Effect of Grain Size, Grain Shape and Subgrain Size on High Temperature Creep Behaviour

Several strengthening machanisms, namely solute strengthening, precipitation and fine particle strengthening, grain size as well as shape control and substructure strengthening, have been utilised in the development of creep resistant alloys. Of these, the effect of polycrystal grain size, grain shape and subgrain size on viscous as well as power-law (climb) creep processes are considered here. Viscous creep processes include Nabarro-Herring (N-H), Coble and Harper-Dorn (H-D) creep. N-H and Coble creep processes are strongly influenced by grain size whereas creep rates under H-D creep are independent of grain size. Though it is usually assumed that power-law creep is independent of grain size, in practice it is not so. Several investigations have revealed that power-law creep does depend on grain size. There are detailed and well understood results pertaining to the role of grain size in viscous creep. On the other hand, subgrain size has been seen to have effect only on the climb creep behaviour which is yet to be clearly understood

Detection of Antibodies Against Trypanosoma evansi in Sheep by Indirect ELISA in Rayalaseema Region of Andhra Pradesh

The present research was carried out with an objective to improve the diagnostic tools for detection of antibodies against Trypanosoma evansi infection using indirect enzyme-linked immunosorbent assay (ELISA) in sheep. In this study standardized the Indirect ELISA for detection of T. evansi in sheep. The optimum concentration of antigen, test sera and conjugate were determined as 5µg per well, 1:10 and 1: 4000 dilutions, respectively. 464 serum samples were collected from sheep in different parts of the Rayalaseema region of Andhra Pradesh for screening of T. evansi infection. Out of 464 serum samples 46 (9.91%) were found positive by indirect ELISA

Basic studies leading to the development of an ultrahigh strength, high fracture toughness low-alloy steel

Ultrahigh strength steels have been used increasingly in recent years for critical aircraft and aerospace structural applications. In such applications, though materials performance is of prime consideration, cost and availability makes the low-alloy steels an attractive option. This paper describes the development of an ultrahigh strength NiSiCrCoMo low-alloy steel, supported by significant findings obtained from the basic studies that were aimed at understanding how solute additions influence fracture resistance of iron, with and without the presence of carbon. The results of the basic studies, in combination with the work of Garrison (1986) on a NiSiCr steel, have profitably been employed in the development of a NiSiCrCoMo low-alloy steel possessing a strength-toughness combination quite comparable to the highly alloyed 250-grade maraging steel. Reproducibility of attractive strength and toughness properties has been established in tonnage scale melts. This steel, in the softened condition, has good formability and machinability. Weld parameters have also been established. The NiSiCrCoMo low-alloy steel thus meets the requirements of performance and cost rendering it an attractive option for advanced structural applications

A first report on fracture toughness of bcc iron alloys as influenced by solutes: opposite effects of silicon and cobalt

The effect of solutes on resistance to fracture of body centred cubic iron single-phase solid-solution alloys has been investigated. The J-integral method has been used for the measurement of ductile fracture toughness. The JIC values so determined quantitatively indicate the extent of degradation in fracture toughness due to the addition of hardening solute silicon. Cobalt addition results in alloy softening. The measured JIC values clearly demonstrate the toughening effect of cobalt addition as a solute, which result renders the case of Fe-Co solid-solution alloys interesting

Creep of alpha-titanium at low stresses

Low-stress creep behaviour of alpha-titanium has been investigated over the range of temperature from 823-1088 K (0.43-0.56 Tm), grain size from 34-443 μm and stresses upto 2.0MN/m2. Resistance heated helical specimens were crept under their own weight and the deflections of individual coils were monitored in situ as a function of time by direct observation through a transparent vacuum chamber using a cathetometer. Analysis of the results has been made with the objective of establishing regimes of stress, temperature and grain size within which a specific creep mechanism controls deformation. Coble, Nabarro-Herring, Harper-Dorn and power-law creep mechanisms have been considered. It is shown that low temperatures and fine grain sizes are conducive to the dominance of Coble creep while Nabarro-Herring and Harper-Dorn creep processes respectively predominate in the intermediate temperature-intermediate grain size and intermediate temperature-coarse grain size regimes. In all these situations Newtonian viscous behaviour with negligible threshold stress has been observed. Analysis of the strain rate vs stress data above a transition stress, corresponding to the stress at which an upward deviation from linearity occurs, has established the operation of power-law creep. On the basis of the experimental results, Ashby- and Langdon-type creep mechanism maps have been constructed for alphatitanium

Influence of polycrystal grain size on fracture toughness of and fatigue threshold in Armco iron

Influence of polycrystal grain size on ductile fracture toughness of and fatigue threshold stress intensity in Armco iron has been studied over a grain size range 40 to 1050 μm. Both ductile fracture toughness and fatigue threshold stress intensity have been found to decrease with increasing grain size and the variation in either case follows a relationship similar to that proposed by Hall-Petch for strength. The variation of toughness with grain size can be understood in terms of plastic zone size whereas the fatigue threshold behaviour in Armco iron appears to be controlled by the critical value of crack tip opening displacement range

Fracture toughness of f.c.c. nickel and strain ageing b.c.c. iron in the temperature range 77-773 K

Ductile initiation fracture toughness J<SUB>IC</SUB> of b.c.c. Armco iron and f.c.c. nickel has been measured in the temperature range 77-773 K. Armco iron exhibits dynamic strain ageing (DSA) in the temperature range 383-573 K while nickel of the purity used does not evince DSA. Load vs load line displacement (LLD) plots during fracture toughness testing of Armco iron show serrations in the temperature range 383-573 K similar to those observed in the tensile stress-strain curves. DSA is found to have a beneficial effect on the fracture toughness J<SUB>IC</SUB>. A marked increase in tensile strength and fracture toughness occurs in Armco iron in the DSA regime. The strain hardening exponent, known to have a bearing on the plastic zone size and the void growth rate, seems a clear parameter in terms of which the observed J<SUB>IC</SUB> variation with temperature can be understood. Remarkably, the variation of n with temperature is found to closely follow the observed trend in<SUB>IC</SUB>. However, the slope of the J-R curve, dJ/da, decreases in the DSA regime with a minimum at 423 K. The decrease has been related to the fracture propagation process which is shown to occur by an alternate fast fracture and the ductile dimpled mode in the DSA regime. In the case of nickel, free from DSA, J<SUB>IC</SUB> or dJ/da are found to be largely unaffected by the test temperature. At room temperature f.c.c. nickel, at closely matching strength levels, possesses higher fracture toughness as compared to b.c.c. Armco iron. The crystal structure effect is more pronounced at temperatures below the ambient. At 77 K, the fracture toughness of iron is drastically reduced due to the onset of cleavage while nickel, not prone to a change in the fracture mode, maintains the same level of J<SUB>IC</SUB>, as at the ambient

Strength differential in Al---Li alloy 8090

Cyclic strength differential (SD) data have been derived from the variation of the tensile and the compressive stress amplitude with elapsed cycles during low cycle fatigue (LCF) for the quaternary Al---Li alloy 8090 in the T8E51 condition. LCF test specimens were machined out of the rolled plate such that the direction of stressing was along the longitudinal (L), L+45° and long transverse (LT) directions. The analysis of cyclic stress amplitude data corresponding to the half-life revealed an SD effect at all strain levels in the L+45° and LT directions. In the case of the L direction, at strain levels below 8.5 × 10<SUP>−3</SUP>, the SD is more than compensated by the Bauschinger effect resulting from the prestretch. The alloy exhibits a similar SD behaviour under monotonic loading conditions. The magnitude of cyclic SD at intermediate strain amplitudes in all the three test directions decreases upon cycling, which behaviour can be attributed to the relaxation of prestretch-related residual stresses leading to a decrease in σ<SUB>T</SUB> only in the L direction and σ<SUB>C</SUB> only in the L+45° and LT directions. As microstructural features vary in the differently heat-treated conditions, a comparison has been made of the monotonic SD data in the stretched and aged (T8E51) condition with those in the solution-treated, in the peak-aged (T6) and in the overaged (T7) conditions

On 'apparent pop-in' during ductile fracture toughness testing being related to the yield-point phenomenon in Armco iron

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