Influence of isothermal annealing in the 600 to 750 °C range on the degradation of SAF 2205 duplex stainless steel

We studied the effect of isothermal annealing (600–750 °C, 1 to 1000 min) on the microstructure and mechanical properties of SAF 2205 duplex stainless steel. Impact toughness was found to be significantly more affected than hardness by annealing. Annealing at 750 °C for 1000 min resulted in a more than 90% decrease in impact toughness, while hardness only increased by 25%. Tensile strength increased up to 100 MPa, but elongation decreased by more than 50% under the same conditions. Sigma phase formation was minimal at lower temperatures (650 °C and below) but increased significantly at higher temperatures. At 750 °C and 1000 min of annealing, the ferrite content dropped from 50% to 16%. These findings suggest that annealing temperature and time need to be carefully controlled to avoid a reduction in impact toughness and ductility caused by sigma phase precipitation. The harmful effect of sigma phase precipitation on mechanical properties was directly shown

High-Temperature Oxidation of Boiler Steels at 650 °C

This study presents a comprehensive investigation of the formation, composition and behaviour of oxide layers during the high-temperature oxidation of four different steel alloys (16Mo3, 13Cr, T24 and P91) at a uniform temperature of 650 °C. The study is aimed at assessing the oxidation damage due to short-term overheating. The research combines CALPHAD (CALculation of PHAse Diagrams) calculations, thermogravimetric analysis (TGA) and advanced microscopy techniques, including scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), to elucidate the complex mechanisms controlling oxidation kinetics and oxide layer development. CALPHAD calculations were used to determine the thermodynamically stable phases for each steel type at 650 °C and different oxygen activities. The results showed different phase compositions, highlighting the importance of the chromium content in steel for the formation of oxide layers. The different oxidation kinetics and oxide layer compositions are presented and associated with the increased risk of material degradation due to overheating. These results have significant implications for industrial applications, mainly the susceptibility to oxidation of low-alloyed steels like 16Mo3 and 13 Cr and contribute to a deeper understanding of oxidation processes in steels

Analysis of Factors Influencing Measurement Accuracy of Al Alloy Tensile Test Results

In order to properly use materials in design, a complete understanding of and information on their mechanical properties, such as yield and ultimate tensile strength must be obtained. Furthermore, as the design of automotive parts is constantly pushed toward higher limits, excessive measuring uncertainty can lead to unexpected premature failure of the component, thus requiring reliable determination of material properties with low uncertainty

Homogenisation efficiency assessed with microstructure analysis and hardness measurements in the EN AW 2011 aluminium alloy

When extruding the casted rods from EN AW 2011 aluminium alloys, not only their homogenized structure, but also their extrudable properties were significantly influenced by the hardness of the alloy. In this study, the object of investigations was the EN AW 2011 aluminium alloy, and the effect of homogenisation time on hardness was investigated. First, homogenisation was carried out at 520 °C for different times, imitating industrial conditions. After homogenisation, the samples were analysed by hardness measurements and further characterised by microscopy and image analysis to verify the influence of homogenisation on the resulting microstructural constituents. In addition, non-equilibrium solidification was simulated using the program Thermo-Calc and phase formation during solidification was investigated. The homogenisation process enabled more rounded shape of the Al$_2$Cu eutectic phase, equilibrium formation of the phases, and the precipitation in the matrix, leading to a significant increase in the hardness of the EN AW 2011 aluminium alloy. The experimental data revealed a suitable homogenisation time of 4–6 h at a temperature of 520 °C, enabling optimal extrusion properties

Elastocaloric effect vs fatigue life

Structural fatigue is the major obstacle that prevents practical applications of the elastocaloric effect (eCE) in cooling or heat-pumping devices. Here, the eCE and fatigue behaviour of Ni-Ti plates are systematically investigated in order to define the fatigue strain limit and the associated eCE. Initially, the eCE was evaluated by measuring adiabatic temperature changes at different strain amplitudes and different mean strains along the loading and unloading transformation plateaus. By comparing the eCE with and without pre-strain conditions, the advantages of cycling an elastocaloric material at the mean strain around the middle of the transformation plateau were demonstrated. In the second part of this work, we evaluated the fatigue life at the mean strain of 2.25% at the loading plateau and at the unloading plateau after initial pre-straining up to 6% and 10%, respectively. It is shown that on polished samples, durable operation of 10 5 cycles can be reached with a strain amplitude of 0.50% at the loading plateau, which corresponds to adiabatic temperature changes of approximately 5 K. At the unloading plateau (after initial pre-strain of 10%), durable operation was reached at a strain amplitude of 1.00%, corresponding to adiabatic temperature changes of approximately 8%K. The functional fatigue was analysed after the cycling and it is shown that once the sample has been stabilized there is no further degradation of the eCE, even after 10 5 cycles. These results present guidelines for the design and operation of efficient and durable elastocaloric devices in the future

Influence of surface preparation of aluminum alloy AW-5754 and stainless steel X5CRNI18-10 on the properties of bonded joints

Adhesive bonding has proven to be a reliable method of joining materials, and the development of new adhesives has made it possible to use bonding in a variety of applications. This article addresses the challenges of bonding metals such as the aluminum alloy EN AW-5754 and the stainless steel X5CrNi18-10. In this study, the effects of laser cleaning and texturing on the surface properties and strength of two bonded joints were investigated and compared with mechanical preparation (hand sanding with Scotch-Brite and P180 sandpaper). The bonded joints were tested with three different epoxy adhesives. During the tests, the adhesion properties of the bonded surface were determined by measuring the contact angle and assessing the wettability, the surface roughness parameters for the different surface preparations, and the mechanical properties (tensile lap-shear strength). Based on the strength test results, it was found that bonded joints made of stainless steel had 16% to 40% higher strength than aluminum alloys when using the same adhesive and surface preparation. Laser cleaning resulted in maximum shear strength of the aluminum alloy bond, while the most suitable surface preparation for both materials was preparation with P180 sandpaper for all adhesives

Thin-walled Ni-Ti tubes under compression

Elastocaloric cooling is emerging as one of the most promising alternatives to vapor-compression cooling technology. It is based on the elastocaloric effect (eCE) of shape memory alloys (SMAs), which occurs due to a stress-induced martensitic transformation (superelasticity). In recent years, several elastocaloric proof-of-concept devices have been developed and the best of them have already achieved commercially relevant cooling characteristics. However, the proposed devices are not yet ready for commercialization, mostly due to their short fatigue life, which is a consequence of the tensile loading. The fatigue life can be significantly improved if the material is instead subjected to compressive loading, but mechanical instabilities (buckling) and the poor heat transfer of bulky geometries (favorable for compression) are the major challenges to overcome when designing compressed elastocaloric elements. Here, we show for the first time that thin-walled Ni-Ti tubes, which allow for the rapid heat transfer, can withstand more than 10$^6$ compressive loading cycles without any degradation of the eCE while maintaining high efficiency (coefficient of performance) and adiabatic temperature changes as high as 27 K. This is the largest, directly measured, durable eCE for any elastocaloric material in the high-cycle fatigue regime to date, and so opens up new avenues in the development of durable and efficient elastocaloric devices

Mechanisms of premature fracture in modular neck stems made of CoCrMo/Ti6Al4V and Ti6Al4V/Ti6Al4V alloy

In this paper, we present the mechanisms of premature fracture of modular neck stems in two case studies: (I) when the neck and stem are both made of the same Ti6Al4V alloy, and (II) when the neck and stem are made from two different alloys, CoCrMo and Ti6Al4V alloy. Our study integrates two orthopedic patients who have undergone primary uncemented THA for usual indications in two orthopedic centers (Community Health Centre and University Medical Centre). Both centers are part of the national public health care system. Both surgeries were performed by two skilled orthopedic surgeons with more than 10 years of experience in THA. The survivorship of the modular neck of cast CoCrMo alloy was 24 months. The survivorship of the modular neck from Ti6Al4V alloy was 84 months. Multivariate analyses were performed to assess the differences in the fretting, corrosion, and fatigue of the two prematurely failed modular neck stems: stereo light microscopy (SLM), scanning electron microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). Patient demographic information, including sex, age, body mass index, survivorship of implants, and reason for the revision, was collected from medical records. We found that fretting and fatigue occurred on both neck-stem retrievals due to additional galvanic corrosion, but the CoCrMo/Ti6Al4V alloy system suffered more corrosion due to additional galvanic corrosion and fractured earlier than the Ti6Al4V/Ti6Al4V metal alloy system. Both metallic alloy systems used in this application are known to be highly corrosion-resistant, but the bio-tribo-corrosion processes need to be understood in detail and characterized so that appropriate improvements in design and materials can be made