Positive Enthalpy Of Mixing Research Articles

Role of minor addition of metallic alloying elements in formation and properties of Cu–Ti-rich bulk metallic glasses

The effect of minor addition (MA) of metallic alloying elements in Cu–Ti-rich Cu–Ti–Zr–Ni–Si bulk metallic glasses (BMGs) has been investigated. MA of elements having a relatively small positive enthalpy of mixing (partial substitution of Zr with Nb) leads to enhancement of compressive plasticity (up to about 5% of fracture strain) when the addition leads to improvement in glass-forming ability (GFA). If the GFA is reduced (partial substitution of Ni with Ag or Co), the plasticity is also reduced. On the one hand, the MA of elements having a relatively large positive enthalpy of mixing (partial substitution of Zr with Y) can lead to the liquid-state phase separation in Cu–Ti–Zr–Ni–Si(–Sn) BMGs, although the addition can lead to drastic deterioration in GFA and plasticity. This concept would be considered to be effective even in design of other BMG systems with tailored properties.

Solid‐Solution Phase Formation Rules for Multi‐component Alloys

The factors of the atomic size difference Delta and the enthalpy of mixing ΔHmix of the multi-component alloys were summarized from the literatures. The formation zones of solid-solution phases, intermediate phases, and bulk metallic glasses were determined and the validity was verified by experimental results. For forming the solid solution, the alloys should have high entropy of mixing, lower Delta, and not too negative and positive enthalpy of mixing.

Improvement of plasticity by tailoring combination of constituent elements in Ti-rich Ti–Zr–Be–Cu–Ni bulk metallic glasses

The effect of replacement of Ti with Y or Nb in Ti-rich Ti–Zr–Be–Cu–Ni bulk metallic glasses (BMGs) has been investigated. The minor addition (MA) of Y (Y–Ti: +58 kJ/mol) induced phase separation into Y-rich crystalline particles and Ti-rich amorphous matrix, while the MA of Nb (Nb–Ti: +10 kJ/mol) led to nanocrystallization in Ti-rich BMGs with icosahedral nuclei. This result indicates that MA of elements having positive enthalpy of mixing can induce a different degree of instability in the single amorphous matrix depending on the amount of repulsive interaction energy. In particular, MA of Nb (up to 4 at.%) significantly increased the compressive fracture strain (ϵf) up to ∼9.35 ± 0.2%, which indicates that the plasticity of BMGs can be enhanced by the size-modulated icosahedral phase embedded in the amorphous matrix.

Plasticity criterion for bulk amorphous alloys

A plasticity criterion ( p-parameter), which enables the prediction of the plasticity of monolithic bulk amorphous alloys, is proposed. This criterion, which is defined as | λ − 0.18|Δ T x/ T x, takes into account the local packing state and structural stability characterized by the thermal properties of amorphous alloys. Bulk amorphous alloys with large p-values have more loosely packed and mechanically less stable microstructures, so that local structural evolution including nanocrystallization is promoted during the process of loading, leading to enhanced. The proposed criterion can be used as a means for predicting the plasticity of monolithic bulk amorphous alloys, which are devoid of a minor element with positive mixing enthalpy.

Synthesis of CaMg-based bcc alloys and their hydrogenation properties

We have demonstrated in our previous reports that the excess energy created during mechanical alloying (MA) yielded alloying of couples with even positive mixing enthalpy forming new ternary bcc phases. It was noticed that the important issue there was the structural correlation between the Laves and bcc structures. In this study, we successfully synthesized new ternary CaMg 2X (X = V, Fe, Mo) bcc alloys from the viewpoint of the structure correlation between CaMg 2 Laves and the bcc elements. It was found that the mechanical alloyed bcc phases were composed of nano-structured bcc grains and the synthesized alloys were able to absorb hydrogen.

Structure formation in liquid and amorphous metallic alloys

Bulk metallic glasses developed in last 15 years represent a new class of amorphous metallic alloys. These multi-component metallic alloys can be obtained at relatively low cooling rates, which allow the production of large-scale materials by conventional casting processes. Furthermore, bulk metallic glasses show a glass transition well below the crystallization temperature enabling hot deformation, but also to investigate the glass transition phenomenon in a metallic system. The thermal behavior of Zr- and Pd-based bulk metallic glasses was studied by in situ X-ray diffraction at elevated temperatures. The temperature dependence of the X-ray structure factor of the glassy state can be well described by the Debye theory. At the caloric glass transition the temperature dependence of the structure alters, pointing to a continuous development of structural changes in the liquid state. The short-range order of the glass, of the super-cooled liquid, and of the equilibrium melt is found to be very similar. The existence of complex chemically ordered clusters in the melt is supposed to be related to the high glass-forming ability of the alloys. The microstructure of metallic glasses consisting of elements with negative enthalpy of mixing is homogeneous at dimensions above 1 nm. Phase separation in the liquid state appears in metallic systems with large positive enthalpy of mixing of the elements like Nb–Y. Thermodynamic calculations of the Ni–Nb–Y phase diagram show that the miscibility gap of the monotectic binary Nb–Y system extends into the ternary up to large Ni content. Experimental evidence of the phase separation in ternary Ni–Nb–Y melts is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melt can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure depends on the chemical composition and consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer dimension. The experimental results confirm the close relationship between the structure of metallic glasses and the corresponding under-cooled liquids.

On the Effect of Nanocrystallization and Disorder on the Magnetic Properties of Cu-Rich, FeMnCu Alloys

We report on the temperature dependencies of the Mössbauer spectra and the AC magnetic susceptibility measured in Cu-rich, FeMnCu samples prepared by mechanically alloying, using, on the one side, Cu and prealloyed FeMn powders and, on the other, pure element Cu, Mn, and Fe powders. From the correlation of the Mössbauer and susceptibility data we conclude about the basic characteristics of the phase distributions present in the different studied samples. Those distributions are a consequence of both the nanostructure induced upon milling and of the different signs of the Mn/Cu (negative) and Fe/Cu (positive) enthalpies of mixing. The proposed phase distributions are significantly different in the samples prepared from different precursors and this fact is analyzed in terms of the disproportion of the precursor FeMn alloy and in those of the favoured Mn/Cu interdiffusion and the hindered Fe/Cu one.

On the Effect of Nanocrystallization and Disorder on the Magnetic Properties of Cu-Rich, FeMnCu Alloys

We report on the temperature dependencies of the Mössbauer spectra and the AC magnetic susceptibility measured in Cu-rich, FeMnCu samples prepared by mechanically alloying, using, on the one side, Cu and prealloyed FeMn powders and, on the other, pure element Cu, Mn, and Fe powders. From the correlation of the Mössbauer and susceptibility data we conclude about the basic characteristics of the phase distributions present in the different studied samples. Those distributions are a consequence of both the nanostructure induced upon milling and of the different signs of the Mn/Cu (negative) and Fe/Cu (positive) enthalpies of mixing. The proposed phase distributions are significantly different in the samples prepared from different precursors and this fact is analyzed in terms of the disproportion of the precursor FeMn alloy and in those of the favoured Mn/Cu interdiffusion and the hindered Fe/Cu one.

Calorimetric measurements of energetics of defect interactions in fluorite oxides

Direct measurement by oxide melt solution calorimetry of energetics of mixing in rare earth and yttrium doped zirconia, hafnia, and ceria systems provides support for spectroscopic and computational studies of the location and clustering of vacancies in these systems. Strongly negative heats of mixing are seen when the vacancy is transferred from being nearest neighbor to Y or RE in the sesquioxide to being nearest neighbor to Zr or Hf in the cubic solid solution. In the absence of such redistribution, small positive enthalpies of mixing are seen in CeO2-YO1.5 and CeO2-REO.15 systems. Strongly positive enthalpies of mixing are seen in CeO2-ZrO2, which has a large difference in cation sizes and no vacancy formation. The system Ce0.8Y0.2O1.9-Zr0.8Y0.2O1.9 shows small positive heats of formation with less destabilization in the Ce-rich region, suggestive of "scavenging" of oxygen vacancies by Zr. The calorimetric data obtained in these studies offer direct comparison with the results of computations on defect clusters and their binding energies.

The effect of Zr addition in glass forming ability of Ni–Nb alloy system

The effect of Zr addition on glass forming ability (GFA) of Ni–Nb alloys has been investigated. Ni 60Nb 40− x Zr x ( x = 0–40) alloys were prepared by melt-spinning and injection-casting. Crystallization behavior of the alloys was studied by differential scanning calorimetry (DSC), differential thermal analyzer (DTA) and X-ray diffractometry (XRD). The GFA increased with the addition of Zr, thus, a rod with the diameter of 2 mm was obtained in Ni 60Nb 30Zr 10 which has the highest reduced glass transition T rg value of 0.619 among the alloys investigated. When Zr content is more than 10 at.%, the GFA deteriorated significantly. The atomic structure of Ni–Nb–Zr alloys investigated by means of extended X-ray absorption fine structure (EXAFS), indicated presence of chemical inhomogeneity in the amorphous structure due to the positive mixing enthalpy between Zr and Nb.

Fe-based bulk glassy alloy composite containing in situ formed α-(Fe,Co) and (Fe,Co)23B6 microcrystalline grains

We report on an {[(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4}99.75Cu0.25 bulk glassy alloy (BGA) composite containing in situ formed α-(Fe,Co) and fcc-(Fe,Co)23B6 microcrystalline grains produced by copper mold casting. By adding 0.25 at. % Cu exhibiting positive mixing enthalpy with Fe, α-(Fe,Co) and fcc-(Fe,Co)23B6 phases with a grain size of 3μm precipitated from the melt during casting, yielding an in situ Fe-based BGA composite. The resulting Fe-based BGA composite shows not only a superhigh fracture strength of 4050MPa but also distinguished yielding combined with an enhanced plastic strain of 0.6% to failure in a compression relative to the [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 single-phase BGA.

Phase separation in liquid and amorphous Ni–Nb–Y alloys

Phase separation in Ni–Nb–Y metallic melts is treated by thermodynamic calculations. The miscibility gap of the monotectic binary Nb–Y system extends into the ternary Ni–Nb–Y system up to 70 at.% Ni as a consequence of the positive mixing enthalpy between Nb and Y. Experimental evidence of phase separation in the melt of Ni 58.5Nb 20.25Y 21.25 (numbers indicate at.%) is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melts of Ni 58.5Nb 20.25Y 21.25 and Ni 58Nb 10.1Y 30.9 can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer-dimension. The two amorphous phases crystallize separately. In a first step the cubic Ni 2Y phase is formed by a polymorphous reaction from the Y-rich amorphous phase. At much higher temperature the remaining Nb-rich amorphous phase crystallizes into Ni 7Nb 6.

The effect of Ta addition on the glass forming ability and mechanical properties of Ni–Zr–Nb–Al metallic glass alloys

The effect of Ta substitution for Zr in the Ni–Zr–Nb–Al metallic glass alloys has investigated by using thermal analysis, X-ray diffractometry and transmission electron microscopy. Small additions of Ta improve the glass forming ability (GFA) of the alloys Ni 61Zr 28− x Nb 7Al 4Ta x ( x = 0, 2, 4, 6, 8). The glass transition and crystallization temperatures increase with increasing Ta content up to 8 at.%. The liquidus temperature also increases, but less rapidly, resulting in the increase of T rg and γ values compared with the quaternary Ni–Zr–Nb–Al alloy. Fully glassy rods with the diameter of up to 2 mm can be fabricated by a copper mold casting method. The addition of Ta remarkably improves the plasticity under compression. The compressive strength and macroscopic plastic strain of the bulk metallic glass Ni 61Zr 24Nb 7Al 4Ta 6 alloy are 3080 MPa and 5%, respectively. The enhanced plasticity can be ascribed to local chemical inhomogeneity resulting from the existence of atom pair with positive enthalpy of mixing in the liquid state.

Microstructure and thermal behavior of two-phase amorphous Ni–Nb–Y alloy

A novel phase-separated Ni-based amorphous alloy is prepared in a glass forming system by rapid quenching of a Ni58.5Nb20.25Y21.25 melt. The microstructure consists of two amorphous Nb-enriched and Y-enriched regions with a size distribution from micrometer dimensions down to several nanometers. Decomposition takes place in the melt prior to solidification. The miscibility gap in the solid and liquid state of the binary Nb–Y system extends into the ternary Ni–Nb–Y alloy as a consequence of the positive mixing enthalpy between Nb and Y. The two-phase amorphous Ni58.5Nb20.25Y21.25 alloy crystallizes in a first step by the primary precipitation of the cubic Ni2Y phase from the Y-rich amorphous phase. At much higher temperatures the remaining Nb-rich amorphous phase crystallizes into Ni7Nb6.

Intermixing tendencies in garnets: Pyrope and grossular

The composition-temperature-pressure phase diagram of pyrope-grossular solid solutions, in which Mg and Ca are guest species in a highly rigid aluminosilicate framework, is predicted using simple models for the mixing enthalpy and the vibrational excitations. The models are expected to elucidate general phase separation tendencies in mixtures of guest species. Size mismatch of guest species is shown to give a positive enthalpy of mixing, and vibrational effects are shown to favor phase separation at the side of the smaller guest species, while at the side of the larger guest species mixing is favored. This gives rise to asymmetrical miscibility gaps that are displaced toward the side of the smaller species; in the case of pyrope-grossular, toward pyrope. It is shown that a realistic description of phase stability can be obtained using very few electronic density functional total energy calculations, making it of particular relevance for large unit-cell systems.

Nanosized mechanocomposites and solid solution in immiscible metal systems

Mechanical alloying is the most perspective method for preparation of nanosized mechanocomposites in non-mixed liquid and solid metals. It is known that plastic metals get fragile in the presence of the liquid metal phase, and this phase spreads over the surface of solid metal due to good wettability. Temperature of milling bodies can rise by several hundred degrees in the high-energy planetary ball mill. Low melting metals, such as gallium, indium, tin, and bismuth can melt on the surface of the balls. So, a pair composed of liquid and solid metal can form in an activator. In such cases, it can be assumed that mechanical activation of metal systems with positive mixing enthalpies, can allow forming a morphologically metastable structure with unusually high concentration of inter-phase boundaries due to easy-melting component spreading along the boundaries of particles formed by comminution of the solid component. Nanosized mechanocomposites were prepared for Cu-Bi and Fe-Bi systems. The content of Bi in these systems must be lower than 3 at. (10 wt.)%. The obtained mechanocomposites represent nanosized particles of copper (or iron) coated with a layer of bismuth in 2-3 atoms. Mechanical activation of the formed composites leads to the formation of supersaturated solid solutions.

Synthesis by ball milling and characterization of Al-Zn alloys

Mechanical ball milling is a useful technique for systems with positive enthalpy of mixing. With this technique solubility of a solute in a solid solution can be enhanced. Al-Zn system has positive heat of mixing. High energy ball milling has been employed to produce four alloys of Al with 2.5 to 10 wt% Zn. Powders of Al (1–125 μm) and Zn (0.7–5.0 μm) were mixed together in the desired proportion and milled with a powder to ball weight ratio of 1:20. The size and shape of the particles of as-received and alloy powders were examined in a scanning electron microscope (SEM) while their microanalysis was performed by energy dispersive system (EDS) attached with SEM. It has been observed that 120 h of milling of the powders produced homogeneous alloys. X-ray diffraction (XRD) patterns confirm complete solubility up to 10 wt% Zn in Al. Using the quasi-chemical theory of binary solid solutions, the enthalpy of mixing of 10 wt% Zn in Al has been determined to be 276 cals/mol. It is shown that stress exerted by very high density of dislocations, generated by mechanical milling, plays a major role in the enhancement of solubility. Hardness has been measured and it increases with increasing solute content.

Thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions

The thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions was investigated. Members of these series were either coprecipitated or synthesized hydrothermally and were characterized by XRD, FTIR, electron microprobe analysis, ICP-MS, and thermal analysis. Partial alkali substitution and vacancies on the Fe/Al sites were observed in all cases, and the solids studied can be described by the general formula K 1-x-yNa y(H 3O) xFe zAl w(SO 4) 2(OH) 6–3(3-z-w)(H 2O) 3(3-z-w). A strong preferential incorporation of Fe over Al in the jarosite/alunite structure was observed. Heats of formation from the elements, ΔH° f, were determined by high-temperature oxide melt solution calorimetry. The solid solutions deviate slightly from thermodynamic ideality by exhibiting positive enthalpies of mixing in the range 0 to +11 kJ/mol. The heats of formation of the end members of both solid solutions were derived. The values ΔH° f = −3773.6 ± 9.4 kJ/mol, ΔH° f = −4912.2 ± 24.2 kJ/mol, ΔH° f = −3734.6 ± 9.7 kJ/mol and ΔH° f = −4979.7 ± 7.5kJ/mol were found for K 0.85(H 3O) 0.15Fe 2.5(SO 4) 2(OH) 4.5(H 2O) 1.5, K 0.85(H 3O) 0.15Al 2.5(SO 4) 2(OH) 4.5(H 2O) 1.5, Na 0.7(H 3O) 0.3Fe 2.7(SO 4) 2(OH) 5.1(H 2O) 0.9, and Na 0.7(H 3O) 0.3Al 2.7(SO 4) 2(OH) 5.1(H 2O) 0.9 respectively. To our knowledge, this is the first experimentally-based report of ΔH° f for such nonstoichiometric alunite and natroalunite samples. These thermodynamic data should prove helpful to study, under given conditions, the partitioning of Fe and Al between the solids and aqueous solution.

Enthalpy of formation of LiNiO 2, LiCoO 2 and their solid solution, LiNi 1− xCo xO 2

LiCoO 2, LiNiO 2 and their solid solution, LiNi 1− x Co x O 2, are important cathode materials for lithium ion batteries. Samples in this system were synthesized by solid state reaction of Co 3O 4, NiO and Li 2CO 3 or LiOH·H 2O. Their lattice parameters were determined by Rietveld refinement. High temperature drop solution calorimetry in molten 3Na 2O·4MoO 3 and 2PbO·B 2O 3 solvents at 974 K was performed to determine the enthalpy of formation from the constituent oxides plus oxygen and the enthalpy of mixing in the solid solution series. There are approximately linear correlations between the lattice parameters, the enthalpy of formation from oxides (Li 2O, NiO and CoO) plus O 2 and the Co content in the compounds. The solid solution of LiCoO 2 and LiNiO 2 is almost ideal, showing a small positive enthalpy of mixing. The enthalpy of formation of LiCoO 2 from oxides (Li 2O, NiO and CoO) and oxygen at 298 K is −142.5±1.7 kJ/mol (from sodium molybdate calorimetry) or −140.2±2.3 kJ/mol (from lead borate calorimetry). That of LiNiO 2 is −56.2±1.5 kJ/mol (from sodium molybdate calorimetry) or −53.4±1.7 kJ/mol (from lead borate calorimetry). The cobalt compound is thus significantly more stable than its nickel analogue. The phase assemblage LiCoO 2, Li 2O and CoO is seen at a lower oxygen pressure at constant temperature than the assemblage Co 3O 4/CoO, reflecting the stabilization of Co(III) in the ternary Li–Co–O system.

Accelerated mechanical alloying of mutually insoluble metals: Co-Cu system

We propose a new method for accelerating the process of mechanical alloying in the Co-Cu system possessing positive enthalpy of mixing and exhibiting no mutual solubility of components under equilibrium conditions. For this purpose, highly disperse powders of composite particles representing a Co-P amorphous alloy core covered with a nanocrystalline copper shell were prepared by chemical deposition. These composite powders were mechanically alloyed by processing in a ball mill. Investigation of the atomic structure and magnetic properties of the composite powders upon milling showed that the formation of supersaturated Co-Cu solid solutions under such conditions requires a much shorter milling time as compared to that for the conventional mechanical alloying processes.