Cyclic Deformation of Powder Metallurgy Stainless Steel/Mg‐PSZ Composite Materials

In this study, the low‐cycle fatigue (LCF) behavior of powder metallurgy stainless steel/MgO partially stabilized zirconia (Mg‐PSZ) composite materials is presented. The steel matrix based on conventional AISI 304 steel (1.4301) is reinforced with Mg‐PSZ. The investigated composite materials were manufactured using the spark plasma sintering (SPS) technique. Total strain‐controlled LCF tests were performed on materials containing 0, 5, and 10 vol% Mg‐PSZ, respectively, in order to evaluate the influence of the ceramic reinforcement. Electron backscatter diffraction (EBSD) measurements were applied to identify the locations where the martensitic phase transformations in the steel matrix and stress‐assisted as well as athermal martensitic phase transformations of the Mg‐PSZ ceramic reinforcement take place. The resulting cyclic deformation behavior is correlated with the microstructural features of the composite material.     

Reinforcing Mechanism of Mg‐PSZ Particles in Highly‐Alloyed TRIP Steel

Dense TRIP‐matrix composites containing 5 vol.% Mg‐PSZ as reinforcing phase were produced by employing the spark plasma sintering technique. A continuous and seamless interface between the ceramic particles and the steel matrix was achieved. Compression tests revealed better mechanical properties of the 5 vol.% Mg‐PSZ‐TRIP steel composites in comparison with both, pure and Al₂O₃ reinforced TRIP steel. The underlying deformation mechanism within the austenitic matrix entailed a pronounced martensite formation. An additional phase transformation was observed within the ZrO₂ particles. The enhanced mechanical properties of the 5 vol.% Mg‐PSZ composite are dedicated to the transformation strengthening of the ceramic particles. Finally a model of the reinforcing mechanism is proposed.     

Interfacial Reactions in TRIP‐Steel/ZrO2 Composites

The interfaces between stainless TRIP steel and three types of zirconia ceramics were investigated after sintering the samples for 48 h at 1400 °C. The samples were prepared by embedding compact pieces of ceramics into steel powder and densifying them with a spark plasma equipment. In all experiments the same steel was used but the zirconia ceramics was stabilised with CaO, MgO, or non‐stabilised, respectively. It was found that no reaction product was formed in the samples with non‐stabilised or CaO‐stabilised ceramics. At the interface of MgO‐stabilised zirconia and steel, a layer of forsterite, Mg₂SiO₄, was formed. The silicate layer resulted from the reaction of MgO which was dissolved in the zirconia with Si which was an impurity in the steel. In a thermodynamic analysis the reasons are explained which lead to the formation of a silicate in the samples with MgO‐stabilised zirconia. Accordingly, thermodynamic calculations are in accord with the absence of a reaction layer in the cases of the non‐stabilised and the CaO‐stabilised zirconia, even though the Gibbs energy of formation of calcium silicates is much higher than that of the corresponding magnesium silicates. The maximum impurity level for silicon in the given steel is calculated which avoids the formation of a silicate layer at the interface of the MgO‐stabilised ceramics.     

Ceramic Processing for TRIP‐Steel/Mg‐PSZ Composite Materials for Mechanical Applications

Composite materials are up‐to‐date products in a growing range of applications and markets. Due to the advantageous combination of two or more materials new generations of materials can be generated. Closely linked to expanding variations of material combinations are increasing numbers of manufacturing techniques. The combination of ductile metals with hard and brittle ceramics offers a range of applications in the field of crash‐absorber and structural products with high mechanical load. This paper deals with the challenges of powder metallurgical processing of TRIP‐steel/Mg‐PSZ composite materials. The presented results are a contribution to improvements in plastic processing especially for lightweight honeycomb structures.     

A Material Model for TRIP‐Steels and its Application to a CrMnNi Cast Alloy

A material model is presented that accounts for strain rate dependent inelastic deformation and strain‐induced phase transformation in TRIP‐steels. Modifications for the kinetics equations of the strain‐induced phase transformation, introduced by Stringfellow, are proposed to overcome a drawback of Stringfellow's model. A parameter identification strategy that relies on Gauss‐Markov estimates is used to determine the model parameters from experimental data of a recently developed cast TRIP‐steel. Good agreement is observed between experimental results of the compression test and the corresponding finite element simulation employing the proposed model. This forms the basis for future applications of the material model in the design of composites and structures.     

Dynamic and Static Softening of Sintered MgO‐PSZ / TRIP‐matrix Composites with up to 10 vol.‐% ZrO2

A metal matrix composite (MMC) consisting of AISI 304 austenitic stainless steel with up to 10 vol.‐% MgO‐PSZ was produced by a powder metallurgic process through sintering at 1300 °C and 1390 °C. The hot working of sintered samples was conducted between 900 °C and 1100 °C. The behaviour of softening kinetics was investigated using flow curve recording methods (dynamic softening) and the double‐hit method (static softening). The influence of the deformation parameters such as temperature, strain rate, inter‐pass time and relative density of the samples was determined. The microstructure development of the sintered composite after hot forming was determined by optical microscopy and SEM and was interpreted with the help of qualitative microstructure analysis. The results show a general acceleration of softening processes with increasing temperature and strain rate, with the addition of ZrO₂ particles and a decrease in the density of composite materials. A mathematical‐physical model was developed to predict the softening behaviour and optimize the forming processes of the composite in the light of these results.     

Microstructure and Local Strain Fields in a High‐Alloyed Austenitic Cast Steel and a Steel‐Matrix Composite Material after in situ Tensile and Cyclic Deformation

The tensile and cyclic deformation behaviour of a new metastable austenitic stainless cast TRIP (TRansformation Induced Plasticity) steel and a composite material consisting of austenitic steel matrix (AISI 304) with 5% MgO partially stabilized ZrO₂ (MgO‐PSZ) was studied in‐situ in a scanning electron microscope (SEM). In‐situ tests in the SEM show the evolution of the microstructure with the strain for uniaxial deformation and the number of cycles during fatigue, respectively. Initially, deformation bands develop. In these bands, the face‐centred cubic austenite transforms into the hexagonal ε‐martensite and subsequently to the body‐centred cubic α'‐ martensite. This evolution was studied by different SEM techniques. Electron backscatter diffraction (EBSD) was applied for phase and orientation identification. The dislocation arrangement was investigated applying the electron channelling contrast imaging (ECCI) technique to different deformation stages. The studies are completed with measurements of local displacement fields using digital image correlation (DIC).     

Strain‐Rate‐Dependent Flow Stress and Failure of an Mg‐PSZ Reinforced TRIP Matrix Composite Produced by Spark Plasma Sintering

A composite consisting of 5 vol% MgO‐partially stabilized ZrO₂ particles (Mg‐PSZ) and a TRIP‐steel‐matrix (CrNiMn steel; transformation induced plasticity) was produced through Spark Plasma Sintering. The processed material was tested under compression at various nominal strain rates (4 × 10^?4 s^?1; 10^?3 s^?1; 1 s^?1, 10² s^?1). Both, the pure steel and the composite showed a considerable plasticity and high strength due to the very fine grained steel matrix. The addition of 5 vol% ceramic particles led to a rise in the offset yield strength of 60 MPa till 90 MPa according to the applied strain rate. Up to a strain rate of 1 s^?1, no change in offset yield strength was measured. A strain‐rate of 100 s^?1 leads to a rise in the offset yield strength of approx. 100 MPa. Both, the ceramic and an increase in the strain rate implicate to an early generation of microdeterioration. Limited by the interfacial strength of steel and Mg‐PSZ, failure occurs early at the interfaces, which is shown in a decrease in the work hardening. During the compression, especially at higher strain‐rates, adiabatic heating occurred and counteracted to the martensitic transformation.     

Anisotropy of Retained Austenite Stability during Transformation to Martensite in a TRIP‐Assisted Steel

Retained austenite as a key constituent in final microstructure plays an important role in TRansformation Induced Plasticity (TRIP) steels. The volume fraction, carbon concentration, size, and morphology of this phase are the well‐known parameters which effects on the rate of transformation of retained austenite to martensite and the properties of steel, are studied by many researchers. Of the transformation of retained austenite to martensite under strain in a TRIP steel is studied in this paper. The experimental results show that the transformation rate of retained, austenite with similar characteristics, to martensite in differently processed TRIP steel samples, exhibits an anisotropic behavior. This phenomenon implies a kind of variant selection of martensitic reaction of retained austenite under strain and is explained by ferrite texture developed in steel.     

Strength and Failure Behaviour of Spark Plasma Sintered Steel‐Zirconia Composites Under Compressive Loading

Several composites, consisting of a metastable austenitic steel matrix and varying amounts of MgO partially stabilized zirconia particles (Mg‐PSZ), were produced through spark plasma sintering (SPS). Compression tests were carried out at room temperature in a wide range of strain rate (4 · 10^?4 s^?1, 2 · 10^?3 s^?1, 10^?1 s^?1, 1 s^?1, 10² s^?1). In conjunction with subsequent microstructural investigations, the mechanical material behaviour was clarified. All composites showed a good ductility and a high strength. The strength increased with an increase of the ceramic content and with higher strain rates. Both, the martensitic transformation of the steel matrix and of the ceramic particles, could be proved at all strain rates. In this study no significant influence of the strain rate on the amount of transformed ceramic could be detected while the steel matrix showed less α′‐martensite after compression at rising strain rates. Local material failure occurred around 0.3 true compressive strain depending on the applied strain rate and the amount of the Mg‐PSZ powder. The main reason for the damage is the relatively weak ceramic‐ceramic interface within the ceramic clusters.     

残余奥氏体中的碳元素在TRIP钢变形前后的分布

利用TEM和EPMA对TRIP钢中残余奥氏体形貌以及碳元素的分配进行了研究,发现TRIP钢中的残余奥氏体以多种形态分布,且碳在残余奥氏体中的浓度显著高于其他两相中的浓度,此时残余奥氏体可以通过EPMA中的贫硅区表示;变形之后的残余奥氏体将会发生相变,通过TEM发现残余奥氏体在受到应力作用而发生相变之后转变为细小的立方马氏体,且由于碳原子来不及扩散,马氏体中的含碳量和奥氏体中的含碳量基本相同。     

Investigation of the Effect of Deformation on γ‐α Phase Transformation Kinetics in Hot‐Rolled Dual Phase Steel by Phase Field Approach

Dual phase steels, consisting of hard martensite particles in a ductile ferritic matrix, offer high strength and deformability at the same time. Additionally, they are cost effective by a dilute alloying concept. In industrial production, two manufacturing concepts have been implemented: intercritical annealing of cold rolled sheet, or hot rolling. The current work has investigated the effect of deformation on the γ‐α phase transformation kinetics in the dual phase steel production using the hot rolling scheme. The pancaked austenite grains containing denser nucleation sites have a strong influence on the ferrite transformation kinetics. In addition, the multiplication of dislocations which results in the increase in elastic strain energy and dislocation core energy contributes to some acceleration in ferrite growth kinetics. A modelling approach for the γ‐α phase transformation kinetics in dual phase steels has been developed employing the phase field theory. The nucleation behaviour, i.e. the number and size of nuclei developed after an elapsed time as well as their nucleation sites which were evaluated from microstructure analysis, and the increase in the driving force of grain growth were integrated into this model.     

In‐Situ Study of the Microstructure Development in a CrMnNi TRIP Steel During Plastic Deformation

The microstructure development in CrMnNi TRIP steel during the onset of the plastic deformation was investigated with the aid of in‐situ X‐ray diffraction experiments. The analysis of the shift and broadening of the X‐ray diffraction lines allowed the elastic and the plastic components of the lattice deformation to be separated from each other. This separation made possible to follow the formation of the microstructure features like stacking faults, deformation bands and local lattice rotations that were afterwards confirmed by X‐ray diffraction with high resolution, scanning electron microscopy and transmission electron microscopy.     

Model of Mean Flow Stress of Ti‐IF Steel Considering Effect of Phase Transformations

The aim of this work was to describe the deformation resistance of austenitized Ti‐IF steel in the temperature range of 650 to 1100°C by a complex equation. The mean flow stress was determined by an original procedure, based on laboratory rolling of flat samples. Deformation resistance was described by a single formula, using a cumulative function in which particular terms are multiplied by a coefficient of 1 or 0, in dependence on a specific temperature. Calculation of specific coefficients had to be proposed in such a way that they could react to exceeding temperature boundaries between individual phase regions. The developed model can be used for off‐line predictions of power/force parameters in the forming of Ti‐IF steel, in a wide range of conditions of hot and warm deformation.     

Isochronal Phase Transformations of Low‐Carbon High Strength Low Alloy Steel upon Continuous Cooling

Aimed to acquire optimum comprehensive properties for the oil and gas pipeline steels, thermal treatment should be controlled to achieve ideal microstructures. Effects of cooling rates on transformation kinetics and microstructures of the low‐carbon high strength low alloy (HSLA) steel were investigated to obtain an optimized thermal treatment technology. Dilatometric measurements, light microscopy, scanning electron microscopy, and transmission electron microscopy were employed in present work. The transformed microstructures contained polygonal ferrite + pearlite, acicular ferrite (AF), and bainitic ferrite (BF) due to the cooling rates increasing from 5 to 3000°C min^?1, in present investigated HSLA steel. The result shows that, the increase of cooling rate accelerates AF transformation and refines the steel's matrix. The morphology of martensite/austenite structures transformed from islands in AF to films in BF with the increase of cooling rate.     

Effect of Thermo‐Mechanical Process on Phase Transformation Behaviors of V–Ti–N Microalloyed Steel

Thermo‐mechanical simulation tests were performed on V–Ti–N microalloyed steel under three hot working conditions by using Gleeble‐3800 thermo‐mechanical simulator to study the effects of hot deformation and post‐deformation holding process on the continuous cooling transformation behaviors of overcooled austenite. The continuous cooling transformation diagrams (CCT diagrams) were determined by thermal dilation method and metallographic method. The effects of the hot deformation, post‐deformation holding, and cooling rate on the microstructure evolution were analyzed. The results show that deformation promotes ferrite and pearlite transformation. In addition, deformation leads to an increase in bainite start temperature, which becomes more markedly with the increase in cooling rate. The post‐deformation holding process is much favorable to promote carbonitride precipitation of the microalloying elements, which contributes to ferrite nucleation and smaller austenite grains. As a result, an increase in ferrite quantity and a decrease in ferrite grain size can be observed. And further more, the post‐deformation holding process reduces the effect of hot deformation on the bainite start temperature.     

In‐situ Synchrotron XRD Analysis of the Effect of Static Strain Aging on the Elasto‐plastic Transition in CMnSi TRIP Steel

In situ synchrotron X‐ray diffraction was used to investigate the martensitic transformation kinetics, lattice straining and diffraction peak broadening in cold‐rolled TRIP steel during tensile testing. Direct evidence of stress‐strain partitioning between different phases, dislocation pinning and differences in yielding behaviour of the different phases were clearly observed. The TRIP steel was subjected to a bake‐hardening treatment and a pronounced static strain aging effect was observed. In the present work, the martensitic transformation kinetics and the elastic micro‐strain evolution for both ferrite and retained austenite during the elasto‐plastic transition are reported with an emphasis on bake‐hardening with and without pre‐straining.