Pressure–composition–temperature hysteresis in C14 Laves phase alloys: Part 2. Applications in NiMH batteries

This part of our series of papers reports our efforts to predict sealed cell cycle life from lattice constant a/c ratio and/or PCT hysteresis based on correlations established from our previous studies reported in Part 1. In the case of Ti₂₁Zr_12.5V₁₄Ni_21.5−xCr₁₀Mn₂₁Sn_x alloys, as x is increased the a/c ratio increases and the PCT hysteresis decreases resulting in extended cycle life for the battery. This is in agreement with the results from Part 1. In the case of Ti₁₈Zr_15.5V₁₄Ni_24.2−xCr₁₀Mn_18−ySn_0.3Co_xAl_y alloys, the a/c ratio trend was not observed due to existence of more than one C14 phase, but the correlation between PCT hysteresis and cycle life was maintained. In the case of Ti₂₁Zr_12.5V₁₀Ni_40.2Cr_8.5−xMn_5.6−yCo_1.5+x+yAl_0.4Sn_0.3 alloys, the cycle life was dominated by the corrosive nature of the alloy affected by the Cr content. As a consequence, predictions from a/c ratio and PCT hysteresis can only be applied to the degree of pulverization in gas phase cycling and not to the sealed cell cycle life.     

Pressure–composition–temperature hysteresis in C14 Laves phase alloys: Part 1. Simple ternary alloys

In Part 1 of a series of two papers, the a/c lattice constant ratio of ZrCr₂-based ternary alloys are shown to be strongly correlated not only to the number of outside electrons of substitutional elements but also to the PCT absorption/desorption hysteresis and the degree of pulverization during hydride/dehydride cycling of the alloy. In differentiation from AB5 alloys in which elongating the c-axis dimension of the unit cell extends the alloy's electrochemical cycle life, flattening the unit cell of an AB2 alloy extends its cycle life. This difference can be explained by the different hydrogen occlusion sites of the two structures. Adding small amounts (10%) of substituents such as Zn, Cr, Mo, Si, or Cu, was generally found to help the prevention of the alloy hydride/dehydride pulverization by maintaining a relatively high a/c lattice constant ratio. Application of these principles to more complicated electrochemical hydrogen storage alloys can be found in Part 2 of this series.     

Point defects in binary Laves phase alloys

Point defects in the binary C15 NbCr₂ and NbCo₂, and C14 NbFe₂ systems on both sides of stoichiometry were studied by bulk density and X-ray lattice parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000°C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with model predictions. Thermal vacancies exhibiting a maximum at the stoichiometric composition were observed in NbCr₂ Laves phase alloys after quenching from 1400°C. However, there are essentially no thermal vacancies in NbFe₂ alloys after quenching from 1300°C. Anti-site hardening was found on both sides of stoichiometry for all the three Laves phase systems studied. Neither the anti-site defects nor the thermal vacancies affect the fracture toughness of the Laves phases significantly.     

Pressure–composition–temperature hysteresis in C14 Laves phase alloys: Part 3. Empirical formula

In Part 1 and Part 2 of this series of papers, the pressure–concentration–temperature (PCT) isotherms hysteresis was found to be closely related to the axial ratio a/c for both simple ternary and more complicated multi-element C14 Laves phase based alloys. Furthermore, the particle pulverization rate, which is the major determining factor in the duration of metal hydride electrode cycling, was found to correlate well with PCT hysteresis. In the current Part 3, we discuss an empirical equation which was developed to predict the PCT hysteresis of battery alloys through the study of the lattice constant ratios of a series of ZrCr₂-based ternary alloys. The empirical formula can then be used to estimate the pulverization rate of metal hydride electrode. To fit the empirical formula, an equivalent number of outer shell electrons for some non-transition metals was calculated from the axial ratio of ZrCr_1.8M_0.2 ternary alloys, where M is an element from the group of Al, Si, Ga, Ge, and Sn. Other factors, such as the amount of substitution, the difference in A and B element electronegativities, atomic size, and the choice of A element, were also investigated.     

Local metal and deuterium ordering in the deuterated ZrTiNi C14 Laves phase

Local structure of the hexagonal C14 Laves phase in an annealed approximately equiatomic ZrTiNi alloy was studied before and after deuteration using neutron total scattering. Rietveld refinements of the P6₃/mmc AB₂ model demonstrate that the A-sites are shared by Zr and Ti, whereas the two non-equivalent B-sites are occupied by a mixture of Ti and Ni. Reverse Monte Carlo refinements using a joint fit of the neutron total scattering data in real and reciprocal spaces revealed significant short-range ordering of Ti and Ni on the B-sites. The refined Ti-Ni nearest-neighbor distances are appreciably shorter than the corresponding Ti-Ti and Ni-Ni distances. Nevertheless, the differences in the effective sizes of chemically distinct tetrahedral interstices (i.e. [Zr₂Ti₂], [Zr₂TiNi], [Zr₂Ni₂], etc.), which are important for hydrogen absorption, remain rather small. Rietveld refinements of the structure of the deuterated sample TiZrNiD_2.14 confirmed that deuterium occupies various [A₂B₂] tetrahedral interstices. RMC refinements suggested a strong preferential occupation of deuterium in the [Zr₂Ti₂] and [Zr₂TiNi] tetrahedra whereas the [Zr₂Ni₂] sites remained nearly empty. These results support earlier models which predicted preferential occupancy by hydrogen in sites coordinated by metals that form the most stable binary hydrides. The deuterium atoms in the [Zr₂TiNi] tetrahedra are displaced toward Ni. X-ray absorption near-edge structure measured for Ti, Zr, and Ni K-edges demonstrated that deuteration is accompanied by the reduction in the density of unoccupied electronic states (just above the Fermi level) associated with Ti and Zr.     

Laves phase in Fe-Cr-Co alloys containing molybdenum and niobium

Conclusion Additional alloying of Fe–Co–Cr alloys containing molybdenum with niobium in an amount greater than 1% leads to formation of MgNi₂ type Laves phases in them, which markedly restrains grain growth during annealing in the temperature range 1000–1250°C.Institute of Steels and Alloys, Moscow. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, p. 58, January, 1987.     

Cobalt-free over-stoichiometric Laves phase alloys for Ni–MH batteries

The charge–discharge cycling behavior of the over-stoichiometric Laves phase alloy Zr_0.75Ti_0.25V_0.9Mn_0.4Cr_0.3Ni_1.4 as hydride electrode has been studied in a negative electrode-limited sealed cell. This cobalt-free alloy shows a maximum discharge capacity C_max=373 mAh g⁻¹ at 160 mA g⁻¹ discharge current and a high rate dischargeability of 285 mAh g⁻¹ at 1500 mA g⁻¹ discharge rate. After 600 cycles the discharge capacity is 81% of the C_max; the alloy also shows good charging efficiency (98%) and low temperature discharge rate.     

Laves phase hydrogen storage alloys for super-high-pressure metal hydride hydrogen compressors

Ti-Cr- and Ti-Mn-based alloys were prepared to be low- and high-pressure stage metals for a double-stage super-high-pressure metal hydride hydrogen compressor. Their crystallographic characteristics and hydrogen storage properties were investigated. The alloy pair Ti_0.9Zr_0.1Mn_1.4-Cr_0.35V_0.2Fe_0.05/TiCr_1.55Mn_0.2Fe_0.2 was optimized based on the comprehensive performance of the studied alloys. The product hydrogen with a pressure of 100 MPa could be produced from 4 MPa feed gas when hot oil was used as a heat reservoir.     

Laves phase hydrogen storage alloys for super-high-pressure metal hydride hydrogen compressors

Ti-Cr- and Ti-Mn-based alloys were prepared to be low- and high-pressure stage metals for a double-stage super-high-pressure metal hydride hydrogen compressor. Their crystallographic characteristics and hydrogen storage properties were investigated. The alloy pair Ti<,0.9>Zr<,0.1>Mn<,1.4>- Cr<,0.35>V<,0.2>Fe<,0.05>/TiCr<,1.55>Mn<,0.2>Fe<,0.2> was optimized based on the comprehensive performance of the studied alloys. The product hydrogen with a pressure of 100 MPa could be produced from 4 MPa feed gas when hot oil was used as a heat reservoir.     

Physical metallurgy and mechanical properties of transition-metal Laves phase alloys

This paper provides a comprehensive review of the recent research on the phase stability, point defects, and fracture toughness of AB₂ Laves phases, and on the alloy design of dual-phase alloys based on a soft Cr solid solution reinforced with hard XCr₂ second phases (where X=Nb, Ta and Zr). Anti-site defects were detected on both sides of the stoichiometric composition of NbCr₂, NbCo₂, and NbFe₂, while they were observed only on the Co-rich side of ZrCo₂. Only thermal vacancies were detected in the Laves phase alloys quenched from high temperatures. The room-temperature fracture toughness cannot be effectively improved by increasing thermal vacancy or reducing stacking fault energy through control of phase stability. Microstructures, mechanical properties, and oxidation resistance of dual-phase alloys based on Cr–NbCr₂, Cr–TaCr₂, and Cr–ZrCr₂ were studied as functions of heat treatment and test temperature at temperatures to 1200°C. Among the three alloy systems, Cr–TaCr₂ alloys possess the best combination of mechanical and metallurgical properties for structural use at elevated temperatures.     

含TiCr2 Laves相过共析钛铬合金的制备

通过非自耗磁控电弧炉熔铸和 70 0℃ ,2 0h高温退火处理 ,制备出了含铬量为 18%～ 30 % (质量分数 )的含TiCr2 Laves相过共析钛铬合金 ,并研究了合金的组织变化规律。结果表明 ,稳定化系数为 2 .5 7～ 4 .6 2的过共析钛铬合金经过熔炼后 ,在随炉冷却条件下得到的是单相 β Ti组织 ;铸态合金在 70 0℃保温退火时 ,金属间化合物TiCr2 不仅沿晶界生成并形成连续分布 ,还在基体内部弥散析出 ;在随后的空冷过程中合金内局部发生 β Ti→α Ti TiCr2 共析分解。合金含铬量越高 ,在高温退火时析出的TiCr2 量越多 ,尺寸越粗大 ,合金的硬度也越高。电弧熔炼 70 0℃ ,2 0h高温退火是一种制备含Laves相过共析钛铬合金的可行工艺     

Electrode characteristics of C14-type Zr-based laves phase alloys

The thermodynamic and electrochemical properties of ZrCr_0.8M_0.2NiMm_0.05 (M V, Mn, Fe, Co; Mm, misch metal) and Zr_1−χTi_χCrNiMm_0.05 (χ = 0, 0.1, 0.2) are investigated by measuring the pressure—composition isotherms using a Sieverts-type apparatus and the electrode characteristics of the Ni—metal hydride (MH) battery. From the results it is found that the substituted alloying elements Fe, Co and Ti are effective in modifying the hydrogen desorption pressure. When Mn or V is substituted for Cr, although it does not have a significant influence on these properties, the dischargeability at low temperatures and the rate capability are greatly improved. This improvement may be due to the increase in the surface area of the metal hydride alloy in the electrode, which is caused by the decrease in particle size or the dissolution of V into the KOH solution. On the basis of the effects of the alloying elements on the thermodynamic and electrochemical properties of ZrCrNiMm_0.05, the alloy Zr----Ti----Cr----Mn----V----Fe(or CO)----NiMm_0.05 is found to be suitable for the anode material in Ni----MH batteries. This alloy, which has the C14 hexagonal structure, shows high discharge capacities (300–370 mA h g⁻¹), good dischargeabilities at low temperatures and good rate capabilities.     

The native point defects in C14 Mg2Ca Laves phase: A first-principles study

The native point defects in C14 Mg₂Ca Laves phase are studied from the first-principles density functional theory calculations within GGA approximation. The defect formation energies indicate that anti-site defects are energetically favored over vacancies. Under Mg-rich and even general Ca-rich condition, defect Mg_Ca of Mg anti-site on Ca sublattice is favorable owing to the lowest formation energy. The Ca_Mg2 defect of Ca anti-site on Mg2 sublattice is also likely dominant only under extreme Ca-rich environment. The present results could explain reasonably the asymmetric off-stoichiometry of Mg₂Ca. The effective point defect concentrations of Mg₂Ca as a function of composition and temperature at experimental range are also calculated from a canonical statistical model, and the derived results show a linear relationship between the logarithm of defect concentration and T⁻¹. Geometrical factor is further studied, and it is found that atomic size possesses an obvious influence on the structure of point defect in Mg₂Ca. The electronic feature is further studied to reveal underlying mechanism for formation of point defects.     

HRTEM observation of silicides and Laves phase precipitates in Nb-Ti-Si based alloys

The morphologies, crystallographic characteristics and formation processes of silicides and Laves phase precipitates in Nb-Ti-Si based alloys have been systematically investigated by high-resolution transmission electron microscopy (HRTEM) observation. Silicide precipitate of δNb₁₁Si₄ usually exhibits homogeneous distribution with acicular and tetrapod morphologies, possessing uniform orientation relationships (ORs) with Nbss. The occurrence of these morphologies might be caused by the symmetry decrease during the phase transformation of Nbss ⟶ δNb₁₁Si₄ + Nbss₁. In some cases, the δNb₁₁Si₄ precipitates also adopt heterogeneous distribution with plate morphology and identical ORs as tetrapod precipitates with Nbss. The Laves phase precipitate, Cr₂Nb, possesses C15 structure and contains high densities of stacking faults (SFs). It always forms along defects or Nbss/γNb₅Si₃ interfaces. The coupling precipitation behaviors and precipitation reactions of δNb₁₁Si₄, Cr₂Nb and γNb₅Si₃ in the Nb-Ti-Si based alloys have been discussed.     

High temperature oxidation behaviors of Ti-Cr alloys with Laves phase TiCr2

1　ＩＮＴＲＯＤＵＣＴＩＯＮＷｉｔｈａｈｉｇｈｍｅｌｔｉｎｇｔｅｍｐｅｒａｔｕｒｅ ,ｃｈｒｏｍｉｕｍｉｓｔｈｅｓｏｌｅｒｅｆｒａｃｔｏｒｙｍｅｔａｌｅｌｅｍｅｎｔｗｉｔｈｇｏｏｄｈｉｇｈｔｅｍ ｐｅｒａｔｕｒｅｏｘｉｄａｔｉｏｎｒｅｓｉｓｔａｎｃｅ[1] ,ｓｏ     

Hydrogen diffusion studies in Zr-based Laves phase AB2 alloys

The diffusion of hydrogen in the tetrahedral interstitials of bulk spherical Laves phase ZrMn_0.85Cr_0.1V_0.05Fe_0.5Ni_0.5 and ZrMn_0.85Cr_0.1V_0.05Fe_0.5Ni_0.5 + 1 wt% B alloys has been studied in the α-phase (solid-solution) region over the temperature range 450–650 °C, for hydrogen pressures up to 100 mbar using Sieverts-type apparatus. The diffusion constants have been determined from the gas–solid reaction, where the gas pressure dependence on time has been measured at fixed temperature. The results have been discussed on the basis of Fick's law of diffusion. The dependence of diffusion constant on alloy composition and initial pressure has been evaluated. Activation energy has been obtained from the temperature dependence of diffusion using Arrhenius relation.     

Strengthening of iron aluminide alloys by atomic ordering and Laves phase precipitation for high-temperature applications

The ternary system Fe–Al–Ta allows the formation of the hard and brittle ternary Laves phase Ta(Fe_0.5+x,Al_0.5−x)₂ with hexagonal C14 structure. The present study concentrates on Fe–Al–Ta alloys with small Ta contents between 2 and 6 at.% and various Al contents between 0 and 45 at.%. The phase equilibria in the ternary Fe–Al–Ta system at 1000 °C are studied experimentally for determination of the solubility limits of Ta in iron aluminide matrices and types of phases and structures which may occur at high temperatures. It is observed that small amounts of Laves phase together with atomic ordering increase the yield stress and affect ductility in a complex way.     

合金化对ZrMn2基Laves相贮氢合金相组成的影响

研究了Ni、V、Cr、Co、Fe、Cu和Ti等合金化元素取代ZrMn2 基Laves相贮氢合金的B侧或A侧对合金相组成的影响。结果表明 ,采用不同的元素对A侧或B侧进行部分取代 ,将引起ZrMn2 基合金相组成的变化。采用Ni取代Mn后 ,ZrMn2 合金的主相结构转变为C15型Laves相 ,表明Ni为C15相稳定元素。对Zr Mn Ni三元合金 ,V为C14相稳定元素 ,而Co、Fe、Cu则为C15相稳定元素。取代量较少时 ,Cr为C15相稳定元素 ,取代量增加时 ,C15相稳定作用减弱。Ti元素为C14相稳定元素 ,Ti对Zr的部分取代将导致合金主相结构转变为C14型Laves相。合金化元素对ZrMn2 合金的相组成的影响与元素的电子浓度和原子尺寸不同有关     

Microstructure and mechanical properties of Mg-Zn-Gd-based alloys strengthened with quasicrystal and Laves phase

One kind of Mg3.5Zn0.6Gd-based alloy strengthened with quasicrystals was designed, and the effect of alloying elements on microstructure and mechanical properties of as-cast Mg-Zn-Gd alloy at room temperature and elevated temperatures were studied. The results indicate that MgZnCu Laves phase, which coexists with quasicrystal at grain boundary, emerges with the addition of copper element in Mg-Zn-Gd alloy. The strength of alloys exhibits the parabola curve with the increase of copper content. The alloy with 1.5% （mole fraction） Cu shows better mechanical properties at room temperature： tensile strength 176 MPa, yield strength 176 MPa and elongation 6.5%. The existence of MgZnCu Laves phase can effectively improve the heat resistance and elevated temperature properties of the alloy. The alloy with 1.5% Cu has better mechanical properties at 200℃ ： tensile strength 130 MPa and elongation 18.5%. The creep test of the alloys at 200℃ and 50 MPa for 102 h indicates that Mg3.5Zn0.6Gd alloy reinforced with quasicrystal has better creep properties than AE42, which can be further improved with the introduction of Laves phase in the alloy.