Transmission electron microscopy and thermodynamic studies of CaO-added AZ31 Mg alloys

We investigated the microstructural evolution of Mg–3Al–1Zn (AZ31) alloy systems, with Ca or CaO added, by carrying out microstructural characterizations in conjunction with thermodynamic calculations. A calculated phase diagram of the Mg–Ca–O ternary system showed that CaO can be dissolved in liquid Mg so as to have 12.6 wt.% Ca content in the liquid Mg at 700 °C. Therefore, for a 0.3 wt.% CaO-added AZ31 alloy, our thermodynamic calculation predicted a similar precipitation pathway to that of a 0.3 wt.% Ca-added AZ31 alloy during the solidification process. In fact, a thermodynamic analysis of the precipitation pathway assuming the Scheil model showed that the major precipitates in both alloys were Al₈Mn₅, CaMgSi, Laves C15 and Laves C36, in good agreement with our experimental observation. However, a microstructural characterization of the as-cast alloys using transmission electron microscopy revealed that the spatial distribution of the precipitates was significantly different in the two alloy systems; unlike in the Ca-added AZ31 alloy, the Ca-containing precipitates in the CaO-added AZ31 alloy exhibited strong agglomeration tendencies. Moreover, in an alloy solidified at a faster cooling rate, undissolved CaO particles were observed in the precipitate agglomerates that were connected to the other Ca-containing precipitates. These results suggest that an incomplete dissolution of CaO particles in the liquid results in the agglomeration of precipitates, as the undissolved CaO particles can act as local sources, supplying Ca to the liquid, and can thus act as preferential nucleation sites for the Ca-containing precipitates forming during the solidification of the alloy.     

Effect of iron content on the formation of β-Al5FeSi and porosity in Al–Si eutectic alloys

A preliminary investigation has been carried to evaluate the influence of Fe on Sr-modified and unmodified eutectic Al–Si alloys in as-cast and heat treatment conditions. The castings were produced in zircon-coated steel permanent mold and were solutionized at 500 °C for 8 h and followed by artificial aging at 155 °C for 5 h, i.e., T6-temper. The microstructure changes in the β-Al₅FeSi particle morphology were analyzed. The results indicate that dendrite arm spacing is strongly related to the cooling rate rather than the chemical composition, increasing the iron content leads to increase porosity and hardness either in the as-cast condition or after T6-temper. The Sr-modified alloys have higher hardness than unmodified at all Fe-added values. The precipitated long branched β-platelets result in the formation of large shrinkage cavities due to the inability of liquid metal to feed the space between them during solidification.     

In situ and real-time 3-D microtomography investigation of dendritic solidification in an Al–10 wt.% Cu alloy

The microstructural evolution of an Al–10 wt.% Cu alloy was investigated during solidification at constant cooling rate by in situ synchrotron X-ray microtomography with a resolution of 2.8 μm. Solidification of this alloy leads to a coarse dendritic microstructure which was fully characterized in terms of variation with temperature of the solid fraction, the specific surface area of the solid–liquid interface and the local curvatures of the solid phase. By analysing the evolution with solid fraction of individual dendrites, at least two coarsening mechanisms were clearly identified in addition to solidification growth. The first mechanism involves remelting of small secondary dendrite arms to the benefit of bigger adjacent arms. The second is the coalescence of adjacent secondary arms, with progressive filling of the inter-arm spacing and coalescence at the tips. Although this mechanism preferentially occurs at high solid fractions, these results show that the evolution of the dendritic microstructure during solidification is complex and involves the occurrence of various mechanisms operating concurrently. In situ X-ray tomography thus allows revisiting the various models which have been proposed to account for dendrite coarsening during solidification.     

Microstructure and magnetic properties of FeMoBCu alloys: Influence of B content

Fe_91–xMo₈Cu₁B_x (x = 12, 15, 17, 20) amorphous and nanocrystalline alloys were studied to examine the influence of B content on their microstructure and magnetic behaviour. Changes in the magnetic properties provoked by microstructural evolution upon thermal treatments of as-cast samples were also analyzed. Nanocrystallization kinetics can be described by an isokinetic approach except for the 20 at.% B content alloy. The Curie temperature of the amorphous as-cast samples increases with the alloy’s B content. Mössbauer results suggest the presence of Mo atoms in the nanocrystals. Crystalline volume fraction and mean grain size of the nanocrystals at the end of the nanocrystallization process are higher for the lowest B content alloy. The 20 at.% B content alloy develops a boride phase just after the early stages of the nanocrystallization process, which provokes a magnetic hardening in this alloy. The softest magnetic behaviour of the studied compositions corresponds to nanocrystallized 17 at.% B content alloy.     

Three-dimensional study of Ni aluminides in an AlSi12 alloy by means of light optical and synchrotron microtomography

The three-dimensional (3D) microstructure of an AlSi12Ni alloy in as-cast and in solution-treated conditions is characterized by light optical and synchrotron tomography. Eutectic Al–Si alloys containing 1 wt.% Ni in as-cast condition present networks of connected Si lamellae as well as complex 3D shapes of Ni-containing aluminides. The eutectic Si networks disintegrate during solution treatment in the binary Al–Si alloy while they remain connected in the Al–Si–Ni alloy. The contiguity between eutectic Si and Ni-containing aluminides is maintained, when the alloy is solution treated at 540 °C for 24 h. The sphericity of the aluminides is only slightly increased by the solution treatment. The reinforcing role of eutectic Si together with the Ni-containing aluminides is governed by a stable interconnectivity and contiguity of these rigid phases accumulating ～20 vol.%. The 3D data obtained by synchrotron tomography quantify connectivity, shape and volume fraction of eutectic Si and aluminides, whereas their contiguity is verified by light optical sectioning tomography.     

The effect of Mg on the structure and properties of Type 319 aluminum casting alloys

The precipitation hardening behavior of a Type 319 aluminum alloy (Al–6.7 wt.% Si–3.75 wt.% Cu) with and without 0.45 wt.% Mg has been investigated. It is shown that both θ and Q phase exist in the as-cast samples and are dissolved during solution heat treatment. Subsequent artificial aging results in the precipitation of both the metastable θ′ phase typical of aged alloy 319 and, in the case of the Mg-containing alloy, an additional and extremely small phase, which was identified as Q′ based on previous literature. Both types of precipitates have been characterized using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) tomography. The Mg-containing alloy exhibits higher strength and lower ductility, presumably due to both solid solution strengthening and the presence of the fine Q′ phase.     

The evolution of the growth morphology in Mg–Al alloys depending on the cooling rate during solidification

The comprehensive microstructural evolution of Mg–3, 6 and 9 wt.% Al alloys with respect to the solidification parameters such as thermal gradient (G), solidification velocity (V), cooling rate (G·V) and solute (Al) content were investigated in the present study. Various solidification techniques, including directional solidification, wedge casting, sand and graphite mould casting, gravity casting in a Cu mould and water quenching, were employed in order to obtain wide ranges of cooling rates between 0.05 and 1000 K s^–1. The microstructural length scales of Mg–Al alloys, such as secondary dendrite arm spacing and primary dendrite arm spacing, were determined experimentally and compared with published models. In addition, the solidification parameters of morphological transitions such as cellular to columnar dendrite and columnar to equiaxed dendrite were also determined. Based on all the experimental data and the solidification model, a solidification map was built in order to provide guidelines for the as-cast microstructural features of Mg–Al alloys.     

Investigation the effect of B4C addition on properties of TZM alloy prepared by spark plasma sintering

TZM alloy is one of the most important molybdenum (Mo) based alloy which has a nominal composition containing 0.5–0.8 wt.% titanium (Ti), 0.08–0.1 wt.% zirconium (Zr) and 0.016–0.02 wt.% carbon (C). It is a possible candidate for high temperature applications in a variety of industries. However, the rapid oxidation of TZM alloys at high temperature in air is considered to be one of the drawback. In this study, TZM alloys with additions of 0–5 wt.% B₄C were prepared by spark plasma sintering (SPS) at 1420 °C utilizing 40 MPa pressure for 5 min under vacuum. The effects of B₄C addition on oxidation, densification behavior, microstructure, and mechanical properties were investigated. The TZM alloy with 5 wt.% B₄C have exhibited an approximately 66% reduction in mass loss under normal atmospheric conditions in oxidation tests made at 1000 °C for 60 min. And an increase from 1.9 GPa to 7.8 GPa has been determined in hardness of the alloy.     

Columnar to equiaxed transition in directional solidification of inoculated melts

Experiments on transient directional solidification were carried out with Al–3% Si and Al–7% Si alloys to study the modification by melt inoculation of the as-cast ingot macrostructure with columnar and equiaxed zones into a completely refined equiaxed structure. Without inoculation the macrostructure consists of typical columnar and equiaxed zones, separated by a relatively thin region of columnar to equiaxed transition. As the inoculant Al–3% Ti–1% B was added to the melt in a series of experiments relatively small equiaxed grains were observed in the as-cast macrostructure. For larger inoculant additions the number of these equiaxed grains increased. The macrostructure became completely refined and equiaxed as the Ti concentration in the melt increased in the small range 0.002 < %Ti < 0.01 for Al–3% Si, and of 0.029 < %Ti < 0.075 for Al–7% Si. The larger inoculant additions for the Al–7% Si alloy are attributed to Si poisoning of the inoculant. This macrostructure modification occurred with an increasingly large fraction of fine equiaxed grains mixed with columnar grains, rather than by a significant decrease in the length of the columnar grains. Solidification paths were calculated from the measured cooling curves and superimposed on dimensionless growth maps, enabling good qualitative predictions of the macrostructure, especially the existence of the mixed region. The maps, however, predict that the position where columnar grains are completely blocked is closer to the ingot base than that observed experimentally. This discrepancy might be related to the restrictive hypothesis assumed to construct the growth maps, not valid in the present experiments, of steady-state solidification conditions.     

A DSC analysis of thermodynamic properties and solidification characteristics for binary Cu–Sn alloys

The liquidus temperatures and enthalpies of fusion for Cu–Sn alloys are systematically measured across the whole composition range by differential scanning calorimetry (DSC). The liquidus slope vs. Sn content is derived on the basis of the measured results. The measured enthalpy of fusion is related to the Sn content by polynomial functions, which exhibit one maximum value at 55 wt.% Sn and two minimum values around 28.9 wt.% Sn and 90 wt.% Sn, respectively. The undercoolability of those liquid alloys solidifying with primary α (Cu) solid solution phase is stronger and can be further enhanced by increasing the cooling rate. However, other alloys with the preferential nucleation of intermetallic compounds display smaller undercoolings and are not influenced by cooling rate. Microstructural observations reveal that peritectic reactions can rarely be completed. With the increase in undercooling, the primary α (Cu) dendrites are refined in the peritectic Cu–22 wt.% Sn alloy. For the hyperperitectic Cu–70 wt.% Sn alloy, typical peritectic cells are formed in which the peritectic η(Cu₆Sn₅) phase has wrapped the primary ε(Cu₃Sn) phase. The DSC curves of metatectic-type Cu–Sn alloys indicate that the metatectic transformation γ → ε + L upon cooling is an exothermic event, and a large undercooling of 70 K is required to initiate this transformation in metatectic Cu–42.5 wt.% Sn alloy. The metatectic microstructures are characterized by (ε + η) composite structures. The η phase is mainly distributed at the grain boundaries of the coarse ε phase, but are also dispersed as small particles inside ε grains. The volume fraction of the η phase increases with the Sn content.     

Low temperature superplasticity in a friction-stir-processed ultrafine grained Al–Zn–Mg–Sc alloy

Friction stir processing (FSP) was used to create a microstructure with ultrafine grains (0.68 μm grain size) in an as-cast Al–8.9Zn–2.6Mg–0.09Sc (wt.%) alloy. The ultrafine grained alloy exhibited superplasticity at relatively low temperatures and higher strain rates. Optimum ductility of 1165% at a strain rate of 3 × 10⁻² s⁻¹ and 310 °C was obtained. Enhanced superplasticity was also achieved at a temperature as low as 220 °C. Experimentally observed parametric dependencies and microstructural examinations indicated that the operating deformation mechanism might be the Rachinger grain boundary sliding accommodated by intragranular slip. The FSP microstructure became highly unstable at 390 °C onwards, thus, affecting ductility adversely. In situ transmission electron microscopy heating was used to understand the instability phenomenon, which has been attributed to the drop in particle pinning forces due to the dissolution of metastable precipitates and microstructural heterogeneity.     

Equilibrium phase of high-entropy FeCoNiCrCu0.5 alloy at elevated temperature

The phase transformations of FeCoNiCrCu_0.5 alloy with the as-cast structure and heat-treated structures were studied. The as-cast alloy specimens were first heated at 1050 °C with a holding time of 1 h. Serial heat-treatment processes at 350 °C, 500 °C, 650 °C, 800 °C, 950 °C, 1100 °C, 1250 °C and 1350 °C with a holding time of 24 h were then carried out to understand the phase evolution and the relationship between the microstructure and the hardness of the specimens. The microstructures were investigated and chemical analyses performed by optical microscopy (OM), scanning elector microscopy (SEM), X-ray diffractometer (XRD) and transmission elector microscopy (TEM). The results show that FCC peaks were observed from the X-ray diffraction of the as-cast specimens and a precipitate phase was present in the specimens that had been heated to 950 °C. The hardness of the FeCoNiCrCu_0.5 alloy remained unchanged in the specimens that underwent various heat treatments that were applied in this study.     

Microsegregation in high Nb containing TiAl alloy ingots beyond laboratory scale

Microsegregation in big ingots of Ti–45Al–(8–9)Nb–(W, B, Y) alloy had been studied. The composition and microstructural morphology of the large ingot exhibited significant microinhomogeneity. Three types of microsegregation were observed in as-cast microstructure of the large ingot. First is the solidification segregation (S-segregation) at interdendritic area, in which the composition is characterized by higher Al, B (boride), and Y (oxide) contents and lower Nb and W contents. Second is the β-segregation at the boundary and triple junctions among α grain due to the phase transformation of β → α. The composition at the segregation area is characterized by higher Nb and W additions that lead to the formation of β particles and γ phase. Third is the α-segregation that forms local lamellar structure composed of β, γ and α plates due to phase transformation of α → α₂ + β + γ. The microsegregation for the PAM ingot is lower than that for SM ingot in terms of the volume fraction of β phase. The reason is that the PAM melting can offer better control of pouring temperature and rather fast cooling rate by water-cooled copper crucible.     

Alloy design concepts for refined gamma titanium aluminide based alloys

The influence of the Al content and the addition of further alloying elements on the cast microstructure of γ(TiAl) + α₂(Ti₃Al) alloys has been examined. The results show that particularly fine and homogeneous microstructures without strong segregation can be obtained for certain alloy compositions solidifying through the β phase. This behavior can be attributed to the avoidance of peritectic solidification and to the alloying influence on the kinetics of the β ⇒ α transformation following solidification. The experimental findings were used to propose a design concept for γ-TiAl + α₂-Ti₃Al alloys. This concept aims at the production of high-quality castings as well as at ingot material for wrought processing routes because the chemically homogeneous and fine-grained microstructures would be a good precondition for improved workability.     

Thermal properties of W–Cu composites manufactured by copper infiltration into tungsten fiber matrix

W–Cu composites were produced by the technique of copper infiltration into tungsten fiber preforms (CITFP) under vacuum circumstance. Fibrous structure preforms with various volume fraction of tungsten fiber were fabricated by the process of mold pressing and sintering. The molten copper was infiltrated into the open pores of the preforms under vacuum at 1473 K to 1573 K for 1 h to produce W–Cu composites with compositions of 10–30 wt.% copper balanced with tungsten. The microstructure, relative densities, and thermal properties of the composites were investigated and measured. The relative as-sintered density was enhanced with the increase of the sintering temperature. The thermal conductivity of the W–Cu30 composite with 28.2 wt.% Cu was 241 W/(m · K) at 298 K, 10% higher than that of the W–Cu alloy with similar copper content produced by conventional powder metallurgy process. The thermal expansion of the composites was decreased with the increase of tungsten content, keeping the same tendency as the prediction by the rule of weighted average of volume ratio of compositions.     

Transition of solidification mode and the as-cast γ grain structure in hyperperitectic carbon steels

Formation processes of as-cast γ grain structures during casting of hyperperitectic carbon steels with 0.15–0.45 mass% carbon concentrations have been studied by means of a rapid unidirectional solidification technique. In steels with 0.15–0.41 mass% carbon concentrations, coarse columnar γ grains (CCGs) with a minor axis diameter of 1–3 mm developed along the direction of temperature gradient. In a steel with 0.38 mass% carbon, importantly, columnar γ grains (CGs) whose minor axis diameter is less than 500 μm form before the formation of CCGs and the grain structure changes discontinuously from CG to CCG. The fraction of the CG region increases with an increase in the carbon concentration. In the samples with a carbon concentration higher than 0.43 mass%, the as-cast structure consists of CGs over almost the entire ingots. Analyses of the relation between γ grain and dendrite structures and their crystallographic orientations indicate that the formation of CGs originates from the primary solidification of γ phase instead of δ phase. This is supported by numerical analysis of the dendrite growths.     

Influence of cooling rate on the microstructure and ageing behavior of as-cast Al–Sc–Zr alloy

Al–0.3Sc–0.15Zr alloy was cast using copper die, insulated alumina mould, and conventional investment shell mould to obtain a wide range of cooling rates. A novel method of quenching the investment shell mould along with the liquid metal in oil was also used which resulted in a significant increase in the cooling rate. The order in increasing average cooling rate is 0.16, 0.78, 1.28, 5.93, 7.69 °C/s. The as-cast samples were aged isothermally at 300 °C and various temperatures for 2 h. Slow cooled samples (in alumina-insulated mould) showed the presence of as-cast primary precipitates as well as rod shaped discontinuous precipitates with high density of interfacial dislocation. The amount of as-cast precipitates decreased with increase in the cooling rate. These as-cast precipitates grew at the expense of Sc in solid solution reducing the number of precipitates formed during ageing process. This results in lower increment in hardness on ageing.     

Hot tearing in polycrystalline Ni-based IN738LC superalloy: Influence of Zr content

Zirconium is always present in Ni base superalloys as it enhances their creep properties. In the present study, the influence of very small Zr additions, 100–400 ppm, i.e. 0.01–0.04 wt.%, on hot tearing of IN738LC superalloy is experimentally investigated using dedicated turbine blade castings. Although the Zr content remains very small, it has a strong effect on hot tearing tendency. Microstructure of hot tear in as-cast samples reveal that grain size and secondary dendrite arm spacing have no significant effect on hot tearing. On the other hand eutectic phase volume fraction and its dispersion or spreading along grain boundaries drastically affect the hot tearing propensity and strongly increase with increasing amounts of Zr. Hence grain coalescence becomes impossible at grain boundaries covered with eutectic phase films. With increasing Zr content, gain coalescence between two distinct grains with no interdendritic phase requires more undercooling. Coalescence is retarded and occurs deeper in the mush zone, i.e. at lower temperatures resulting in a higher sensitivity to hot tearing. Finally, it is shown that a reduction of Zr content to 0.02 wt.% is required to fully suppress hot tearing in polycrystalline IN738LC blades.     

Co–Cr–Mo-based composite reinforced with bioactive glass

The powder metallurgy (PM) technology was used to produce a porous Co–Cr–Mo-based composite material with the bioactive glass (S2) addition of 5 wt.%, 10 wt.% and 15 wt.%. The results show that the addition of bioglass to the matrix of Co–Cr–Mo alloy, as well as rotary cold repressing and heat treatment of sintered specimens can cause significant changes in the microstructure, mechanical and corrosion properties of composite materials in comparison with the pure porous Co–Cr–Mo alloy. A significant increase in the hardness, yield strength and corrosion resistance of the composites was observed with increasing the bioglass volume fraction. Although all PM samples are in a passive state, the higher corrosion resistances were obtained in the case of the composites with bioglass additions. Superior mechanical properties were achieved in the case of composite with 10 wt.% of bioglass.