Papers by Author: John J. Jonas

Abstract: The problems associated with the use of conventional rolling mill models are described. These include the unavoidable variations in temperature and strain rate (rolling speed) during rolling. They are exacerbated by the wide variety of mill types and configurations found in industry and their correspondingly broad ranges of interpass time. Finally, a major limitation arises from the approach currently employed to model the “strain accumulation” attributable to incomplete softening between passes, particularly during the processing of microalloyed steels.

Abstract: It is shown that the kinetics of softening between mill passes can be modeled more simply when the normalized strain (reduction) per pass is employed rather than the conventional strain. This method requires a second important input, namely the strain hardening rate at the end of preloading. Using this approach, the number of input parameters and experiments required for their determination are drastically reduced. The use of the Law of Mixtures to describe the behaviors of the recrystallized and unrecrystallized volume fractions is then illustrated. Finally, the approach required for quantifying the precipitation kinetics (in microalloyed steels) is described.

Abstract: The crystallographic relationship between the g and a phases in samples of the Gibeon meteorite and a TRIP steel was investigated by means of EBSD techniques. The orientations of the two phases were measured and are represented in pole figures. The results are compared to predictions made on the basis of the Bain, Kurdjumov-Sachs (K-S), Nishiyama-Wassermann (NW), Greninger-Troiano (G-T) and Pitsch orientation relationships. The local misorientation between individual fcc and bcc crystals along their common interface was measured to demonstrate the way in which the exact orientation relationship varies along the boundary. The local orientations within lamellae and laths of kamacite and bainite are compared to that in recrystallised ferrite polygons. The occurrence of variant selection during the transformation of deformed austenite is analyzed using a recent dislocation-based model.

Abstract: Orientation relationships between individual crystals can be readily represented in Rodrigues-Frank space because of the one-to-one correspondence between each misorientation and a vector in the fundamental zone of this space. This is done by integrating the rotation angle and axis into a three-component vector. In this study, the three classical orientation relationships describing the γ-to-α transformation, namely the Bain, Kurdjumov-Sachs and Nishiyama- Wassermann, are represented in Rodrigues-Frank space. Also considered are the somewhat less common Pitsch and Greninger-Troiano relationships. The misorientations between these types of transformation variants are displayed in R-F space based on alternative reference systems to highlight the differences. Examples of the various crystallographic relationships between fcc and bcc crystals during the γ-to-α transformation are given to demonstrate the advantages of the use of this space.

Abstract: The preferential initiation of dynamic recrystallization (DRX) at triple junctions (TJs) in stainless steel polycrystals was investigated in compression at 1123 K to 1323 K at a strain rate of 2 x 10-4 s-1. Nucleation appeared at TJs at strains as low as 0.1. This strain is only about 1/5 to 1/2 of the peak strain at which DRX is conventionally believed to occur extensively. Furthermore, DRX nucleation was not observed to take place at grain boundaries or in the matrix at this strain. The probability of DRX nucleation at TJs increased monotonically with strain and temperature. It also depended on the angle, y, between the compression axis and the sliding boundary. That is, when the angle, y, approaches 45 degrees, the probability of DRX nucleation at TJs is higher. These results reveal the important role of grain-boundary sliding (GBS) on DRX nucleation at TJs. It should also be noted that more than 90% of the grains nucleated at TJs were twins. Such dynamic twinning suggests that the essential DRX nucleation mechanism is twinning.

Abstract: The orientation relationships operating during the austenite (FCC) to BCC phase transformation were investigated in a high performance steel using X-ray and electron diffraction techniques and employing several length scales. These steels contain some retained austenite that permits the direct comparison of the textures of the parent (austenite) and product (ferrite, bainite, martensite) phases. X-ray diffraction allowed the global texture of the rolled parent austenite phase to be determined as well as that of its transformation product. EBSD techniques permitted study of the orientation relationships on a local scale. The observed correspondence relations are expressed in Rodrigues-Frank space. The exact Kurdjumov-Sachs relation was never found. The local spread of orientation in the parent austenite (due to deformation) is seen to be inherited by the bainite. This is attributed to the displacive mode of transformation to bainite. The influence of prior deformation of the austenite on the occurrence of variant selection was also studied. It appears that a critical strain is necessary to produce a significant amount of variant selection.

Abstract: The effect of warm and cold rolling parameters on the development of annealing textures was studied in two low carbon steels containing additions of chromium. Two warm rolling temperatures (640 and 700°C) were employed together with a reduction of 65%. The effects of an additional cold rolling reduction of 40% and of decreasing the heating rate during annealing were also studied. The ND fiber, <111>//ND, of the recrystallization texture was strengthened as the warm rolling temperature was decreased. However, all the warm rolled steels contained a retained RD fiber, <110>//RD. A noticeable improvement in both the continuity and intensity of the ND fiber was obtained when the sample was submitted to an additional 40% cold rolling reduction. The ND fiber was even more continuous and intense when a low heating rate was utilized, yielding r-values of 1.1 and 1.3 for the warm rolled and warm plus cold rolled samples, respectively.

Abstract: The mechanisms governing the formation of transformation textures during the austenite-to-ferrite transformation are the subject of major debate. In this study, two extreme cases were examined: those of undeformed and deformed austenite. The first involves the transformation of austenite into Widmanstätten ferrite under "equilibrium" conditions in the Gibeon iron-nickel meteorite. This meteorite passed through the transformation at the rate of a few degrees per million years. Such cooling rates cannot of course be reached under laboratory conditions. The second concerns the transformation of hot rolled austenite after a quench into the bainite temperature range. These two behaviors were investigated by means of optical microscopy and electron backscatter diffraction (EBSD) techniques. The orientations of both the parent and product phases were measured and the orientation relationships are represented in Rodrigues-Frank (R-F) space. From the orientation of a particular FCC crystal, the crystallographic orientations of the product BCC crystals can be predicted according to the Bain, Kurdjumov-Sachs (K-S) and Nishiyama- Wassermann (N-W) correspondence relationships. Comparison of the predicted and measured orientations reveals that the Bain rotation is never observed; the K-S and N-W relationships are both observed and there is a continuous distribution of orientations between the exact K-S and N-W positions. The formation of preferred orientations under non-equilibrium conditions is scrutinized. These results are compared to recent models accounting for variant selection.

Abstract: Particle-stimulated nucleation (PSN) was investigated in magnesium alloy AZ31 to study the effect of the evolution of second-phases during extrusion and other metal forming processes. Compression tests were carried out on samples taken from coarse-grained as-cast magnesium alloy billets containing a lamellar Mg17All2 eutectic phase and (Al, Mn) particles. These revealed that particle-stimulated DRX nucleation (PSN) was taking place during hot deformation and that this is facilitated by the fragmentation of the Mg17All2. When Mg17All2 dissolves into the matrix at about 350°C, the (Al, Mn) particles remain effective in producing PSN at temperatures up to at least 400°C. This suggests that alloy design leading to a suitable distribution of second-phase particles can improve the properties and formability of wrought magnesium alloys.