V.V. Kutumbarao

On the discontinuous precipitation of Cr2N in Cr-Mn-N austenitic stainless steels

Abstract Discontinuous precipitation of CrzN in the Fe-Cr-Mn-N system at temperatures in the range 700–900 °C occurs by long range diffusion of nitrogen from the untransformed matrix to the precipitate cell. The rate of discontinuous precipitation has a direct effect on the nitrogen concentrations in the untransformed matrix as well as in the cell. Nitrogen diffusion is faster in the steel containing higher initial nitrogen concentration.

Tensile behaviour of powder metallurgy processed (Al-Cu-Mg)/SiCp composites

Abstract The tensile behaviour of Al–Cu–Mg alloy matrix composites produced by a powder metallurgy process was investigated as a function of particle size in the as extruded, homogenised, and peak aged conditions. The tensile behaviour of the corresponding matrix alloy which was produced in a similar manner, designated as Control, was also studied. There was a significant increase in the 0.2% yield strength of Control and all the metal matrix composites (MMCs) after homogenisation treatment (53–68%) and peak aging (93–109%), as compared to their values in the as extruded condition. The ultimate tensile strength (UTS) of Control as well as the MMCs also increases considerably after homogenisation treatment (39–70%), however, subsequent peak aging did not result in any further increase in UTS in case of any of the MMCs. It was found that the finer the reinforcement size, the higher the 0.2% yield strength and UTS in all the conditions. On the other hand, ductility decreased considerably after homogenisation treatment and subsequent peak aging. The results are discussed in the light of dislocation strengthening as well as reinforcement damage.

In-plane anisotropy in the fracture toughness of an Al-Li 8090 alloy plate

Abstract Fracture toughness of an Al-Li 8090 alloy plate was studied in L-T, L + 45° and T-L orientations. A significant in-plane anisotropy exists, with the fracture toughness in the L + 45° orientation being 32% lower and that in the T-L orientation being 45% lower than the fracture toughness in the L-T orientation. The observed behaviour is explained on the basis of Chen and Knotts model that relates the toughness of the alloys to the tensile properties and the characteristics of the void initiating particles. The analysis has revealed that the anisotropy in the in-plane fracture toughness arises because of the variation in yield strength, work hardening exponent and interparticle spacing with notch orientation. The variation in yield strength and work hardening exponent is caused by the crystallographic texture, whereas the variation in interparticle spacing primarily results from the highly unrecrystallized pancake microstructure of the alloy.

Aluminium Alloys for Aerospace Applications

This chapter starts with a brief overview of the historical development of aerospace aluminium alloys. This is followed by a listing of a range of current alloys with a description of the alloy classification system and the wide range of tempers in which Al alloys are used. A description is given of the alloying and precipitation hardening behaviour, which is the principal strengthening mechanism for Al alloys. A survey of the mechanical properties, fatigue behaviour and corrosion resistance of Al alloys is followed by a listing of some of the typical aerospace applications of Al alloys. The Indian scenario with respect to production of primary aluminium and some aerospace alloys, and the Type Certification process of Al alloys for aerospace applications are described. Finally there is a critical review of some of the gaps in existing aerospace Al alloy technologies.

An analysis based on plastic strain energy for bilinearity in Coffin-Manson plots in an AlLi alloy

Fatigue failure of a component occurs through an irrecoverable energy dissipating process. The alloy dissipates most of the plastic strain energy as heat, the other forms being vibration and acoustic emission. Some energy will be absorbed by immobile life defects and by surface damage processes. In order to gain greater understanding of the material response to fatigue damage, it is necessary to propose a physical quantity with energy dissipation as a fatigue damage parameter. Two such widely used fatigue damage parameters are the average plastic strain energy per cycle ([Delta]W[sub p]), the area under the hysteresis loop during low cycle fatigue and the total dissipated energy (W[sub f]), the sum of the areas of all the loops before failure. Each alloy has a certain capacity to dissipate the plastic strain energy. When this limit is attained, the cracks which originated during the earlier cycling will propagate and failure occurs. This paper presents the results of the analysis of the low cycle fatigue data obtained in the case of a quaternary Al-Li-Cu-Mg alloy. The aim of this analysis is to provide further understanding on the bilinear nature of the Coffin-Manson power law relationship, which has already been reported by several researchers.

On the dual slope Coffin-Manson relationship during low cycle fatigue of Ni-base alloy IN 718

Abstract A detailed TEM study has been conducted on specimens from LCF tests interrupted after different number of cycles at two strain amplitudes to find the reason for the observed dual slope C-M plot at room temperature in the alloy IN 718. The following conclusions emerge from this study: (a) In the low strain region the deformation mode is essentially twinning, (b) In the high strain region deformation principally occurs by slip band formation, (c) Change in mode of deformation has resulted in dual slope C-M plot.

Cyclic Stress Response of Al-Cu-Mg Alloy Matrix Composites with SiCp of Varying Sizes

High specific stiffness and strength, isotropic property and ease of production has made the family of discontinuously reinforced metal matrix composites (MMCs) attractive candidate material for aerospace components. Many of the components employed for such applications are subjected to fluctuating stress and strain conditions where an understanding of the low cycle fatigue (LCF) behavior of the material is important. In recent years, study of LCF behavior of discontinuously reinforced aluminum matrix composites has received considerable attention. The cyclic stress response which characterizes the mechanical stability of the intrinsic microstructural features and plastic strain distribution during fully reversed, total strain controlled cycling is an important LCF feature. Some systematic investigations have been carried out to study the effect of variation in particulate/whisker volume fraction on the cyclic stress response of particulate reinforced aluminum matrix composites. The present work reports influence of different sizes of SiCp on the cyclic stress response of Al-Cu-Mg alloy matrix composites.

On the micromechanisms responsible for bilinearity in fatigue power-law relationships in aluminium-lithium alloys

Aluminium-lithium alloys, like many other aerospace structural alloys, exhibit bilinearity in power-law relationships between high strain, low cycle fatigue life (in terms of number of reversals to failure, 2N{sub f}) and plastic strain amplitude ({Delta}{epsilon}{sub p}/2) or average stress amplitude ({Delta}{sigma}/2) or average plastic strain energy per cycle ({Delta}W{sub p}). In the present paper the micromechanisms leading to bilinear fatigue power-law relationships, especially the Coffin-Manson (C-M) relationship, in aluminium-lithium alloys are compiled and discussed.