Defects in High Entropy Alloy HfNbTaTiZr Prepared by High Pressure Torsion

High entropy alloy HfNbTaTiZr was successfully processed by severe plastic deformation using high pressure torsion (HPT) and ultrafine grained microstructure was achieved. The microstructure of HPT-deformed HfNbTaTiZr alloy was characterized by X-ray diffraction and compared with conventionally cast ingots. The lattice defects introduced by HPT processing were characterized by positron annihilation spectroscopy. The X-ray diffraction profiles of HTP-deformed samples were extremely broadened due to small sizes of coherently diffracting domains and a high microstrain introduced by severe plastic deformation.11Ysciescopu

Microstructure and Mechanical Properties of Lead-Free Solder Joints

In this study we investigated the morphology and growth kinetics of the interfacial intermetallic compound layers formed between a Cu substrate and Sn-Cu based solders. We simulated the wave soldering process and subsequent ageing both at different temperatures and reaction times. The near eutectic Sn-0.7Cu alloy and Ni enriched Sn-0.7Cu-0.05Ni alloy were studied. Moreover, we measured the tensile strength of the simulated solder joints and analyzed fracture surfaces

Mechanisms of Plastic Deformation in Ti-Nb-Zr-Ta Based Biomedical Alloys with Fe and Si Content

Specialized beta titanium alloys containing biocompatible elements (Nb, Zr, Ta) are increasingly considered as a material for orthopaedic implants. In this study, small additions of Fe and Si are used to increase the strength of commercial Ti-35Nb-7Zr-5Ta (TNZT) alloy. Six different advanced alloys with iron content up to 2 wt% and silicon content up to 1 wt% were manufactured by arc melting and hot forging. Flow curves were determined from tensile tests carried out at room temperature. The yield stress is increased from 450 MPa to 700 MPa due to small Fe and Si additions. Fe causes solid solution strengthening exhibited by sharp yield point and significant work hardening. (Ti,Zr)₅Si₃ intermetallic particles further increase the strength via precipitation hardening. An unusual serrated yielding behaviour of benchmark TNZT alloy is caused by twinning as shown by acoustic emission measurement and electron backscattered diffraction analysis

Phase Transformations in Ti-15Mo Investigated by in situ Electrical Resistance

In this study phase transformations in metastable beta Ti-15Mo alloy were investigated by an in situ electrical resistance measurement in a wide range of temperatures from -196°C to 850°C. Different temperature ranges of the evolution of electrical resistance were correlated with underlying phase transformations. In the low temperature range, stage I (from -196°C to 220°C) the decrease of electrical resistance with increasing temperature is caused by the dissolution of ω_{ath} (formed during quenching by athermal shuffle transformation) which is accompanied by the relaxation of lattice strain, while the diffusional assisted growth of ω_{iso} in the range from 220°C to 380°C (stage II) is the main mechanism causing the increase of resistance. Another decrease of the resistance in the range from 380°C to 550°C (stage III) is explained by the dissolution or transformation of ω_{iso}. The increase of resistance above 550°C (stage IV) is related to the growth of α-phase particles. The fully reversible character of ω_{ath} growth and dissolution during heating and cooling in the stage I up to 100°C was confirmed by temperature cycling during repeated in situ resistance runs from RT. Pre-ageing of samples at 300°C promotes the formation of ω_{iso} particles. Subsequently, ω_{ath} particles are not created, which is fully consistent with electrical resistance measurements. The presence of ω_{ath} and the orientation relationship between ω and β were identified by the electron diffraction

Achieving high strength and low Young’s modulus by controlling the beta stabilizers content in Ti-Nb-Ta-Zr-O alloys

High strength and low Young’s modulus is the desired combination of mechanical properties for the endoprostheses material. Metastable beta titanium alloys are promising materials for this application. In this study, four Ti-xNb-6Ta-7Zr-0.7O (wt.%) alloys were prepared where Nb content ranged from 26 to 35 wt. %. All alloys contained pure beta phase. The high oxygen content causes high microhardness (330 HV), hence also the strength, while decreasing content of Nb leads to lower electrons per atom (e/a) ratio. The e/a ratio affects the Young’s modulus which is highest (76 GPa) in Ti-35Nb-6Ta-7Zr-0.7O alloy with e/a=4.31 and the lowest (64 GPa) in Ti-26Nb-6Ta-7Zr-0.7O with e/a=4.24. Such evolution of Young’s modulus is in accordance with existing literature data, however, in comparison with other works, the Ti-26Nb-6Ta-7Zr-0.7O alloy shows double microhardness when compared to alloys with similar Young’s modulus. Therefore, the approach of using controlled oxygen content for alloy design is very promising for development of biocompatible metastable beta Ti alloy for endoprostheses production

Alpha Variant Selection Determined from Grain Misorientations in Ti-6Al-7Nb Alloy with a Duplex Microstructure

Titanium occurs in two structures; a high temperature body-centered cubic structure which is known as β phase and an ambient temperature α phase which has the hexagonal closed-packed structure. In the present study a biomedical Ti-6Al-7Nb alloy was investigated. The so-called duplex structure consisting of α lamellae and equiaxed primary α-grains was prepared by a thermal treatment. The α lamellae are created during cooling from a β-field according to the Burgers relation. This relation allows the formation of the α lamellae with different crystallographic orientations - so-called variants. The preferential misorientation between α lamellae was studied by a detailed electron backscattered diffraction analysis. The misorientation of grains in the duplex structure was modelled by a sum of random Mackenzie distribution and Gaussian peaks related to the preferred misorientations according to the Burgers relation. The preferred misorientations based on the Burgers relationship were identified in the biomedical Ti-6Al-7Nb alloy with duplex structure. It is confirmed that the variant selection of α lamellae is not random

Evolution of distance between omega particles in metastable beta Ti alloy determined from in situ small angle neutron scattering

The evolution of distance between omega particles in metastable beta Ti 15Mo alloy 8.1 in at. was determined from in situ small angle neutron scattering SANS . SANS data were recorded during heating of the material from room temperature to 600 C with the heating rate of 1 C min. The results agree with previously determined ordering of omega particles in a cubic three dimensional array with the axes along the cubic axes [100]beta of the host lattice. The distance between particles, which increases with temperature, was investigated in three orientations with the incident beam parallel to [100]beta, [110]beta and [111]bet

Achieving high strength and low Young’s modulus by controlling the beta stabilizers content in Ti-Nb-Ta-Zr-O alloys

High strength and low Young’s modulus is the desired combination of mechanical properties for the endoprostheses material. Metastable beta titanium alloys are promising materials for this application. In this study, four Ti-xNb-6Ta-7Zr-0.7O (wt.%) alloys were prepared where Nb content ranged from 26 to 35 wt. %. All alloys contained pure beta phase. The high oxygen content causes high microhardness (330 HV), hence also the strength, while decreasing content of Nb leads to lower electrons per atom (e/a) ratio. The e/a ratio affects the Young’s modulus which is highest (76 GPa) in Ti-35Nb-6Ta-7Zr-0.7O alloy with e/a=4.31 and the lowest (64 GPa) in Ti-26Nb-6Ta-7Zr-0.7O with e/a=4.24. Such evolution of Young’s modulus is in accordance with existing literature data, however, in comparison with other works, the Ti-26Nb-6Ta-7Zr-0.7O alloy shows double microhardness when compared to alloys with similar Young’s modulus. Therefore, the approach of using controlled oxygen content for alloy design is very promising for development of biocompatible metastable beta Ti alloy for endoprostheses production

The Very High Cycle Fatigue Behaviour of Ti-6Al-4V Alloy

The high cycle and very high cycle fatigue properties of the titanium alloy Ti-6Al-4V with a duplex microstructure were investigated at room temperature. High cycle fatigue tests were performed in the range from 10⁴ to 10⁷ cycles by rotating bending at the frequency of 30 Hz. The very high cycle fatigue tests were carried out in the range from 10⁷ to 10¹⁰ cycles in tension-compression on an ultrasonic fatigue testing machine at the frequency of 20 kHz. The stress amplitude was found to decrease with increasing number of cycles in the whole range from 10⁴ up to 10⁹ cycles and only at the highest number of cycles $(N_{F}=10^9)$ the alloy exhibits the fatigue limit of 460 MPa. The detail fractographic analysis was performed to characterize the fatigue failure mechanisms. Both subsurface and surface crack initiation were observed in very high cycle fatigue region. No inclusions, but only local chemical inhomogeneity in microstructure was observed at the locations of subsurface fatigue crack initiation in alpha-grains