (HIP) + recoating heat treatment, and a HIP + controlled cooling + recoating heat treatment. The blade microstructure undergoes significant change during service, and this leads to a loss in creep properties exhibited by specimens machined from the blade airfoils. Good correlations were observed between the rupture time and the amount of blade airfoil untwist and between the minimum creep rate and the amount of untwist. The recoating heat treatment and the HIP + controlled cooling + recoating treatment were moderately successful in restoring the microstructure and creep properties of the service-exposed blades. In comparison, the HIP + recoating treatment was very successful in rejuvenating creep properties but only for blades having a chemical composition with a lower propensity to form σ phase. For the blades with an unfavorable composition, σ phase was found to form preferentially near the grain boundaries during creep testing, and this had a detrimental effect on the creep properties. Nonetheless, the degree of rejuvenation for these blades was always at least as good as that obtained through the recoating heat treatment alone.

Microstructure, creep properties, and rejuvenation of service-exposed alloy 713C turbine blades

The high temperature flow behavior of a nickel-base superalloy powder compact, prepared by hot isostatic pressing has been examined by means of uniaxial compression testing in terms of the microstructures developed during plastic flow. The tests were done isothermally at 1050 and 1100 °C and at constant true strain rates between 10-5 s-1 and 1 s-1. The fine grained compact exhibits some degree of superplasticity which always increases with compressive flow as the grain structure is refined. The faster the rate of deformation, the finer is the grain size produced at high strains, when steady state conditions of flow appear to develop. By deforming to different strains at a given strain rate or into the steady state regimes at various strain rates, grain sizes in the range 1 to 5 pun were produced. By unloading and restraining the test pieces in situ, the effect of grain size on the onset of plastic flow has been examined and the yield stress observed to increase with grain size. It is shown that, in this material, hardening or softening occurs during flow depending on the size of the initial grains. The changes in microstructure and flow stress observed during deformation are analyzed and the potential offered for control of the microstructure during isothermal forging is discussed.

Microstructural instabilities during superplastic forging of a nickel-base superalloy compact

Interest has evolved recently in thermomechanical processing of the cast Co-Cr-Mo surgical implant alloys such as Vitallium and Vinertia. Work has shown that the wrought forms of these alloys exhibit much improved properties over their as-cast counterparts. In this paper, the response of as-cast Vinertia to isothermal forging is examined by means of isothermal and isostrain-rate compression testing. The effects of temperature, strain rate, and strain on the breakdown of the as-cast micro-structure are examined in detail. The effects of prior heat treatment on plastic flow and microstructure achieved are also considered. It is shown that the interaction between the carbide phase and the recrystallization induced during hot working governs the degree of homogeneity that can be achieved in the forged product. Control of carbide volume fraction, size, and distribution by appropriate prior processing can lead to a fine grain equiaxed structure with uniformly distributed carbides. The potential offered by isothermal forging for control of the microstructure in this type of alloy is discussed, as well as the limits imposed on the process by the starting material and by the strain gradients expected during the forging of implants.

Microstructural Changes during Isothermal Forging of a Co-Cr-Mo Alloy

Hot isostatically pressed Ti-48%Al-2%W powder compacts were isothermally compressed between 900 °C and 1200 °C, and at constant strain rate in the range 10 −1 to 10 −4 s −1 . The deformation processes occurring are determined by correlation of the stress-strain response, strain rate sensitivity exponent m and activation energy, with the deformed microstructures. The resulting “deformation map” indicates that at low temperatures and high strain rates, mechanical twinning and dislocation generation cause a high flow stress, low m and high activation energy. Increasing compression temperature causes dynamic recrystallization, flow softening and increased m . Concurrent dynamic recrystallization and superplasticity can occur, with m as high as 0.44. At high temperatures and strain rates −3 s −1 , superplastic deformation is accompanied by grain coarsening. A “forging map” indicates that cracking after 120% engineering strain can be avoided if appropriate compression parameters are applied.

The isothermal compression response of a near γ-TiAl + W intermetallic

The high-temperature deformation behaviour of Mar M200 superalloy powder compacts, pressed below the γ′ solvus, has been examined. Compression tests were carried out at temperatures between 950 and 1200°C and at constant true-strain rates between 10−1 and 10−5 S−1. Superplastic behaviour was found at all the temperatures used and at strain rates below 10−2 S−1. The strain rate sensitivities at low strain rates are in the range 0.4–0.6 while at higher strain rates the exponents fall to between 0.1 and 0.3. In each range the exponent increases with temperature, indicating higher degrees of plasticity. Variations in strain-rate sensitivities are related to the various deformation mechanisms which are thought to contribute to flow. The effects of powder particle mesh size and size distribution on the flow properties of the compacts are also considered. Coarser mesh sizes produce the highest peak flow stresses at all strain rates and temperatures investigated. Compacts made predominantly from fine mesh...

J. P. A. Immarigeon

Papers

Microstructure, creep properties, and rejuvenation of service-exposed alloy 713C turbine blades

Microstructural instabilities during superplastic forging of a nickel-base superalloy compact

Microstructural Changes during Isothermal Forging of a Co-Cr-Mo Alloy

The isothermal compression response of a near γ-TiAl + W intermetallic

Flow behaviour of Mar M200 powder compacts during isothermal forging