Effect of grain size on deformation twinning behaviour of high MN steel

The effect of grain size on the mechanical properties and deformation twinning behaviour in high manganese steel was investigated. In order to generate different grain sizes, the samples were subjected to hot rolling, cold rolling and annealing. Room temperature tensile testing of the steel with different grain sizes (5-50 µm) indicated the occurrence of twinning induced plasticity (TWIP) in all the samples. Also, changes in work-hardening behaviour accompanied changes in the grain size. The results are discussed in terms of the enhanced sensitivity of twinning to the grain size.

Microstructural analysis of pre-strained TRIP steel after fatigue testing

The widespread introduction of multiphase sheet steels in the automotive industry has led to considerable interest in the fatigue properties of these materials. The different microstructural phases within matelials such as TRIP steels can influence the fatigue behaviour due to the manner in which the cyclic strain is accommodated within these phases. In this study fully reversed straincontrolled fatigue tests were perfonnrmed on a commercially-produced uncoated TRIP 780 steel both in the as-received and 20 % prestrained condition. The pre-strained TRIP steel showed significant cyclic softening at higher strain amplitudes, whereas some initial work hardening was observed at lower strain amplitudes before cyclic softening. The cyclic stabilised strength of the pre-strained TRIP steel was independent of strain amplitude, while the cyclic stabilised strength of the as-received TRIP steel increased with strain amplitude. Transmission Electron Microscopy TEM was used to examine the effect of the cyclic deformation on the microstructure of the different conditions, with the differences in fatigue behaviour explained based on the differences in the deformation structure formed within the steel (i.e. dislocation density and sub-structure and microband formation).

Constitutive modeling of TWIP steel in uni-axial tension

High Mn steels demonstrate an exceptional combination of high strength and ductility due to their high work hardening rate during deformation. The microstructure evolution and work hardening behavior of Fe18Mn0.6C1.5Al TWIP steel in uni-axial tension were examined. The purpose of this study was to determine the contribution of all the relevant deformation mechanism : slip, twinning and dynamic strain aging. Constitutive modeling was carried out based on the Kubin-Estrin model, in which the densities of mobile and forest dislocations are coupled in order to account for the continuous immobilization of mobile dislocations during straining. These coupled dislocation densities were also used for simulating the contribution of dynamic strain aging on the flow stress. The model was modified to include the effect of twinning.

A comparative assessment of austempered ductile iron as a substitute in weight reduction applications

Manufacturing engineering has had to undergo drastic changes in the approach to material selection in order to meet new design challenges. In the automotive industry, researchers in their effort to reduce emissions and satisfy environmental regulations, have shifted their focus to new emerging materials such as high-strength aluminium alloys, metal matrix composites, plastics, polymers and of late, Austempered Ductile Iron (ADI). ADI is a good choice for design where the criterion is high performance at reduced weight and cost. The unique, ausferrite microstructure gives the material desirable material properties and an edge over other materials. A comparative study of ADI in terms of materials properties and machining characteristics with other materials is desirable to highlight the potential of the material. This paper focuses on a comparative assessment of material and machining characteristics of ADI for different applications. The properties under consideration are machinability, weight and cost savings and versatility. ADI has a higher strength-to-weight ratio than aluminium making it a ready alternative for material selection. In terms of machinability, there are some problems associated with machining of ADI due to its work hardening nature. This paper attempts to identify the possible potential applications of ADI, by critically reviewing specific applications such as machinability, overall economics and service.

Tribological behaviour of pure Ti with a nanocrystalline surface layer under different loads

Surface mechanical attrition treatment (SMAT), a novel surface severe plastic deformation method, was carried out for titanium (Ti) to create a gradient-structured Ti (SMAT Ti). The tribological behaviour was studied under different loads and dry sliding conditions. The results showed that the deformation layer of SMAT Ti was about 160 μm. The friction and wear results showed that the wear resistance of SMAT Ti was enhanced compared to the coarse-grained (CG) counterpart. SMAT Ti showed abrasive wear under 1 and 5 N, and exhibited abrasive and adhesive wear under 2 N. While CG Ti showed abrasive and adhesive wear under 1-2 N, and exhibited abrasive wear under 5 N for the work hardening effects.

The biomedical application of Ti after severe plastic deformation and thermal oxidation, especially the friction and wear properties of Ti

Surface mechanical attrition treatment (SMAT), a novel surface severe plastic deformation method, was carried out for titanium (Ti) to create a gradient-structured Ti (SMAT Ti). The tribological behaviour was studied under different loads and dry sliding conditions. The results showed that the deformation layer of SMAT Ti was about 160 lm. The friction and wear results showed that the wear resistance of SMAT Ti was enhanced compared to the coarse-grained (CG) counterpart. SMAT Ti showed abrasive wear under 1 and 5 N, and exhibited abrasive and adhesive wear under 2 N. While CG Ti showed abrasive and adhesive wear under 1–2 N, and exhibited abrasive wear under 5 N for the work hardening effects.

Recrystallization of 304SS during and after high strain rate deformation

During the hot working of austenitic stainless steels the shape of the flow curve is strongly influenced by the strain rate. Low strain rate deformation results in flow curves typical of dynamic recrystallization (DRX) but as the strain rate increases the shape changes to a ‘flat-top’ curve. This has traditionally been thought to indicate no DRX is taking place and that dynamic recovery (DRV) is the only operating softening mechanism. Examining the work-hardening behaviour and corresponding deformation microstructures showed this is not the case for austenitic stainless steel, as clear evidence of dynamic recrystallization process can be seen. The post-deformation recrystallization kinetics can be modelled using a standard Avrami equation with an Avrami exponent, n, of 1.15. With an increasing value of the Zener-Hollomon parameter it was found that the kinetics of recrystallization become less strain rate sensitive until at the highest values (highest strain rates/lowest temperatures) the recrystallization kinetics become strain rate insensitive.

A physically based phenomenological model for deformation twinning in magnesium alloy

A partial differential equation is developed that captures the evolution of key characteristics of tensile twinning in magnesium base alloys. The objective is to provide a framework for ascertaining the effects of hardening – due to grain refinement, precipitation and dislocation substructure – on twin volume fraction, thickness and length. The model is developed with the help of observations made on alloy AZ31. It is shown that it is necessary to consider the nucleation of twins at locations where neighbouring twins impinge on the grain boundary. The model provides a reasonable approximation for the role of grain size on twinning. It predicts a period of low apparent work hardening following yielding and shows that this should be more extensive for finer grain sizes, in agreement with experiment. Finally, some predictions are made on the effect of changing the resistance to twinning.

Characterization of microstructures and tensile properties of TRIP-aided steels with different matrix microstructure

Three different heat treatment processes have been proposed as a fundamental method to produce three kinds of TRIP-aided steels with polygonal ferritic matrix (F-TRIP), bainitic matrix (B-TRIP) and martensitic matrix (M-TRIP) in a newly designed low alloy carbon steel. By means of dilatometry study and detailed characterization, the relationships among transformation, microstructure and the resulting mechanical behavior were compared and analyzed for the three cases. The work hardening of the samples was evaluated by calculating the instantaneous n value as a function of strain. The M-TRIP sample exhibits the highest strength with the highest work hardening rate at low strains and subsequent rapid descending at high strains. In contrast, the B-TRIP sample has relatively high continuously constant work hardening behavior over strain levels greater than 0.067. The difference in work hardening behavior corresponds directly to the rate of the retained austenite-martensitic transformation during straining, which can be attributed to the carbon content, the morphology of the retained austenite and the matrix microstructure in the respective TRIP-aided samples. © 2014 Elsevier B.V.

Constitutive analysis of hot deformation behavior of a Ti6Al4V alloy using physical based model

The effect of deformation parameters on the flow behavior of a Ti6Al4V alloy has been studied to understand the deformation mechanisms during hot compression. Cylindrical samples with partially equiaxed grains were deformed in the α+β phase region at different thermo-mechanical conditions. To develop components with tailored properties, the physically based Estrin and Mecking (EM) model for the work hardening/dynamic recovery combined with the Avrami equation for dynamic recrystallization was used to predict the flow stress at varying process conditions. The EM model revealed good predictability up to the peak strain, however, at strain rates below 0.01s^-1, a higher B value was observed due to the reduced density of dislocation tangles. In contrast, the flow softening model revealed higher value of constants a and b at high strain rates due to the reduction in the volume fraction of dynamic recrystallization and larger peak strain. The predicted flow stress using the combined EM+Avrami model revealed good agreement with the measured flow stress resulted in very low average absolute relative error value. The microstructural analysis of the samples suggests the formation of coarse equiaxed grains together with the increased β phase fraction at low strain rate leads to a higher flow softening.

Strain gradients in Cu-Fe thin films and multilayers during micropillar compression

Plastic strain gradients can influence the work-hardening behaviour of metals due to the accumulation of geometrically necessary discolations at the micron/submicron scale. A finite element model based on the conventional theory of mechanism-based strain-gradient plasticity has been developed to simulate the micropillar compression of Cu–Fe thin films and multilayers. The modelling results show that the geometric constraints lead to inhomogeneous deformation in the Cu layers, which agrees well with the bulging of Cu layers observed experimentally. Plastic strain gradients develop inside the individual layers, leading to extra work-hardening due to the accumulation of geometrically necessary dislocations. In the multilayer specimens, the Cu layers deform more severely than the Fe layers, resulting in the development of tensile stresses in the Fe layers. It is proposed that these tensile stresses are responsible for the development of micro-cracks in the Fe layers.

A preliminary study on machinability of super austenitic stainless steel

Stainless steel is the most widely used alloys of steel. The reputed variety of stainless steel having customised material properties as per the design requirements is Duplex Stainless Steel and Austenitic Stainless Steel. The Austenite Stainless Steel alloy has been developed further to be Super Austenitic Stainless Steel (SASS) by increasing the percentage of the alloying elements to form the half or more than the half of the material composition. SASS (Grade-AL-6XN) is an alloy steel containing high percentages of nickel (24%), molybdenum (6%) and chromium (21%). The chemical elements offer high degrees of corrosion resistance, toughness and stability in a large range of hostile environments like petroleum, marine and food processing industries. SASS is often used as a commercially viable substitute to high cost non-ferrous or non-metallic metals. The ability to machine steel effectively and efficiently is of utmost importance in the current competitive market. This paper is an attempt to evaluate the machinability of SASS which has been a classified material so far with very limited research conducted on it. Understanding the machinability of this alloy would assist in the effective forming of this material by metal cutting. The novelty of research associated with this is paper is reasonable taking into consideration the unknowns involved in machining SASS. The experimental design consists of conducting eight milling trials at combination of two different feed rates, 0.1 and 0.15 mm/tooth; cutting speeds, 100 and 150 m/min; Depth of Cut (DoC), 2 and 3 mm and coolant on for all the trials. The cutting tool has two inserts and therefore has two cutting edges. The trial sample is mounted on a dynamometer (type 9257B) to measure the cutting forces during the trials. The cutting force data obtained is later analyzed using DynaWare supplied by Kistler. The machined sample is subjected to surface roughness (Ra) measurement using a 3D optical surface profilometer (Alicona Infinite Focus). A comprehensive metallography process consisting of mounting, polishing and etching was conducted on a before and after machined sample in order to make a comparative analysis of the microstructural changes due to machining. The microstructural images were capture using a digital microscope. The microhardness test were conducted on a Vickers scale (Hv) using a Vickers microhardness tester. Initial bulk hardness testing conducted on the material show that the alloy is having a hardness of 83.4 HRb. This study expects an increase in hardness mostly due to work hardening may be due to phase transformation. The results obtained from the cutting trials are analyzed in order to judge the machinability of the material. Some of the criteria used for machinability evaluation are cutting force analysis, surface texture analysis, metallographic analysis and microhardness analysis. The methodology followed in each aspect of the investigation is similar to and inspired by similar research conducted on other materials. However, the novelty of this research is the investigation of various aspects of machinability and drawing comparisons between each other while attempting to justify each result obtained to the microstructural changes observed which influence the behaviour of the alloy. Due to the limited scope of the paper, machinability criteria such as chip morphology, Metal Removal Rate (MRR) and tool wear are not included in this paper. All aspects are then compared and the optimum machining parameters are justified with a scope for future investigations

Comportamento plástico do aço inoxidável austenítico em baixa temperatura

Ensaios de tração uniaxiais foram empregados para deformar aços inoxidáveis austeníticos do tipo 304, em diferentes temperaturas abaixo da ambiente (de 77 K a 300 K). A relação entre a estabilidade da austenita e o encruamento, em função da temperatura de teste, é discutida quanto à transformação martensítica induzida por deformação e ao deslizamento de discordâncias na austenita. em curvas tensão-deformação que assumem a equação de Ludwik sigma = sigmao + képsilonn, na qual sigma é a tensão verdadeira e e a elongação plástica verdadeira, um modo conveniente para analisar o encruamento é por meio do diagrama log dsigma / dépsilon versus log épsilon. O aspecto significativo é a variação da taxa de encruamento dsigma / dépsilon com a elongação plástica verdadeira nas diferentes temperaturas. As mudanças no comportamento do encruamento motivando até três estágios de deformação são associadas a diferentes processos microestruturais. A transformação martensítica pode ser considerada como um processo de deformação que compete com o processo usual de deslizamento. A investigação desses estágios, na região plástica, produz uma referência qualitativa de como diferentes fatores, tais como o grau de deformação, temperatura e composição química da austenita, afetam a transformação austenita-martensita.