Behrang Poorganji

Crystallographic Analysis of Proeutectoid Ferrite/Austenite Interface and Interphase Precipitation of Vanadium Carbide in Medium-Carbon Steel

To clarify the mechanism of interphase precipitation of vanadium carbide (VC) in a medium-carbon steel, orientation relationships (ORs) and plane orientations of ferrite/austenite interfaces were investigated. It was found that a large part of grain boundary ferrite holds near-K-S OR with at least one side of austenite adjacent to grain boundary regardless of V addition. By the V addition, a fraction of grain boundary ferrite holding near the K-S OR with both sides of austenite is decreased remarkably. Furthermore, only non-K-S ferrite/austenite interfaces migrate dominantly in the V-added alloy in contrast to the V-free alloy. Ferrite/austenite interface orientations are not fixed crystallographically but are randomly distributed in terms of ferrite and austenite orientations. Those results do not agree with the ledge mechanism originally proposed by Honeycombe. Thus, it is proposed that the ledge mechanism is extended to the non-K-S interface, which partially consists of coherent and less-mobile interfaces.

Formation of Ultrafine Grained Ferrite by Warm Deformation of Tempered Lath Martensite in Low Alloy Steels

Microstructure change during warm deformation of tempered lath martensite in Fe-2mass%Mn-C alloys with different carbon contents in the range between 0.1 and 0.8mass%C was investigated. Specimens of the alloys after being quenched and tempered at 923K for 0.3ks were compressed by 50% with a strain rate varying from 10-3 to 10-4s-1 at 923K. EBSD analysis of the deformed microstructures has revealed that fine equiaxed ferrite (α) grains surrounded by high-angle boundaries are formed by dynamic recrystallization (DRX). As carbon content increases, the DRX α grain size decreases. This could be attributed to the change in volume fraction of the cementite (θ) phase as boundary dragging particles. The sub-micron θ particles can suppress the coarsening of the DRX α grains by exerting a pinning effect on grain boundary migration. Furthermore, the fraction of recrystallized region increases by increasing carbon content, presumably due to a decrease in the martensite block width as an initial α grain size and a larger volume fraction of hard second phase (θ) particles. Both of these should increase inhomogeneous plastic deformation which promotes the recrystallization. It seems that continuous DRX is responsible for the formation of ultrafine α grains in the tempered lath martensite.

Formation of Ultrafine Grained Ferrite + Cementite Duplex Structure by Warm Deformation

The microstructure change by warm deformation in low alloy steels with different initial ferrite (α) + cementite (θ) duplex structures is discussed in the present paper. In high carbon steels, heterogeneous deformation introduced in pearlite containing lamellar θ promotes dynamic recrystallization (DRX) of α for mild deformation of less than 1.2 in true strain. On the other hand, the original α grains become elongated and only subgrains are formed by dynamic recovery in the case of (α + θ) duplex structure containing equiaxed spheroidized θ. Equiaxed fine α grains, approximately 2 μm in diameter and mostly bounded by high-angle boundaries, are formed with spheroidized θ by DRX during compression of the pearlite by 75%. When the (α + θ) duplex structure containing spheroidized θ was deformed, the original α grains become elongated and only subgrains are formed by dynamic recovery. For the tempered martensite, equiaxed α grains similar to those in the deformed pearlite were obtained after 50% compression. This indicates that the critical strain needed for the completion of DRX is smaller for the tempered martensite than for the other structures. Lath martensite in a higher carbon alloy is more suitable for DRX because of its finer initial grain size. DRX α grain size is finer in a higher carbon alloy because of stronger pinning effect by θ particle.

The Effect of Small Amounts of Yttrium Addition on Static and under Superplastic Deformation Grain Growth in Newly Developed α+β Type, Ti-4.5Al-6Nb-2Mo-2Fe Alloy

New α+β type titanium alloy with Ti-4.5Al-6Nb-2Mo-2Fe was developed on the basis of using biocompatible elements and eliminating the cytotoxic ones such as Vanadium, while achieving the desirable mechanical properties such as appropriate strength, cold workability and low superplastic forming (SPF) temperature. The present study was conducted to investigate the effect of yttrium addition of less than 0.05% into this alloy on static and under superplastic deformation grain growth behavior. The new alloy bar manufactured by α+β processing and annealed at 1073K yielded extremely fine two-phase microstructure with α grain size around 2μm. Specimens were heated at temperatures of 1048, 1073 and 1098K and kept for times between 3.6 to 172.8KS. Yttrium forms in-situ Y2O3 particles, and the presence of these particles yield finer two phase microstructure due to their retardation effect on β phase grain growth. Grain growth behavior during hot deformation was investigated by hot compression test in use of a hot working simulator of THERMEC-Master Z. Strain rate was varied from 2×10-2 to 2×10-4S-1 and strain was 0.69. Grain size of both α and β phases increased with a reduction of strain rate, and Y2O3 particle was also effective to retard grain growth under hot deformation. It was confirmed from comparison of grain growth during isothermal heating with and without hot deformation that grain growth was much accelerated by deformation. All of these results were discussed based on grain growth mechanism or model for two-phase microstructures as well as superplastic deformation mechanism.

Hot Deformation Behavior of Near-α Ti-Fe Alloy in (α+β) Two-Phase Region with Different Fe Content

In the present study, microstructure evolution of Ti-Fe alloys with different Fe content between 0.2-1.5mass% during hot deformation in (α+β) two-phase region is studied with focusing on effect of phase volume fraction at different deformation temperatures and strain rates. Hot deformation was conducted on the specimens quenched after β solutionizing at 1173K for 1.2ks at 1108, 1073 and 948K, by uniaxial compression by 50% at various strain rates ranged from 1 to 10-4 s-1. Initial structures are (α+β) lamellar structures of fine interlamellar spacing and colony sizes. Increase in Fe content results in increasing the fraction of the β phase at the given deformation temperature. Either colony size or interlamellar spacing is coarser at higher temperatures. At the higher deformation temperature where β phase fraction is larger, dynamic recovery of β phase is a major deformation mechanism while at a lower temperature, i.e., a higher α fraction, dynamic recrystallization of α phase occurs predominantly. It is concluded that critical strain needed for occurrence of dynamic recrystallization is decreased by increasing fraction of the α phase at the same deformation temperature, i.e., by decreasing Fe content. Furthermore, by increasing strain rate grain size of the recrystallized α is decreased.

ECAP Processing of High Si Bainitic Steel, Microstructure and Mechanical Properties

Microstructure and mechanical properties of high Si bainitic steel, before and after two passes of equal channel angular pressing (ECAP) at room temperature were investigated. SEM and TEM microscopy were used for microstructural study. Shear punch test and Vickers hardness test of as received and ECAPed samples were carried out to measure the influence of ECAP process on the mechanical properties of the samples. The results showed that tensile strength and shear strain were increased as a consequence of ECAP processing.