Numerical investigation of influence of the Martensite Volume Fraction on dp steels fracture behavior on the basis of digital material representation model

Development of the methodology for creating reliable digital material representation (DMR) models of dual-phase steels and investigation of influence of the martensite volume fraction on fracture behavior under tensile load are the main goals of the paper. First, an approach based on image processing algorithms for creating a DMR is described. Then, obtained digital microstructures are used as input for the numerical model of deformation, which takes into account mechanisms of ductile fracture. Ferrite and martensite material model parameters are evaluated on the basis of micropillar compression tests. Finally, the model is used to investigate the impact of the martensite volume fraction on the DP steel behavior under plastic deformation. Results of calculations are presented and discussed in the paper.

Effect of martensite volume fraction on low cycle fatigue behaviour of dual phase steels: experimental and microstructural investigation

The low cycle fatigue (LCF) behaviour of a dual phase (DP) steel with different martensite volume fractions has been investigated, with particular focus on fatigue life, cyclic hardening/softening behaviour and microstructural evolution. DP steels with martensite volume fractions between 13% and 88% were produced and their monotonic and cyclic deformation behaviours evaluated. The LCF life has been examined in depth and compared with published literature. It has been concluded that, once normalised for plastic strain amplitude, the fatigue life was found to be significantly reduced by an increase in the martensite volume fraction. All alloys were observed to show some initial cyclic hardening followed by cyclic softening. Clear sub-cell formation occurred in ferrite grains irrespective of the martensite volume fraction, and it is suggested that this cell formation and martensite softening are responsible for the cyclic softening behaviour.

Size distribution and volume fraction of T1 phase precipitates from TEM images: Direct measurements and related correction

Transmission Electron Microscopy (TEM) can be used to measure the size distribution and volume fraction of fine scale precipitates in metallic systems. However, such measurements suffer from a number of artefacts that need to be accounted for, related to the finite thickness of the TEM foil and to the projected observation in two dimensions of the microstructure. We present a correction procedure to describe the 3D distribution of disc-like particles and apply this method to the plate-like T1 precipitates in an Al-Li-Cu alloy in two ageing conditions showing different particle morphologies. The precipitates were imaged in a High-Angular Annular Dark Field Microscope (HAADF-STEM). The corrected size distribution is further used to determine the precipitate volume fraction. Atom probe tomography (APT) is finally utilised as an alternative way to measure the precipitate volume fraction and test the validity of the electron microscopy results.

Method for determining reaction rate of mild steel containers during melting of magnesium-aluminium alloys and effect of aluminium content on directionally solidified microstructures

A magnesium alloy of eutectic composition (33 wt-'%Al) was directionally solidified in mild steel tubes at two growth rates, 32 and 580 mum s(-1,) in a temperature gradient between 10 and 20 K mm(-1). After directional solidification, the composition of each specimen varied dramatically, from 32'%Al in the region that had remained solid to 18%Al (32 mum s(-1) specimen) and 13%Al (580 mum s(-1) specimen) at the plane that had been quenched from the eutectic temperature. As the aluminium content decreased, the microstructure contained an increasing volume fraction of primary magnesium dendrites and the eutectic morphology gradually changed from lamellar to partially divorced. The reduction in aluminium content was caused by the growth of an Al-Fe phase ahead of the Mg-Al growth front. Most of the growth of the Al-Fe phase occurred during the remelting period before directional solidification. The thickness of the Al-Fe phase increased with increased temperature and time of contact with the molten Mg-Al alloy. (C) 2003 Maney Publishing.

Effect of deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel

Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure.

Microstructural evolution of ultrafine grained structure in plain carbon steels through single pass rolling

In the present study, the effect of nominal equivalent strain (between 0 and 1.2), deformation temperature (790– 750°C) and carbon content (0.06 – 0.35%C) was investigated on ferrite grain refinement through dynamic strain induced transformation (DSIT) in plain carbon steels in single pass rolling. The microstructural evolution of the transformation of austenite to ferrite has been evaluated through the thickness of the strip. The results showed a number of important microstructural features as a function of strain, which could be classified into three regions; no DSIT region, DSIT region, and ultrafine ferrite (UFF) grain region. Hence, two critical strains; dynamic strain induced transformation (εC, DSIT) and ultrafine ferrite formation (εC, UFF) were determined. These strains were increased significantly with an increase in carbon content. The critical strain for UFF formation reduced with decrease in deformation temperature. The UFF microstructure consisted of ultrafine, equiaxed ferrite grains (<2 μm) with very fine cementite particles. In the centre of the rolled strip, there was a conventional ferrite– pearlite microstructure, although ferrite grain refinement and the volume fraction of ferrite increased with increase in the nominal equivalent strain.

Formation of ultrafine grained structure in plain carbon steels through thermomechanical processing

In the present study, wedge-shaped samples were used to determine the effect of nominal equivalent strain (between 0 and 1.2) and carbon content (0.06--0.35%C) on ferrite grain refinement through dynamic strain-induced transformation (DSIT) in plain carbon steels using single-pass rolling. The microstructural evolution of the transformation of austenite to ferrite has been evaluated through the thickness of the strip. The results showed a number of important microstructural features as a function of strain which could be classified into three regions; no DSIT region, DSIT region and the ultrafine ferrite (UFF) grain region. Also, the extent of these regions was strongly influenced by the carbon content. The UFF microstructure consisted of ultrafine, equiaxed ferrite grains (<2 μ$m) with very fine cementite particles. In the centre of the rolled strip, there was a conventional ferrite-pearlite microstructure, although ferrite grain refinement and the volume fraction of ferrite increased with an increase in the nominal equivalent strain.

The formation of ultrafine grained steel microstructures through thermomechanical processing.

The formation of ultrafine grained steels is an area of intense research around the World. There are a number of methods to produce grain sizes of approximately 1 µm, ranging from extreme thermal and deformation cycles to more typical thermomechanical processes. This paper reviews the status of the production of ultrafine grained steels through relatively simple thermomechanical processing. It is shown that this requires deformation within the Ae3 to Ar3 temperature range for a given alloy. The formation of ultrafine ferrite involves a dynamic transformation of a significant volume fraction of the austenite to ferrite. This dynamic strain induced transformation arises from the introduction of additional intragranular nucleation sites. It is possible that the deformation also hinders the growth or coarsening of the ferrite and may also lead to dynamic recrystallization of the ferrite. The most likely commercial exploitation of ultrafine ferrite would appear to rely on the formation of a critical volume fraction of dynamic strain induced ferrite followed by controlled cooling to ensure this is maintained to room temperature and to also form other secondary phases, such as martensite, bainite and/or retained austenite to improve the formability.

A semianalytical Sachs model for the flow stress of a magnesium alloy

A semianalytical Sachs-type equation for the flow stress of magnesium-base alloys is developed using the Schmid law, power law hardening, and a sigmoidal increase in the twinning volume fraction with strain. Average Schmid factors were estimated from electron backscattered diffraction (EBSD) data. With these, the equation provides a reasonable description of the flow curves obtained in compression and tension for samples of Mg-3Al-1Zn cut in different orientations from rolled plate. The model illustrates the general importance of basal slip and twinning in magnesium alloys. The significance of prismatic slip in room temperature tension testing is also highlighted. This is supported with EBSD slip line trace analysis and rationalized in terms of a possible sensitivity of the critical resolved shear stress for prismatic (cross) slip to the stress on the basal plane.

The effect of Nb, Mo, and A1 additions on the transformation behaviour and mechanical properties of thermomechanically processed C-Si-Mn trip steel

The effect of additions of Nb, A1 and Mo to Fe-C-Mn-Si TRIP steels on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. Laboratory simulations of continuous cooling during TMP were performed using a quench deformation dilatometer, while laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. From this a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in these types of TRIP steels has been developed. All samples were characterised using optical microscopy and XRD. The relationships between the morphology of bainitic structure, volume fraction, stability of RA and mechanical properties were investigated.

Ultrafine grained structure formation in steels using dynamic strain induced transformation processing

The refinement of ferrite grain size is the most generally accepted approach to simultaneously improve the strength and toughness in steels. Historically, the level of ferrite refinement is limited to 5-10 μm using conventional industrial approaches. Nowadays, though, several thermomechanical processes have been developed to produce ferrite grain sizes of 1-3 μm or less, ranging from extreme thermal and deformation cycles to more typical thermomechanical processes. The present paper reviews the status of the production of ultrafine grained steels through relatively simple thermomechanical processing. This requires deformation within the Ae3 to Ar3 temperature range for a given alloy. Here, the formation of ultrafine ferrite (UFF) involves the dynamic transformation of a significant volume fraction of the austenite to ferrite. This dynamic strain induced transformation (DSIT) arises from the introduction of extensive intragranular nucleation sites that are not present in conventional controlled rolling. The DSIT route has the potential to be adjusted to suit current industrial infrastructure. However, there are a number of significant issues that have been raised, both as gaps in our understanding and as obstacles to industrial implementation. One of the critical issues is that it appears that very large strains are required. Combined with this concern is the issue of whether a combination of dynamic and static transformation can be used to achieve an adequate level of refinement. Another issue that has also become apparent is that grain sizes of 1 μm can lead to low levels of ductility and hence many workers are attempting to obtain 2-3 μm grains, or to introduce a second phase to provide the required ductility. There are also a number of areas of disagreement between authors including the role of dynamic recrystallisation of ferrite in the production of UFF by DSIT, the reasons for the low coarsening rate of UFF grains, the role of microalloying elements and the effects of austenite grain size and strain rate. The present review discusses these areas of controversy and highlights cases where experimental results do not agree.

Precipitate characterisation of an advanced high-strength low-alloy (HSLA) steel using atom probe tomography

Increased fuel economy, combined with the need for the improved safety has generated the development of new hot-rolled high-strength low-alloy (HSLA) and multiphase steels such as dual-phase or transformation-induced plasticity steels with improved ductility without sacrificing strength and crash resistance. However, the modern multiphase steels with good strength-ductility balance showed deteriorated stretch-flangeability due to the stress concentration region between the soft ferrite and hard martensite phases [1]. Ferritic, hot-rolled steels can provide good local elongation and, in turn, good stretch-flangeability [2]. However, conventional HSLA ferritic steels only have a tensile strength of not, vert, similar600 MPa, while steels for the automotive industry are now required to have a high tensile strength of not, vert, similar780 MPa, with excellent elongation and stretch-flangeability [1]. This level of strength and stretch-flangeability can only be achieved by precipitation hardening of the ferrite matrix with very fine precipitates and by ferrite grain refinement. It has been suggested that Mo [3] and Ti [4] should be added to form carbides and decrease the coiling temperature to 650 °C since only a low precipitation temperature can provide the precipitation refinement [4]. These particles appeared to be (Ti, Mo)C, with a cubic lattice and a parameter of 0.433 nm, and they were aligned in rows [4]. It was reported [4] that the formation of these very fine carbides led to an increase in strength of not, vert, similar300 MPa. However, the detailed analysis of these particles has not been performed to date due to their nanoscale size. The aim of this work was to carry out a detailed investigation using atom probe tomography (APT) of precipitates formed in hot-rolled low-carbon steel containing additions Ti and Mo.

The investigated low-carbon steel, containing Fe–0.1C–1.24Mn–0.03Si–0.11Cr–0.11Mo–0.09Ti–0.091Al at.%, was produced by hot rolling. The processing route has been described in detail elsewhere [5] European Patent Application, 1616970 A1, 18.01.2006.[5]. The microstructure was characterised by transmission electron microscopy (TEM) on a Philips CM 20, operated at 200 kV using thin foil and carbon replica techniques. Qualitative energy dispersive X-ray spectroscopy (EDXS) was used to analyse the chemical composition of particles. The atomic level of particle characterisation was performed at the University of Sydney using a local electrode atom probe [6]. APT was carried out using a pulse repetition rate of 200 kHz and a 20% pulse fraction on the sample with temperature of 80 K. The extent of solute-enriched regions (radius of gyration) and the local solute concentrations in these regions were estimated using the maximum separation envelope method with a grid spacing of 0.1 nm [7]. A maximum separation distance between the atoms of interest of dmax = 1 nm was used.

The microstructure of the steel consisted of two types of fine ferrite grains: (i) small recrystallised grains with an average grain size of 1.4 ± 0.2 μm; and (ii) grains with a high dislocation density (5.8 ± 1.4 × 1014 m−2) and an average grain size of 1.9 ± 0.1 μm in thickness and 2.7 ± 0.1 μm in length (Fig. 1a). Some grains with high dislocation density displayed an elongated shape with Widmanstätten side plates and also the formation of cells and subgrains (Fig. 1a). The volume fraction of recrystallised grains was 34 ± 8%.

Cement composites reinforced with surface modified coir fibers

An experimental investigation of coir mesh reinforced mortar (CMRM) is conducted using nonwoven coir mesh matting. The main parameters in this study are the fiber volume fraction (number of mesh layers) and fiber surface treatment with a wetting agent. The composites are subjected to the four-point bending test. The short-term mechanical properties of CMRM are discussed. Scanning electron micrograph analysis is used to observe the fiber—matrix interfacial characteristics. The results indicate that the addition of coir mesh to mortar significantly improves the composite post-cracking flexural stress, toughness, ductility, and toughness index, compared to plain mortar materials. The Albatex © FFC wetting agent (2-ethylhexanol) can effectively improve water absorption of coir fiber and enhance the fiber—matrix bonding strength. These coir mesh reinforced composites may be useful in civil engineering applications.

Effect of initial processing on the microstructure-property relationships in bake-hardened C-Mn-Si TRIP steels

The effect of pre-straining (PS) and bake-hardening (BH) on the microstructure and mechanical properties has been studied in C-Mn-Si TRansformation Induced Plasticity (TRIP) steels after: (i) thermomechanically processing (TMP) and (ii) intercritical annealing. The steels were characterised before and after PS/BH by transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile tests. The main microstructural differences were the higher volume fraction of bainite and more stable retained austenite in the TMP steel. This led to a difference in the strain-hardening behavior before and after BH treatment. The higher dislocation density in ferrite and formation of microbands in the TMP steel after PS and the formation of Fe3C carbides between the bainitic ferrite laths during BH for both steels also affected the strain-hardening behavior. However, both steels after PS/BH treatment demonstrated an increase in the yield and tensile strength.