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Abstract: The internal friction (IF) of Ti50Ni50−x Cu x polycrystals, which were normally solution-treated, was systematically studied using dynamic mechanical analysis. As a result, the broad peak appearing in B19 martensite was confirmed to be a relaxation peak with an activation energy of 0.67 and 0.76eV for x =20 and 16, respectively. Then the broad peak was shown to disappear in a single-crystal experiment, if twin boundaries in martensite are largely eliminated. This is direct evidence to show twin boundaries are indispensable for the broad peak. The effect of hydrogen on IF was also examined using a vacuum system with a mass spectrometer, and the broad peak at 250K was observed to disappear, while a new small relaxation peak was found at 178K, when hydrogen was eliminated. These experiments clearly show that there are two essential ingredients to observe the high broad peak: twin boundaries and hydrogen. Based on these results a possible origin for the broad peak is discussed. [Copyright &y& Elsevier]

Abstract: A 30μm thick nanocrystalline coating (NC) with a composition identical to that of conventional coarse-grained 304 stainless steel (SS) was prepared via direct current magnetron sputtering (DCMS). The DCMS NC exhibited a typical columnar microstructure, with a mean grain size of ∼12nm in the plane normal to the growth direction. Comparison of the oxidation of the 304 NC with 304 SS (∼20μm in an average grain size) was investigated at temperatures (700–900°C) in flowing air with 0–40% H2O vapor. Catastrophic breakaway oxidation started from the upstream edge of the coarse-grained sample in wet air after an incubation period, due to the conversion of protective Cr-rich scale into non-protective Fe-rich one as a result of the water-vapor-promoted evaporation of chromia from scale. Increasing temperature or water-vapor concentration accelerated the appearance of breakaway oxidation. In contrast, breakaway oxidation did not occur in wet air for the DCMS NC, where abundant grain boundaries greatly enhanced the Cr diffusion to guarantee the stable growth of the Cr-rich scale. [Copyright &y& Elsevier]

Abstract: A novel method using differential scanning calorimetry (DSC) has been developed to quantify the interface kinetics in a solid/liquid diffusion couple. The Ag–Cu binary eutectic system is investigated by heating an assembly of Ag base metal and Ag–Cu eutectic foil to 800 °C and holding. The fraction of liquid remaining after various isothermal hold periods is measured by comparing the melting endotherms with the solidification exotherms. Detailed analysis of the results show the per cent liquid remaining is inversely proportional to the square root of isothermal hold time; however, effects of the base metal cause an apparent loss of 25% of the liquid immediately after heating. The fundamental understanding of the effects of bi-phase diffusion couple geometry is advanced to manifest the mechanisms resulting in the error. Further carefully devised experiments reveal that primary solidification during cooling is not included in the enthalpy of solidification measured by the DSC. Furthermore, baseline shift across the melting endotherm increases the measured melting enthalpy. These effects combine to systematically underestimate the fraction of liquid remaining. Development of a modified temperature program and application of an appropriate correction can remedy these effects. The experimental results compare well with a prediction generated by an analytical model. Successful quantification of these phenomena has broadened the knowledge of DSC operational characteristics in the solid/liquid diffusion couple treatment, which can now be applied to other material systems. [Copyright &y& Elsevier]

Secondary phases, either introduced by alloying or heat treatment, are commonly present in most high-entropy alloys (HEAs). Understanding the formation of secondary phases at high temperatures, and their effect on mechanical properties, is a critical issue that is undertaken in the present study, using the Al x CoCrFeNi (x = 0.3, 0.5, and 0.7) as a model alloy. The in-situ transmission-electron-microscopy (TEM) heating observation, an atom-probe-tomography (APT) study for the reference starting materials (Al 0.3 and Al 0.5 alloys), and thermodynamic calculations for all three alloys, are performed to investigate (1) the aluminum effect on the secondary-phase fractions, (2) the annealing-twinning formation in the face-centered-cubic (FCC) matrix, (3) the strengthening effect of the secondary ordered body-centered-cubic (B2) phase, and (4) the nucleation path of the σ secondary phase thoroughly. The present work will substantially optimize the alloy design of HEAs and facilitate applications of HEAs to a wide temperature range. [ABSTRACT FROM AUTHOR]

Joining NiTi to Ti6Al4V is of great interest for applications in the biomedical and aerospace fields. Despite the importance, no joining techniques have been developed that avoid the formation of brittle intermetallics to produce high strength joints. In this work, Niobium was used as an interlayer to prevent the formation of these brittle phases when joining NiTi to Ti6Al4V. The presence of this interlayer ensured that crack free welds were obtained and no brittle intermetallic compounds were observed. The Niobium interlayer was of a much higher melting temperature than the base materials so the bulk Niobium did not melt during the joining process, acting as a diffusion barrier between the NiTi and Ti6Al4V. The laser was focused on the Ti6Al4V side of the joint, which joined the Ti6Al4V and Niobium by fusion welding. At this interface a (Ti, Nb) region was formed due to dilution of the Niobium and mixing with the Ti6Al4V. At the NiTi–Nb interface a eutectic reaction was responsible for joining. Mechanical testing of the joints revealed that the minimum tensile strength matched the ultimate tensile strength of the weakest material, Niobium. These results highlight new possibilities for the use of high melting point filler materials when joining NiTi to dissimilar materials, so that the formation of undesired phases can be avoided. [ABSTRACT FROM AUTHOR]

Ternary zinc-aluminum-magnesium (ZnAlMg) alloy coatings are the focus of significant attention in the automotive and steel industries due to several advantages over traditional Zn-based coatings. Currently, the literature on this type of coating is limited and focuses mainly on their corrosion resistance and room temperature tensile properties. To assess the relevance of ZnAlMg coatings in current manufacturing processes such as hot stamping and welding of advanced high strength steels (AHSS), it is essential to understand their high-temperature performance, particularly their resistance to liquid metal embrittlement (LME) cracking. This study showed that the ZnAlMg coating had complete resistance to LME cracking at a temperature of about 900 °C, which is traditionally recognized as the temperature at which the highest levels of LME susceptibility are observed in the different families of AHSS. Elemental distribution analysis confirmed that due to an increase in the testing temperature, the lamellar eutectic microstructure of the coating dissolved into the Zn-matrix, with the constituent elements, Al and Mg, segregating towards the steel substrate and the coating surface, respectively. This led to the in-situ formation of a uniform α-Fe(Zn, Al) layer at the steel/coating interface which prevented the direct contact of liquid metal with the steel substrate, resulting in complete suppression of LME at high temperature. Numerical calculations of interdiffusion flux were used to investigate the diffusion behavior of the elements of interest at the interface which indicated that the α-Fe(Zn, Al) layer formed due to the high diffusion rate of Al towards the Fe substrate at 900 °C. The effectiveness of the α-Fe(Zn, Al) layer in mitigating LME was evaluated by calculating the work of adhesion, which showed that this layer preserved its integrity under an applied tensile load, successfully mitigating the initiation of LME. The findings of this study offer valuable insights into developing new avenues for advancing LME resistant coatings by utilizing ternary Zn-based alloy systems. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: We use phase-field models to characterize the microstructure present in magnesium AZ31 alloy solidified under welding conditions. We focus our attention on the study of the conditions under which a columnar-to-equiaxed transition (CET) is observed in resistance spot welds. Our simulations show how the size and shape of the columnar and equiaxed regions depend on factors such as cooling rate, temperature gradient and the nature of inoculant particles. Our results are compared with experimental observations. In addition, we contrast our findings with predictions from a previously developed steady-state model for the CET. [Copyright &y& Elsevier]

Abstract: The influence of additives on the reaction kinetics and microstructure refinement in LiBH4–MgH2 composites is investigated in detail. Indications of the rate-limiting processes during the reactions are obtained by comparison of the measured reaction kinetics with simulations with one specific rate-limiting process. The kinetics of the sorption reactions are derived from volumetric measurements as well as from in situ X-ray diffraction measurements. During desorption, the hydrogen is released at a constant rate, which is possibly correlated with the one-dimensional growth of MgB2 platelets. In contrast, the kinetic curves of the absorption reactions exhibit the typical shape of contracting-volume controlled kinetics. The microscopical interpretation of kinetic measurements are supported by transmission electron microscopy images confirming the formation of additive-nanostructures in the grain boundaries upon cycling. The present investigations underline the importance of the additives as nucleation substrates and the influence of microstructure on the reaction kinetics. [Copyright &y& Elsevier]

Abstract: In the present work, the “additivity rule” was proved to be compatible with an analytical transformation model assuming mixed nucleation (with site saturation and continuous nucleation as two extremes), for some specified conditions. Combining the analytical model with numerical calculations, it is practicable to allow a conversion of continuous heating or cooling transformation (CHT or CCT) data into isothermal transformation data (TTT), and also a conversion of TTT data into CHT or CCT data, under a restrictive assumption of constant cooling or heating rate. From a comparative study between TTT and CHT data, the effective activation energy was found to increase and decrease, respectively, as compared with that before the CHT–TTT and TTT–CHT conversions; the growth exponent was proved to hold almost invariably, independent of the conversions. Real applications in the crystallization of amorphous Mg–Ni alloy and in the gamma–alpha massive transformation in Fe–Mn alloy show that the above conclusions are reasonable. [Copyright &y& Elsevier]