J.J. Jonas

Microtextural study of orientation change during nucleation and growth in a cold rolled ULC steel

Abstract Texture development during the recrystallization of an ultra low carbon steel is shown to be largely dependent on the rolling reduction. Comparison of the recrystallization process for two different rolling reductions indicates that in each case a different recrystallization mechanism is responsible for the principal features of the annealing texture.

Modern HSLA steels and role of non-recrystallisation temperature

Abstract The use of heavy gauge steel sheets for structural applications often requires a combination of high yield strength and adequate toughness. The most cost effective way to achieve high yield strength and high ductility in low alloyed steels is through grain refinement. In industrial practice, such refinement is commonly obtained by thermomechanical controlled processing (TMCP). This approach comprises slab reheating to well defined temperatures, a large amount of hot deformation below the non-recrystallisation temperature Tnr and accelerated cooling. In practice, the Tnr is generally raised by the addition of microalloying elements such as Nb and Ti. As these elements contribute substantially to the alloying costs, optimisation of their use allows for a decrease in production cost. Better understanding of the Tnr assists in tuning the rolling process so that optimum mechanical properties can be produced. One area of importance is to recognise that the concept of the Tnr was originally developed for reversing mills and the production of plate steels. Methods of defining and determining it must be modified if it is to be applied to strip mills and their associated short interpass times. The main goal of this review is to provide a concise and complete overview of the current understanding of the fundamental mechanisms that control the Tnr and to address the different methods that can be used to determine it.

Nucleation of dynamic recrystallization and variant selection in copper bicrystals

Orientation-controlled copper bicrystals containing [001] symmetrical tilt boundaries aligned parallel to the loading axis were deformed in tension at 923 K and a strain rate of 4.2 × 10−4 s−1. The nucleation of dynamic recrystallization (DRX) was investigated along the grain boundary. For this purpose, both optical and orientation imaging microscopy methods were used. After grain-boundary migration (GBM) and bulging, nuclei appeared behind the most deeply indented grain boundary regions. The critical strain for nucleation was about one-quarter to one-half of the peak strain and depended on the misorientation angle. All the nuclei were twin-related (Σ3) to the matrices. Furthermore, all the primary twin traces were parallel to those of the inactive slip planes of the parent single crystals. Crystallographic analysis revealed the important role of the direction of GBM on twinning-plane variant selection. The characteristics of grain boundary nucleation depended sensitively on grain boundary character and on grain boundary mobility. The observed DRX nucleation mechanism is discussed in relation to the occurrence of GBM and twinning.

Nucleation of dynamic recrystallization at triple junctions in polycrystalline copper

The preferential nucleation of dynamic recrystallization (DRX) was investigated in copper polycrystals. DRX nucleation began to appear preferentially at triple junctions (TJs) at around 0.01 strain irrespective of the testing conditions, i.e. at only about 1/10 to 1/20 of the peak strain. The likelihood of DRX nucleation at TJs increased monotonically with strain. Furthermore, this probability also depended strongly on the strain rate, temperature and initial grain size. More than 80% of the grains nucleated at TJs were twins. The observed likelihood of DRX nucleation at TJs was closely related to the amount of grain-boundary sliding, fold formation at the TJs, the misorientation angle of the grain boundaries composing the TJ, and the angle between the tensile axis and the sliding grain boundary. The mechanisms determining the strain-rate dependence of DRX nucleation and twin formation are discussed in detail in relation to the above factors.

Evolution of microstructure, microtexture and mechanical properties of linear friction welded IMI 834

Abstract Titanium alloys have been of great interest in the aerospace industry for many years. Recently, linear friction welding has also been making strides in conquering a part of the aerospace manufacturing market, with its clear advantages over fusion welding and mechanical fastening methods for integrated bladed rotors. High tech near-α alloy IMI834 (Ti–5·8Al–4Sn–3·5Zr–0·7Nb–0·5Mo–0·35Si) was designed to have improved creep resistance and retains its mechanical properties at temperatures up to 600°C. It balances creep resistance and fatigue strength, making it an excellent material for compressor discs and blades. IMI834 with an initial bimodal α+β microstructure was welded using varying axial pressures during welding and then characterised using both microstructural examination and mechanical testing. Electron backscatter diffraction (EBSD) was used to characterise the texture and phase fraction of the welded IMI834 samples in the weld zone (WZ) and thermomechanically affected zones. The EBSD analysis revealed fine recrystallised grains at the weld centres. The microhardness evaluation of the weldments showed that the recrystallised WZ was slightly harder than the parent material (PM). The local and global tensile properties of the welds, investigated using a tensile testing rig with integrated digital image correlation, revealed higher strength in the WZ and failure in the PM. Les alliages de titane ont suscité un grand intérêt dans l’industrie aérospatiale pendant plusieurs années. Récemment, le soudage par friction linéaire a également fait de grands progrès dans la conquête d’une partie du marché de la fabrication aérospatiale, avec ses avantages clairs par rapport aux méthodes de soudage par fusion et d’assemblage mécanique pour les rotors à lames intégrées. L’alliage de haute technologie quasi-alpha, IMI834 (Ti–5·8Al–4Sn–3·5Zr–0·7Nb–0·5Mo–0·35Si) a été conçu pour avoir une résistance améliorée au fluage et il retient ses propriétés mécaniques à des températures jusqu’à 600°C. Il balance la résistance au fluage et la résistance à la fatigue, ce qui en fait un excellent matériau pour les disques et les lames de compresseur. IMI834, avec une microstructure initiale bimodale α+β, a été soudé en utilisant des pressions axiales variables lors du soudage. On l’a ensuite caractérisé en utilisant tant l’examen de la microstructure que les essais mécaniques. On a utilisé la diffraction d’électrons rétrodiffusés (EBSD) pour caractériser la texture et la fraction des phases des échantillons soudés d’IMI834 dans la zone de soudure (WZ) et dans les zones affectées thermomécaniquement. L’analyse par EBSD a révélé des grains fins recristallisés au centre des soudures. L’évaluation de la microdureté des ensembles soudés a montré que la zone de soudure recristallisée était légèrement plus dure que le matériau de base (PM). Les propriétés de traction locales et globales des soudures, examinées au moyen d’un assemblage d’essai de traction avec corrélation intégrée d’image digitale, ont révélé une résistance plus élevée dans la zone de soudure et la défaillance du matériau de base.

Production of recrystallized nano-grains in a fine-grained Cu–Zn alloy

A fine-grained Cu–30%Zn alloy sheet was rolled at 77 K to induce ultrafine mechanical twins. Subsequent annealing of the rolled alloy at temperatures up to 543 K led to the uniform appearance of recrystallized ultrafine grains (UFGs), which contained numerous annealing twins. Average grain sizes of 150 ∼ 300 nm were produced in this way. The formation of such UFGs during annealing is attributed to the high nucleus density associated with the fine initial grain size as well as to the high densities of mechanical twins and dislocations produced by cryorolling. The high driving force for recrystallization enabled the use of relatively low annealing temperatures, which limited the subsequent grain growth.

Work hardening and kinetics of dynamic recrystallization in hot deformed austenite

The work hardening behaviors of 4 steels deformed under dynamic recrystallization (DRX) conditions were analysed. The h and r parameters necessary to describe the work hardening or dynamic recovery (DRV) curves in the absence of DRX were determined from the experimental flow curves. The fractional softening X attributable to DRX, defined as the difference between the calculated DRV and experimental DRX curves, was used to derive the Avrami kinetics of DRX. Some conclusions are drawn about the effects of deformation temperature, strain rate and steel composition on the kinetics of DRX.