Influence of thermomechanical processing on shear bands formation and magnetic properties of a 3% Si non-oriented electrical steel

The effect of thermomechanical processing on the formation of shear bands and on the magnetic properties of a 3.0 wt% silicon non-oriented steel was investigated by hot rolling samples with different thicknesses at different temperatures, in order to obtain a variation in hot band grain size and cold strain. All the samples were processed in a single-stage cold rolling and finally annealed at 1020 °C. It was found that the increase of the hot band grain size decreases the γ fiber volume fraction and increases the η fiber volume fraction after the final annealing. The increase of the cold strain strongly contributed to this result. A good combination of intense generation of shear bands, and proper crystallographic texture, due to higher nucleation of grains with favorable orientations to magnetization in these bands, can be obtained for the samples hot rolled at 1000 and 1120 °C and submitted to cold strain of 64.3% and 72.2% respectively. However the best combination of B₅₀, W_15/60 and μ_r can be obtained by hot rolling the samples at 1000 °C to the thickness of 1.4 mm, corresponding to 64.3% of cold strain.     

A study of the dynamic impact behaviour of IN 718 and ATI 718Plus® superalloys

The dynamic impact response of IN 718 and ATI 718Plus®, in both the solution heat treated and age-hardened conditions, were investigated at different deformation temperatures and strain rates using a direct impact Hopkinson pressure bar. Analyses of the results provide a vital but previously not reported information that the ATI 718Plus® offers a higher resistance to damage during high strain rate ballistic impact deformation compared to the most widely used Iron-nickel based superalloy, Inconel 718. ATI 718Plus® showed higher strain hardening and strain rate sensitivity, in both heat treatment conditions, than IN 718. The difference in the deformation behaviour of both alloys, in the annealed condition, is attributable to the compositional modification in ATI 718Plus® which has been reported to lower its stacking fault energy and increases the tendency for deformation twinning. However, in the age-hardened condition, the difference is believed to be related to the disparity in the operative strengthening mechanism, of the precipitates present in both alloys. Furthermore, a higher susceptibility to strain location and the formation of adiabatic shear band, in aged IN 718, is attributable to the stronger temperature-softening characteristics observed in the alloy and to the limited strain hardening tendency under dynamic impact loading.     

Slip band formation and mobile dislocation density generation in high rate deformation of single fcc crystals

The mechanisms for the nucleation, thickening, and growth of crystallographic slip bands from the sub-nanoscale to the microscale are studied using three-dimensional dislocation dynamics. In the simulations, a single fcc crystal is strained along the [111] direction at three different high strain rates: 10⁴, 10⁵, and 10⁶?s^??1. Dislocation inertia and drag are included and the simulations were conducted with and without cross-slip. With cross-slip, slip bands form parallel to active (111) planes as a result of double cross-slip onto fresh glide planes within localized regions of the crystal. In this manner, fine nanoscale slip bands nucleate throughout the crystal, and, with further straining, build up to larger bands by a proposed self-replicating mechanism. It is shown that slip bands are regions of concentrated glide, high dislocation multiplication rates, and high dislocation velocities. Cross-slip increases in activity proportionally with the product of the total dislocation density and the square root of the applied stress. Effects of cross-slip on work hardening are attributed to the role of cross-slip on mobile dislocation generation, rather than slip band formation. A new dislocation density evolution law is presented for high rates, which introduces the mobile density, a state variable that is missing in most constitutive laws.     

TEM and HRTEM study of influence of thermal cycles with stress on dynamic recrystallization in Ti46Al8Nb1B during creep

Short-term tension creep and thermal cycles under compressive stress were performed on Ti46Al8Nb1B in order to explore the dynamic recrystallization (DRX) grains formed during the creep and the impact of thermal cycles under stress to the DRX. After 1600 times' thermal cycles from 300 degrees C to 800 degrees C under 300 MPa compressive stress, high density of ledges and thick ledges are found in the interfaces. Two kinds of moiré fringes, instead of 9R structure, can be found in the thick ledges. Ti46Al8Nb1B sample and another sample which was treated by thermal cycles with stress were crept under 300 MPa compressive stress at 800 degrees C. DRX grains are found in the interfaces in those samples. Those grains, formed at the ledges, have an orientation relationship of [101](gamma)//[011](gammaR), (1 1 1)(gamma)//(1 11 )(gammaR) with the matrix of gamma phases. Thermal cycles with stress could lead to more DRX grains during creep.     

On the refinement of carbide precipitates by cryotreatment in AISI D2 steel

Refinement of carbide particles by cryotreatment is often proposed as a major factor for the improvement of wear resistance in tool steels. However, this proposition is not substantiated by experimental evidence. This has been examined in this report by (i) detailed micro-structural analyses of the nature, volume fraction, size, population density and distribution of carbide particles, (ii) XRD and EDX micro-analysis on the bulk samples and electrochemically extracted carbides, and (iii) measurement of hardness and wear rate of a series of differently cryotreated AISI D2 steel. The results conclusively establish that (i) cryotreatment, in comparison to conventional treatment, induces precipitation of finer carbides with higher volume fraction and more uniform distribution, and (ii) population density and the size of secondary carbide particles significantly increases with holding time up to a critical duration at 77 K in cryotreatment. The latter observation indicates the pioneering direction towards optimization of cryotreatment design for techno-economic benefit.     

Atomic-scale study of precipitates (NbC and Cu-rich phase) at the twin boundary in the long time ageing austenitic stainless steel

ABSTRACT

Microstructures of Cu-rich phases and NbC precipitated phases have been studied in a long time ageing austenitic stainless steel by high resolution scanning transmission electron microscopy. The interaction difference between the twins and the second phases found to be dependent on the nature of the precipitates. The Cu-rich phases were identified to be twinned at the twin boundary. Nevertheless, the NbC precipitates not only twinned at the twin boundary but also induced the twin boundary bypass them. A particle size dependence of the generation of misfit dislocations also was detected at interface between precipitates and the austenitic matrix.     

A study on the formation of crystalline phases during solidification and crystallisation in the bulk metallic glass of Zr53Cu21Al10Ni8Ti8 composition

The present paper considers the formation of crystalline phases during solidification and crystallisation of the Zr₅₃Cu₂₁Al₁₀Ni₈Ti₈ alloy. Solidification was carried out by a copper mould casting technique, which yielded a partially crystalline microstructure comprising a ‘big cube phase’ in a dendritic morphology and a bct Zr₂Ni phase. Detailed high-resolution microscopy was carried out to determine possible mechanisms for the formation of the crystalline phases. Based on microstructural examinations, it was established that the dendrites grew by the attachment of atomistic ledges. The bct Zr₂Ni phase, formed during solidification and crystallisation, showed various types of faults depending on the crystallite size, and its crystallography was examined in detail. It has been shown that the presence of these faults could be explained by anti-site occupancy in the bct lattice of the Zr₂Ni phase.