M.C. Somani

Testing and evaluation of material data for analysis of forming and hardening of boron steel components

Finite element modelling and simulation is becoming an increasingly important tool in the development process for structural automotive components, manufactured using thermo-mechanical forming techniques. Accurate and reliable analysis of coupled thermo-mechanical processes requires efficient simulation tools as well as good quality and relevant material data, usually obtained by experimental testing of the mechanical and thermal properties. The work present in this paper concerns methods for obtaining and evaluating the mechanical properties, required for modelling the high-temperature forming of a high-strength boron-alloyed steel. The material data was obtained from high temperature compression tests and dilatometric measurements made using a Gleeble 1500 thermo-mechanical simulator. Two examples of finite element simulations using the data obtained are also presented. The first example is an isothermal finite element simulation of a thin-walled tubular beam subjected to high-temperature bending. The predicted press force showed acceptable agreement with experimental results in the initial part of the process. In the second example, a cylindrical specimen compressed during continuous cooling was simulated, and the press force and radial displacement were compared with experimental results. Again the simulations showed acceptable agreement with experimental results but indicated the need for further improvements in the simulation technology and methods used for material parameter evaluation.

Cellular response of preosteoblasts to nanograined/ultrafine-grained structures

Metallic materials with submicron- to nanometer-sized grains provide surfaces that are different from conventional polycrystalline materials because of the large proportion of grain boundaries with high free energy. In the study described here, the combination of cellular and molecular biology, materials science and engineering advances our understanding of cell-substrate interactions, especially the cellular activity between preosteoblasts and nanostructured metallic surfaces. Experiments on the effect of nano-/ultrafine grains have shown that cell attachment, proliferation, viability, morphology and spread are favorably modulated and significantly different from conventional coarse-grained structures. Additionally, immunofluorescence studies demonstrated stronger vinculin signals associated with actin stress fibers in the outer regions of the cells and cellular extensions on nanograined/ultrafine-grained substrate. These observations suggest enhanced cell-substrate interaction and activity. The differences in the cellular response on nanograined/ultrafine-grained and coarse-grained substrates are attributed to grain size and degree of hydrophilicity. The outcomes of the study are expected to reduce challenges to engineer bulk nanostructured materials with specific physical and surface properties for medical devices with improved cellular attachment and response. The data lay the foundation for a new branch of nanostructured materials for biomedical applications.

Deformation Mechanisms in High-Al Bearing High-Mn TWIP Steels in Hot Compression and in Tension at Low Temperatures

The hot deformation behaviour of two high-Mn (23-24 wt-%) TWIP steels containing 6 and 8 wt-% Al with the fully austenitic and duplex microstructures, respectively, has been investigated at temperatures of 900-1100°C. In addition, tensile properties were determined over the temperature range from -80 to 100°C. It was observed that in spite of the lower Al content, the austenitic steel possessed the hot deformation resistance about twice as high as that of the duplex steel. Whereas the flow stress curves of the austenitic steel exhibited work hardening followed by slight softening due to dynamic recrystallisation, the duplex steel showed the absence of work hardening and discontinuous yielding under similar conditions. Tensile tests at low temperatures revealed that the austenitic grade had a lower yield strength than that of the duplex grade, but much better ductility, the elongation increasing with decreasing temperature, contrary to that for the duplex steel. This can be attributed to the intense mechanical twinning in the austenitic steel, while in the duplex steel, twinning occurred in the ferrite only and the austenite showed dislocation glide.

Nanoscale Deformation Behavior of Phase-Reversion Induced Austenitic Stainless Steels: The Interplay Between Grain Size from Nano-Grain Regime to Coarse-Grain Regime

We have used the recently adopted concept of phase reversion to obtain grain size from the nanograined/ultrafine-grained (NG/UFG) to fine grain (FG) regime by varying temperature–time annealing sequence of cold deformed metastable austenite. The phase-reversion induced NG/UFG structure was characterized by high strength-high ductility combination. The concept of phase reversion involves severe cold deformation of metastable austenite to generate strain-induced martensite. Upon annealing, martensite transforms back to austenite through a diffusional reversion mechanism with NG/UFG, sub-micron grains (SMG) or FG structure, depending on the annealing condition. Depth-sensing nanoindentation experiments were combined with electron microscopy to elucidate the dependence of grain size from nanograin/ultrafine-grain (NG/UFG) to coarse grain (CG) regime on the deformation mechanisms. There was distinct transition in the deformation mechanism from intense mechanical twinning and stacking faults in NG/UFG structure to strain-induced martensite formation at the intersection of shear bands in the CG structure. The transition in the deformation mechanism is discussed in terms of increase in austenite stability with decrease in grain size.

Biological significance of nanograined/ultrafine-grained structures: Interaction with fibroblasts.

Given the need to develop high strength/weight ratio bioimplants with enhanced cellular response, we describe here a study focused on the processing-structure-functional property relationship in austenitic stainless steel that was processed using an ingenious phase reversion approach to obtain an nanograined/ultrafine-grained (NG/UFG) structure. The cellular activity between fibroblast and NG/UFG substrate is compared with the coarse-grained (CG) substrate. A comparative investigation of NG/UFG and CG structures illustrated that cell attachment, proliferation, viability, morphology and spread are favorably modulated and significantly different from the conventional CG structure. These observations were further confirmed by expression levels of vinculin and associated actin cytoskeleton. Immunofluorescence studies demonstrated increased vinculin concentrations associated with actin stress fibers in the outer regions of the cells and cellular extensions on NG/UFG substrate. These observations suggest enhanced cell-substrate interaction and activity. The cellular attachment response on NG/UFG substrate is attributed to grain size and hydrophilicity and is related to more open lattice in the positions of high-angle grain boundaries.

Interplay between grain structure and protein adsorption on functional response of osteoblasts: Ultrafine‐grained versus coarse‐grained substrates

The rapid adsorption of proteins is the starting and primary biological response that occurs when a biomedical device is implanted in the physiological system. The biological response, however, depends on the surface characteristics of the device. Considering the significant interest in nano-/ultrafine surfaces and nanostructured coatings, we describe here, the interplay between grain structure and protein adsorption (bovine serum albumin: BSA) on osteoblasts functions by comparing nanograined/ultrafine-grained (NG/UFG) and coarse-grained (CG: grain size in the micrometer range) substrates by investigating cell-substrate interactions. The protein adsorption on NG/UFG surface was beneficial in favorably modulating biological functions including cell attachment, proliferation, and viability, whereas the effect was less pronounced on protein adsorbed CG surface. Additionally, immunofluorescence studies demonstrated stronger vinculin signals associated with actin stress fibers in the outer regions of the cells and cellular extensions on protein adsorbed NG/UFG surface. The functional response followed the sequence: NG/UFG(BSA) > NG/UFG > CG(BSA) > CG. The differences in the cellular response on bare and protein adsorbed NG/UFG and CG surfaces are attributed to cumulative contribution of grain structure and degree of hydrophilicity. The study underscores the potential advantages of protein adsorption on artificial biomedical devices to enhance the bioactivity and regulate biological functions.

Processing of Submicron Grained Microstructures and Enhanced Mechanical Properties by Cold-Rolling and Reversion Annealing of Metastable Austenitic Stainless Steels

The effects of chemical composition, cold rolling and subsequent annealing parameters on the reversion of strain-induced martensite to austenite were investigated in three experimental Mn and Si-free Cr-Ni austenitic stainless steels and two commercial Type 301 and Type 301LN grades by optical and electron microscopy, X-ray diffraction and magnetic measurements. Hardness and tensile tests were performed to determine the mechanical properties achieved. In cold rolling, completely martensitic structure could be obtained in the experimental heats, but only partially in 301 and 301LN grades at reasonable reductions. Upon annealing, in 301LN the reversion took place by the nucleation and growth mechanism, and submicron austenite grains were formed within a few seconds at temperatures above 700°C. In the other steels, reversion took place by the shear mechanism, and ultra-fine grains were formed by the recrystallization of austenite at temperatures of 900°C or above. Partial reversion resulted in an excellent combination of yield strength and elongation in 301LN, and also in 301 such ones were attained in the reverted structure even before any profound formation of submicron grains.

Phase reversion induced nanograined austenitic stainless steels: microstructure, reversion and deformation mechanisms

Abstract We describe here the attributes of a promising ‘phase reversion’ approach that results in nanograined/ultrafine grained (NG/UFG) structure in austenitic stainless steels with high strength–high ductility combination. The approach involves severe cold deformation (about 45–75%) of metastable austenite to martensite, which, on annealing for short durations, reverts to austenite via diffusional or shear mechanism, depending on the chemical composition of steel. There was, however, a need to optimise the severity of cold deformation and temperature–time annealing sequence to obtain NG/UFG structure. The fundamental criteria to obtain NG/UFG structure via ‘phase reversion’ approach were to obtain dislocation cell type martensite. In high strength NG/UFG steel, mechanical twinning contributed to the excellent ductility, while in low strength coarse grained (CG) steel, ductility was also good, but due to nucleation of strain induced martensite at shear bands. The difference in deformation mechanism between NG/UFG and CG steels was attributed to increase in the stability of austenite with decrease in austenite grain size.

Cellular activity of bioactive nanograined/ultrafine-grained materials

Our recent electron microscopy study on biomimetic nanostructured coatings on nanograined/ultrafine-grained (NG/UFG) substrates [Mater Sci Eng C 2009;29:2417-27] indicated that electrocrystallized nanohydroxyapatite (nHA) on phase-reversion-induced NG/UFG substrates exhibited a vein-type interconnected and fibrillar structure that closely mimicked the hierarchical structure of bone. The fibrillar structure on NG/UFG substrate is expected to be more favorable for cellular response than a planar surface. In contrast, hydroxyapatite (HA) coating on coarse-grained (CG) substrate more closely resembled a film rather than a fibrillar structure. Inspired by the differences in the structure of HA coating, we describe here the cell-substrate interactions of pre-osteoblasts (MC 3T3-E1) on bioactive NG/UFG and CG austenitic stainless steel substrates. NG/UFG austenitic stainless steel was obtained by a novel controlled phase-reversion annealing of cold-deformed austenite. This example provides an illustration of how a combination of cellular and molecular biology, materials science and engineering can advance our understanding of cell-substrate interactions. Interestingly, the cellular response of nanohydroxyapatite (nHA)-coated NG/UFG substrate demonstrated superior cytocompatibility, improved initial cell attachment, higher viability and proliferation, and well-spread morphology in relation to HA-coated CG substrate and their respective uncoated (bare) counterparts as implied by fluorescence and electron microscopy and MTT assay. Similar conclusions were derived from an immunofluorescence study that involved examination of the expression levels of vinculin focal adhesion contacts associated with dense actin stress fibers and fibronectin, protein analysis through protein bands in SDS-PAGE, and quantitative total protein assay. The enhancement of cellular response followed the sequence: nHA-coated NG/UFG>nHA-coated CG>NG/UFG>CG substrates. The outcomes of the study are expected to counter the challenges associated with the engineering of nanostructured surfaces with specific physical and surface properties for medical devices with significantly improved cellular response.

Validation of the New Regression Model for the Static Recrystallisation of Hot-Deformed Austenite in Special Steels

A linear regression model consisting of the weighted sums of certain alloying elements has recently been developed to predict the activation energy (Qrex) and kinetics of static recrystallisation (SRX) for hot-deformed austenite based on stress relaxation test results for over 40 different carbon steels. The validity of the model has been further assessed here by determining the Qrex and the kinetics of SRX of certain high-Nb bearing steels, extra-low and low carbon Nb-Mo bainitic and high-Si dual phase and TRIP steels, and Nb-Ti grades with the varying N content. The validity of the model is shown to be fairly good for the Nb-Ti, Nb-Mo and Cr-Mo grades. The approach of maximum effective concentration of Nb and Si and the weight factor for Cr enable reasonable fit for DP, TRIP and Nb-Cr steels, as well. Possible influences of C and N on Qrex and the kinetics of SRX were checked, but none was observed in microalloyed steels.