Papers by Author: John J. Jonas

Abstract: Seven-pass strip rolling simulations were carried out on a 0.06%C and a 0.09%C-0.036%Nb steel. The rolling loads (mean flow stresses or MFS’s) did not increase as the temperature decreased during the simulation. This is ascribed to the occurrence of dynamic transformation. The simulation results are compared to the high temperature flow curves determined on eight plain C and Nb-modified steels in both compression and torsion and at a series of temperatures and strain rates. When the associated MFS’s are plotted against inverse absolute temperature in the form of Boratto diagrams, the stress drop temperatures, normally defined as the upper critical temperature applicable to rolling, A_r3*, are shown to be about 40 degrees above the paraequilibrium and about 20-30 degrees above the orthoequilibrium A_e3’s. These drops are ascribed to the dynamic transformation of austenite to ferrite, a softer phase. The characteristics of the ferrite produced dynamically are described and the transformation is shown to be displacive in nature, leading to the appearance of fine Widmanstätten plates. These plates coalesce into polygonal grains on further deformation and on holding.

Abstract: High temperature flow curves were evaluated on two Nb steels in both compression and torsion and at a series of temperatures and strain rates. The critical strains for the initiation of dynamic transformation (DT) were determined by the double differentiation method. These are shown to be distinctly lower than those associated with dynamic recrystallization (DRX). It is also evident that the compression critical strains for both DT and DRX are lower than the equivalent torsion critical strains. Mean flow stresses (MFSs) were calculated by integration from the flow curves. When plotted against inverse temperature, stress drops were observed about 30 degrees above the Ae₃. These drops are shown to be caused by the dynamic transformation of austenite to ferrite, a softer phase. The characteristics of the ferrite produced dynamically are described and the transformation is shown to be displacive in nature, leading to the appearance fine Widmanstatten plates.

Abstract: A 0.02%C plain carbon and a 0.22%C TRIP steel were tested in compression in the temperature range 900°C to 1150°C and strain rate range 0.05s^-1 to 1s^-1. Thirty-two experimental flow curves were obtained in this way. The critical conditions for the initiation of dynamic recrystallization were determined by the double differentiation method. Using a dislocation density model to describe the austenite flow stress, the work hardening parameters r and h were derived and are used to model the flow curve in the absence of dynamic recrystallization. The latter was employed to calculate the fractional softening attributable to dynamic recrystallization. The kinetics of dynamic recrystallization are then described using Avrami kinetics. Finally, the dependences of the Avrami and work hardening parameters on Z, the Zener-Hollomon parameter, are used to model compression flow curves at strain rates an order of magnitude greater than the ones employed in the tests.

Abstract: Recent observations regarding the transformation of deformed austenite are reviewed. It is shown that superequilibrium ferrite and pearlite can be formed at temperatures well above the Ae₃ and Ae₁, respectively. The role of the stored energy associated with the introduction of the dislocations introduced by the deformation is discussed. It is shown that the forward dynamic transformation into ferrite and pearlite is several orders of magnitude faster than the reverse static transformation back into austenite. The retarding effect of alloying additions such as niobium is also outlined. The results are interpreted in terms of the effect of deformation on the modified phase diagrams pertaining to the transformation of deformed austenite.

Abstract: Recent observations regarding the transformation of deformed austenite are reviewed. It is shown that superequilibrium ferrite and pearlite can be formed at temperatures well above the Ae₃ and Ae₁, respectively. The role of the stored energy associated with the introduction of the dislocations introduced by the deformation is discussed. It is shown that the forward dynamic transformation into ferrite and pearlite is several orders of magnitude faster than the reverse static transformation back into austenite. The retarding effect of alloying additions such as niobium is also outlined. The results are interpreted in terms of the effect of deformation on the modified phase diagrams pertaining to the transformation of deformed austenite.

Abstract: IMI834 (Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si) is a high-tech near-α titanium alloy with improved creep resistance and mechanical property retention at temperatures up to 600°C [1]. It is used in the aerospace industry for compressor disks and blades due to its excellent balance between creep resistance and fatigue strength [2]. The linear friction welding (LFW) behaviour of IMI834 displaying an initial bimodal α+β microstructure was investigated using varying axial pressures during welding. Electron backscatter diffraction (EBSD) was used to characterize the texture and phase fraction of the welded IMI834 samples in the weld zone (WZ) and thermomechanically affected zones (TMAZ) in relation to the base material. Based on microhardness evaluation of the weldments, the WZ was determined to be slightly harder than the base material.

Abstract: Compression tests were employed to characterize the DSA behaviour of Mg-Ce alloys. Samples were taken from cast billets and extruded bars of Mg-0.5 wt.% Ce. The DSA behavior was examined at temperatures from 150°C to 400°C at strain rates of 0.001/s to 1.5/s. A rate equation was fitted to the experimental results, which is employed to predict whether or not DSA will occur at the strain rates and temperatures involved in the formation of the RE texture component during extrusion.

Abstract: A 0.21% C plain carbon steel was deformed in torsion to strains of ε = 0.15-3.0 at a strain rate of ε ̇= 4.5 s^-1 over the temperature range 722-822°C in a 5%H₂-Ar gas atmosphere. The experimental parameters were varied in order to study the formation of ferrite and pearlite by dynamic transformation (DT) in the intercritical region. This transformation was observed right up to the highest experimental temperature (822°C). The pearlite formed by DT contained cementite spheroids whose size distribution evolved during isothermal holding after deformation. In the first stage, corresponding to the first 800 s of holding, spheroid coarsening took place. When the holding time exceeded 800 s, the spheroids dissolved and the pearlite reverted into the original parent austenite. The results indicate that pearlite can form by DT at temperatures well above the Ae₁ and that the reverse static transformation is much slower than the forward dynamic transformation.

Abstract: A 0.45% C steel was deformed in torsion over the temperature range 762-872°C in a 5%H₂-Ar gas atmosphere. Strains of 0.25-3.0 were applied at a strain rate of ε^. = 4 s^-1. The experimental parameters were varied in order to study the effects of strain and temperature on the formation of ferrite by dynamic transformation (DT) at temperatures above the Ae₃. The critical strain for ferrite formation by DT was about 0.2 and its volume fraction increased with strain. The average ferrite grain sizes were about 1 to 2 µm and were fairly independent of temperature. It was observed that the deformation-induced ferrite remained fairly stable during 800 s of isothermal holding. In general, the experiments showed that DT takes place at temperatures above the Ae₃ and that the reverse static transformation is much slower than the forward dynamic transformation.

Abstract: The linear friction welding (LFW) behavior of Ti-6Al-4V, a commercial α + β titanium alloy, was investigated using oscillation frequencies ranging from 30-70 Hz and axial pressures from 50-110 MPa. LFW samples were examined using electron backscattered diffraction (EBSD) to relate the texture to the welding parameters and to the estimated strain and strain rate. Characterization of the welds included analysis of the microstructure of the weld and of the thermomechanically affected zones (TMAZ) in relation to the parent material.