Valéria Mertinger

Effect of hydrostatic pressure on the martensitic transformation in near equiatomic Ti-Ni alloys

Abstract The effect of hydrostatic pressures on martensitic phase transformations in near-equiatomic Ti-Ni shape-memory alloys has been investigated up to 1.5 GPa. The relative volume changes δV c/V and the entropy changes δS c have also been determined at atmospheric pressure. Supposing that the volume derivative of the elastic energy differs from zero at the martensite start (and austenite finish) temperatures as well, that is e 0 ≠ 0, and that the Clausius-Clapeyron equation is valid for the equilibrium transformation temperature T 0, it was shown that the dissipative energy terms were approximately zero for the B2-R transition. For the R → B19′ transformation this energy was independent of pressure, while for the reverse transformation it had a defmite pressure dependence. The pressure derivatives of elastic energy e m (which corresponds to the martensite finish or austenite start) and e 0 are similar in both directions for the R-B19′ transformations, while for the B2-R transition it is negative for e 0.

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.

Deforming Texture Effect on Phase Transformation in Stainless Steels

In stainless steels during the non-equilibrium transformation of aus tenite the deformed bcc α’ martensite and hexagonal, non-magnetic ε martensite also can form. These transformations can be induced by cooling (under critical temperature) and/or mechani cal deformation. The appearance of ε martensite has an effect on the deformation ability and other prope rties. The experimental results associate the ε martensite formation with the plastic deformation behaviour (TRIP effect), while the transformation of ε martensite into α’ martensite is accompanied with hardening. The present phases hardly determine the cold rolling properties. During the deforming of α’, ε, γ phases in stainless steels the texture also developes. The i nitial strong deforming γ texture changes with the appearance of α’ martensite. We suppose that the transformation takes place at certain places in an anisotropic wa y due to the deforming texture. The texture of α’ martensite develops by the initial anisotropy of parent phase a nd/or the phase deformation. Introduction Martensite has long been used to designate the hard microstructur e found in quenched carbon steel. If the steel with the carbon content higher than 0.6wt% is heated above the austeniti c range and than quenched at a sufficiently high cooling rate the fcc austenite tra nsforms into the metastable body centered tetragonal phase ( ). Perhaps the most important aspect in martensitic transformati n involves a special crystallographic relationship between the marte nsite and austenite, which allows a fast growth mechanism. The crystallography is very simil ar to the crystallography involved in deformation twinning. Thus in general when a martensitic transform ation is coupled with an externally applied stress other phenomena such as stress or strai n induced martensitic transformation, the generation of new nucleation sites, and transformat ion induced plasticity (TRIP) may occur. The most commonly used austenitic steel has a tendenc y to transform into α’ martensite and hexagonal ε martensite during thermomechanical treatment. The ε martensite has a well defined crystallographic relationship to the parent austenite phase, it forms close to the stacking faults with fine lathes on the {111} plate of austenite. The thickness of the ε lath is 300 nm or less. [1] Between Ms and Md temperatures the formation of martensite can be induced by ela stic and/or plastic deformation. Within this temperature intervals the driving force for the reaction consists of 1) the free energy difference between the martensitic and aust enitic states and 2) the externally applied stress. The authors propose to distinguish between the stress-i nduced and strain-induced formation of martensite. Martensite is considered to be stress induced when it forms as a result of elastic stresses from an external load (below the actual yi eld strength of austenite). Martensite is strain induced when the slip in the austenite precedes its format i n. (σA-M>σyield A, the lowest temperature is Ms σ ). The austenitic stainless steels have a wide range of usage i n th chemical industry and also in the processing of household goods. The most commonly used processing for the se goods is the cold working. So during the deformation of metastable austenitic CrNi st eels he and ε martensite can form. According to the chemical composition of steel and the condition of def rmation different transformations take place such as γ→ε, ε→α’ , γ→α’ , γ→ε→α‘. The deformation induced Materials Science Forum Online: 2003-01-15 ISSN: 1662-9752, Vols. 414-415, pp 281-288 doi:10.4028/www.scientific.net/MSF.414-415.281

Melt motions during unidirectional solidification of AlAl3Ni eutectics

Abstract AlNi eutectics and near-eutectics were unidirectionally solidified under normal and microgravity conditions. The effects of gravity-driven melt convection on growth rate, eutectic cell morphology and eutectic spacing were investigated experimentally. In an experiment performed under microgravity a decrease in growth rate and a decrease of the eutectic spacing was found. In interpreting the results an interaction is assumed between solidification rate and melt flow in order to minimize the energy consumption.

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).

Residual Stress Behavior in Hardened Shot Peened 42CrMo4 Specimens during Fatigue Load

The advantages of applied compressive residual stress on fatigue properties of materials is a well-described topic, but not in all respects. Compressive macro residual stresses in the surface region with medium and high hardness increase the fatigue life and the fatigue limit compared to materials that are free from designed compressive residual stresses because of their increased resistance against crack initiation and propagation. For this aim various surface compressing methods such as burnishing, shot peening, rolling have been developed. The monitoring of residual stress variation during fatigue is important. All properties, which exert lifetime, should be analyzed. The residual stress state of machine elements can change during application, therefore it is necessary to describe how these changes are related to the operational parameters. The surface residual stress state evolution of hardened (quenched and tempered)-and shot peened-42CrMo4 steel during fatigue tests was investigated nondestructively by X-ray diffraction. Four fatigue stress levels were applied. The residual stress state was recorded in shot peened state and monitored during the fatigue tests. The fatigue test was stopped after certain cycles until the specimens fractured. The stress state was measured after each fatigue test stops and the stress relaxation is given in the percentage of the initial stress state in function of cycle number. Introduction Shot peening is a widely used surface compressing method to create compressive residual stress in machined elements [1, 2]. The compressive residual stress state has a beneficial effect on the fatigue durability of metals, especially on fatigue strength [3, 4, 5]. For the assessment of the influence of residual stresses on fatigue behavior, the stability of the residual stress state is very important. During fatigue, the macro and micro residual stress states interact with the cyclic loading stresses and the work hardening and softening processes. For example the surface compressive residual stress improves the fatigue resistance properties of all materials. The stability of the surface compressive residual stress during low and high cycle fatigue in annealed micro-alloyed, medium carbon, perlitic and austenitic steels has been already investigated [6,-9]. The authors have shown that the residual stress relaxation is not monotonic in these alloys. However, in general, machine elements, which are tipically exposed to high cycle fatigue loads; (gears, shafts) are mainly made of high strength hardened steels, therefore it is worth to investigate the stability of residual stress in such material. The stability of surcafe compressive residual stress on hardened, shot peened CM45MV specimens during fatigue was investigated in one of our eralier project [10]. Residual stress relaxation was not observable in that steel, because of the effect of microstructure defects and specimen geometry. In this paper, Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 491-496 doi: http://dx.doi.org/10.21741/9781945291173-83 492 our aim was to analyze the variation of compressive residual stress state in shot peened hardened 42CrMo4 steel specimens during fatigue load. This is a widely used steel in the vehicle industry especially in the case of surface compressed machined elements. These measurements are based on our earlier project. Since then, the specimen geometry was improved and the material selection has changed. Materials The specimens were made of EN:42CrMo4 type steel. The chemical composition is given in Table 1. Table 1. Chemical composition of the examined steel

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.

Technological Investigation of Clad Sheet Bonding by Hot Rolling

In this study the bonding properties of three layer-plated aluminum sheets are investigated. The alloys applied in specific layers were as follows: AlMn1Si0.8 (core alloy) and AlSi10 (liner). The bonding was performed on a Von Roll experimental roll mill using hot rolling. The experimental temperatures were 460, 480 and 500 °C, respectively. To qualify bond development, T-peel test was used. The test was performed using an Instron universal material testing machine. T-peel test can be well used for the qualification of bond strength as the peel-off force and bond value developed on contacting surfaces are proportional. In addition to T-peel test, optical micrographs and SEM micrographs were also captured, in which typical bond faults were sought. The study aims at modelling the technology used in industry and exploring some typical bond faults as well as suggesting the causes generating these and their remedy. The impact of surface roughening before heating was studied as well. Also, the study aimed at confirming the suitability of T-peel test to qualify bond strength.