Márton Benke

In Situ Optical Microscope Examinations of the ε↔γ Transformations in FeMn(Cr) Austenitic Steels during Thermal Cycling

A group of austenitic steels exhibit high deformability and strength due to TRansformation Induced Plasticity (TRIP) and/or TWinning Induced Plasticity (TWIP). The phase transformations of the TRIP and TWIP steels have been examined in details in many FeMnX alloy systems (X: Ni, Al, Si). However, less attention was given to the FeMn(Cr) alloys. The γ ↔ ε transformations in the austenitic FeMn(Cr) alloys have been examined during heat cycling by in situ optical microscopy and DSC measurements.

In Situ Optical Microscope Study of the Thermally Induced Displacive Transformations in Cualni-Based Shape-Memory Alloys

In situ optical microscope examinations were carried out on the thermally induced thermoelastic martensitic transformation in an untrained CuAlNi alloy and on the bainitic reaction in a CuAlNiMnFe alloy. It was found that a different martensite variant structure formed after every thermal cycle and the transformation was not accompanied with observable plastic deformation in the CuAlNi alloy. Elastic deformations were observed as the martensite plates reached grain boundaries. The growth of new martensite plates was initiated at these locations. The result are discussed and compared to results of other alloys found in the literature. The bainitic reaction was found to occur under isothermal conditions in the CuAlNiMnFe alloys. The reaction was accompanied with a relief formation much finer than that of the thermoelastic martensitic transformation.

Investigation of the bainitic reaction in a CuAlNiMnFe shape memory alloy

Despite their favorable properties, brittle nature of the CuAlNi shape memory alloys limits their suitability. To increase their ductility, Mn and Fe were added to the base CuAlNi alloy. To reveal the applicability of the developed CuAlNiMn and CuAlNiMnFe alloys as functional materials, the effect of ageing on the thermoelastic martensitic transformation was investigated. During the first heating of the aged samples the thermoelastic γ’ → β transformation occurred, which was followed by a bainitic transformation. This transformation inhibited the further thermoelastic martensitic transformations. The present paper covers heat flux DSC, SEM, and TEM investigations of the bainitic transformation. A feasible mechanism of the bainitic transformation in these alloys is suggested based on the results of the examinations.

Characteristics of Martensitic Transformations Induced by Uni-Axial Tensile Tests in a FeMnCr Steel

Austenitic FeMnCr steels have high strength, high toughness and formability because of the stress-and strain-induced γ→α and γ→ε martensitic phase transformations. These are the so-called TRIP (Transformation Induced Plasticity) and TWIP (Twining induced Plasticity) effects. TWIP steels deform by both glide of individual dislocations and mechanical twinning [1]. The type and mechanism of the austenite→martensite transformation depends on the composition, deformation rate and temperature. The ratio and quantity of the resulting phases determine the properties of the product. It is known that austenitic steels can transform into α and/or ε martensite phases during plastic deformation The characteristics of the martensitic transformations induced by uni-axial tensile tests between room temperature and 200°C in a FeMnCr steel with 2,26 w% Cr content were examined. Mechanical properties as, yield stress were determined from tensile tests. Metallographic examinations, quantitative and qualitative phase analysis by X-ray diffraction were carried out on the uniformly elongated part of the samples (cross, longitudinal sections).

Innovation methods for residual stress determination for the automotive industry

Determination of the residual stress state in a loaded automotive component is highly important because of its strong effect on the lifetime of the element. Nowadays, the residual stress characterization of products became an everyday requirement in the automotive industry, and the quality control is impossible to imagine without it. Forasmuch as every producing process (casting, heat treating, different kinds of metal deformation processes and surface compressing methods, etc.) influences the residual stress state, therefore it can be very complex and various within the materials. If we conscious in the effect of these processes it is possible to reach such a state in the material which can enhance its lifetime and quality, and with an optimized process, the costs could be reduced. Several methods exist for measuring the residual stress and each of them has its own advantage and disadvantage. With this paper our purpose is to introduce nowadays’ available stress measuring methods such as X-ray diffraction, magnetic Barkhausen noise and the hole drilling methods and a few more alternatives. Some useful results from the practice are also presented.

Investigation of the Kinetic of a Bainitic Reaction upon Heating in a CuAlNiMn and a CuAlNiMnFe Shape Memory Alloy

The examination of solid state processes leading to the degradation of the shape memory behaviour is essential with respect to the suitability of shape memory alloys. Besides degradation processes occurring during relatively long periods of time called ageing, bainitic reactions that suddenly degrade the shape memory behaviour were also observed in many Cu-based shape memory alloys. The mechanisms and effects of the bainitic reactions on the shape memory characteristics were investigated in many Cu-based systems, but the kinetic of the reaction was not examined so far. In the present paper, an examination was carried out on a CuAlNiMn and a CuAlNiMnFe shape memory alloy to reveal what kinetic model describes the bainitic reaction occurring and thus completely destroying the shape memory effect during one stage of heating.

Investigation of Ageing Phenomena in CuAlNi Based Shape Memory Alloys

The ageing phenomena and its effect on the thermoelastic martensitic transformation was investigated in three Cu-base SMAs. The transformation temperatures shifted to higher temperatures due to aging in the beta-phase. To increase the alloy’s ductility, a definite amount of Mn (4 wt%) and Fe (2 wt%) were added to the ternary alloy. The thermoelastic martensitic transformation was found in the not-aged samples of the CuAlNiMn and CuAlNiMnFe alloys. This transformation was destroyed due to ageing heat treatments by a fairly unknown exothermic process. The thermoelastic martensitic transformation appeared again in the aged CuAlNiMn and CuAlNiMnFe samples after keeping them on room temperature for a few months. This phenomena was investigated by DSC, SEM, TEM, and XRD.

Significance of the Residual Stress Monitoring in the Automotive Industry

Determination of the residual stress is highly important because it has a strong effect on the lifetime of the component and it can cause deformation or failure during the manufacturing. Nowadays, the residual stress characterization became an everyday requirement in the automotive industry. Forasmuch as every producing process (casting, heat treating, different kinds of metal deformation processes and surface compressing methods etc.) influences the residual stress state, therefore it can be very complex and various within the components. If the effect of these processes are known, it is possible to reach such a stress state in the component which can enhance its lifetime and quality, and with an optimized process, the costs could be reduced.

Correlation Between Deformation Texture and Martensitic Transformation in TWIP/TRIP Steels on Multiscale

In the present manuscript, the texture variation of γ austenite, e martensite and α′ martensite phases are investigated in FeMn (Cr) steels exhibiting both TWIP and TRIP behaviour during uniaxial tensile tests. Samples of three steels with varying Cr content were subjected to tensile tests till fracture on different temperatures ranging from room temperature, at which e martensite and γ austenite are stable to 453 K, where only γ austenite was present prior to tensile stressing. The developed texture of e martensite, α′ martensite and γ austenite was examined on multiscale, by TEM orientation mapping, Electron Backscattered Diffraction (EBSD) orientation mapping and X-ray-diffraction pole figure measurements. Correlations between the developed textures and the formation of the martensitic phases are discussed.

Martensites Formation During Thermomechanical Treatments of Twip Steels

Austenitic steels are attractive for automobile applications including press-formed parts due to their energy absorption or structural reinforcement. Austenitic FeMnCr steels have high strength, high toughness and formability because of the martensitic transformation. In this steels not only the temperature but also strain-induced non thermoelastic martensitic phase transformation takes place. This is the so-called TRIP (transformation induced plasticity) and TWIP (twining induced plasticity) effect. TWIP steel can deform by both glide of individual dislocations and mechanical twinning [1,2]. It has been known that during plastic deformation of austenitic steels it can transform into α’ martensite. Two transformation mechanisms have been observed. The first one describes the direct formation of α’ martensite from austenite, while the second one involves a two-step reaction in which α’ phase is formed from  martensite [3, 4]. The austenite–martensite transformation depends on composition, deformation rate and temperature. During thermomechanical treatments, when a well-controlled combination of the temperature and the deformation rate is applied an extraordinary combination of microstructures can be achieved [5-8]. Because of the ratio and quantity of resulted phases determines the properties of the product an extraordinary mechanical property can be predicted after or during the treatments. Martensite formation in manganese steels with different Cr content were investigated. Uniaxial tensile tests with different temperature and strain rate were performed to achieve thermomechanical treatments. The flow rate the hardening effect and the microstructure were investigated and compare in the different steels.