I. Samajdar

Resistance to sensitization and intergranular corrosion through extreme randomization of grain boundaries

Abstract Two grades of austenitic stainless steel, type 304 and 316L, were cold rolled to different reductions by unidirectional and by cross rolling. Subsequent solutionizing of the cold rolled samples produced noticeable textural differences in type 304, but insignificant differences in type 316L. Both the solutionized materials had however the same trend in grain boundary character distribution (GBCD): an increasing fraction of random boundaries with an increasing pre-solutionizing reduction percentage. The degree of sensitization (DOS) was measured by the double loop electrochemical potentiokinetic reactivation (DL-EPR) test in both the alloys. The susceptibility to intergranular corrosion was assessed by the standard weight loss technique (practice B, A262 ASTM) in type 304 alloy. These increased with increase in random boundary concentration, but then dropped significantly beyond a ‘critical’ concentration—a pattern observed in both the grades. Such a pattern may be explained from a balance between nucleation rate of Cr-carbides and grain boundary Cr-flux, though postulating an exact model is premature at this stage. The present study, however, demonstrates a clear possibility of remarkable improvement in DOS and IGC through extreme grain boundary randomization.

Quantitative Prediction of Cold Rolling Textures in Low-Carbon Steel by Means of the Lamel Model

Rolling textures of low-carbon steel predicted by full constraints and relaxed constraints Taylor models, as well by a self-consistent model, are quantitatively compared to experimental results. It appears that none of these models really performs well, the best results being obtained by the Pancake model. Anew model (“Lamel model”) is then proposed as a further development of the Pancake model. It treats a stack of two lamella-shaped grains at a time. The new model is described in detail, after which the results obtained for rolling of low-carbon steel are discussed. The prediction of the overall texture now is quantitatively correct. However, the γ-fibre components are better predicted than the α-fibre ones. Finally it is concluded that further work is necessary, as the same kind of success is not guaranteed for other cases, such as rolling of f.c.c, materials.

Explaining absence of texture development in cold rolled two-phase Zr–2.5 wt% Nb alloy

Abstract In the present study, two distinct starting microstructures of Zr–2.5 wt% Nb have been used: (1) single-phase α hcp martensitic structure; and (2) two-phase, 10% bcc β and rest hcp α, Widmanstatten structure. In the second case, two types of α were present—near grain boundary predominantly single-phase α (about 5% of the total α) and α plates in an apparently continuous β matrix. Both (1) and (2) had similar starting crystallographic texture of the hcp α phase and were deformed by unidirectional and cross rolling. In the two-phase structure the changes in the bulk texture on cold rolling was found to be insignificant, while in the single-phase material noticeable textural changes were observed. Taylor type deformation texture models predicted textural changes in single-phase structure but failed to predict the observed lack of textural development in the two-phase material. Microtexture observations showed that α plates remained approximately single crystalline after cold rolling, while the β matrix underwent significant orientational changes. Relative hardening, estimated by X-ray peak broadening, was observed mainly in β phase; while aspect ratio of α plates remained unchanged with cold rolling—indicating absence of effective macroscopic strain in the hcp α plates. Based on microstructural and microtextural observations, a simple model is proposed in which the plastic flow is mainly confined to the β matrix within which the α plates are subjected to ‘in-plane rigid body rotation’. The model explains the observed lack of textural developments in the two-phase structure.

Anomalous variation of coercivity with annealing in nanocrystalline NiZn ferrite films

The sputter deposited NiZn ferrite thin films were studied as a function of annealing temperature. The magnetization showed a monotonic increase with increasing annealing temperature. The coercivity shows a minimum at annealing temperature of 400 °C and shows a value of 14 Oe. Transmission electron microscopy study indicated that the grain size increases from ∼3 nm for the as-deposited case to ∼15 nm for the film annealed at 800 °C. The observed coercivity behavior could be attributed to the defects present in the films, the change in cation distribution, and the grain growth.

Annealing induced structural change in sputter deposited copper ferrite thin films and its impact on magnetic properties

Copper ferrite (CuFe2O4) thin films have been prepared by rf sputtering on fused quartz and silicon (111) substrates at ambient temperature. The as-deposited film is found to be in cubic phase, which is stable only at higher temperatures in bulk. The films were annealed at temperatures ranging from 100 °C to 800 °C and slowly cooled. The films had tetragonal structure at annealing temperature above 200 °C. The c/a ratio was observed to increase with increasing annealing temperature. Transmission electron microscopy study confirmed the phase transformation from cubic to tetragonal as a function of annealing temperature. The magnetization values of the films were observed to show a maximum at the annealing temperature of 200 °C and a minimum at 500 °C. The coercivity increased monotonically from 70 Oe for the as-deposited film, to 1450 Oe for the film annealed at 800 °C. The results were explained on the basis of the phase transformation and the grain growth phenomenon.

Evolution and characterization of dynamically recrystallized microstructure in a titanium-modified austenitic stainless steel using ultrasonic and EBSD techniques

A C-scan ultrasonic imaging system was used to investigate the microstructural evolution during dynamic recrystallization (DRX) of a 15Cr–15Ni–2.2Mo–Ti modified austenitic stainless steel (alloy D9). Four specimens were forged at 1273 K to different strains in the range 0.1–0.5. Specimens with true strains of 0.2 or lower did not show any variation in the amplitude of the first back-wall echo. However, a visible variation in the C-scan image was observed at and above the 0.3 strain level. This variation was attributed to the evolution of fine grains. The formation of fine grains was related to DRX, as indicated by electron backscattered diffraction. This study also revealed the characteristics of the DRX or ‘necklace grains’, as opposed to the so-called parent grains or rest of the microstructure.

Precipitation stages in a 316L austenitic stainless steel

Abstract Detailed studies of precipitation in type 316L (UNS S31603) austenitic stainless steel were carried out by using a combination of differential scanning calorimetry, electrical resistivity measurement and transmission electron microscopy. Four distinct stages of precipitation were identified––coherent precipitation, its coarsening and initiation of grain boundary precipitation, σ phase and finally M 23 C 6 precipitation.

Enhanced magnetization in sputter-deposited copper ferrite thin films

Copper ferrite thin films have been prepared by RF sputtering on quartz substrates. By following successive heating and quenching, it was possible to obtain the films, which showed cubic phase in X-ray diffraction. The grain size was found to be ∼30 nm. The magnetization was observed to be of the order of 3000 G with a coercivity of 480 Oe.

Relationship Between Pack Chemistry and Growth of Silicide Coatings on Mo–TZM Alloy

A theoretical model equation has been derived to relate the growth kinetics of silicide coating with the pack chemical composition and other processing conditions for siliconizing of Mo-TZM (Mo-0.5Ti-0.1Zr-0.02C) alloy to improve its oxidation resistance at high temperatures. A series of experiments conducted with varying pack Si (1-10 wt %) and NH 4 F (2-20 wt %) content, time (1-25 h), and temperature (800-1200°C) confirmed the validity of the model. MoSi 2 was the main coating layer formed during the siliconizing process. Optimum processing conditions were derived for doping of Al in MoSi 2 to form Mo(Si,Al) 2 in the outer layer of the coating.

Deformation twinning in AISI 316L austenitic stainless steel: role of strain and strain path

Abstract AISI 316L austenitic stainless steel was deformed at different strain and strain paths. The twin boundaries in the deformed microstructure had two possible origins: decay of original annealing twins and generation of deformation twins. Assuming that rotations of grains, specifically grains on both sides of a twin boundary, are responsible for the twin decay, a simple model was proposed to bring out the domain of relative twin generation. A biaxial strain path, in general, was associated with strong twin generation – an association or dependency linked to the texture estimated values of Taylor factor. Formation of strain induced martensite was also observed to be strain and strain path dependent and was more in biaxial strain path.