The solidification of undercooled Ni-4.5 wt pct B alloy melt was investigated by using the glass fluxing technique. The alloy melt was undercooled up to ΔTp ~ 245 K (245 °C), where a mixture of α-Ni dendrite, Ni3B dendrite, rod eutectic, and precipitates was obtained. If ΔTp < 175 K ± 10 K (175 °C ± 10 °C), the solidification pathway was found as primary transformation and eutectic transformation (L → Ni3B and L → Ni/Ni3B); if ΔTp ≥ 175 K ± 10 K (175 °C ± 10 °C), the pathway was found as metastable eutectic transformation, metastable phase decomposition, and residual liquid solidification (L → Ni/Ni23B6, Ni23B6 → Ni/Ni3B, and Lr → Ni/Ni3B). A high-speed video system was adopted to observe the solidification front of each transformation. It showed that for residual liquid solidification, the solidification front velocity is the same magnitude as that for eutectic transformation, but is an order of magnitude larger than for metastable eutectic transformation, which confirms the reaction as Lr → Ni/Ni3B; it also showed that this velocity decreases with increasing ΔTr, which can be explained by reduction of the residual liquid fraction and decrease of Ni23B6 decomposition rate. 相似文献
The effect of overheating ΔTL+ of the gallium melt on its supercooling ΔTL? during solidification is studied by cyclic thermographic analysis. The obtained data on ΔTL? are used to calculate the thermodynamic and kinetic characteristics of gallium solidification. 相似文献
Ni75B25 alloy was solidified at various undercooling. The formation and subsequent transformation of metastable Ni23B6 phase were clearly identified. If undercooling prior to nucleation is less than a critical value of 240 K (240 °C), the alloy solidifies completely into Ni3B phase. At larger undercooling, metastable Ni23B6 phase primarily forms in the melt but then is decomposed into α-Ni and Ni3B through a eutectoid reaction. The decomposition simultaneously triggers the rapid solidification of residual liquid, due to which a second temperature recalescence occurs. The α-Ni/Ni3B eutectoid is partially remelted if temperature exceeds the eutectic temperature during the second recalescence. Then, residual Ni3B grows into coarse round grains while the remaining liquid re-solidifies into α-Ni/Ni3B eutectic structure in the remelted region. In the case that the eutectic temperature is not reached, the eutectoid product with dot α-Ni distributing in Ni3B matrix is retained in the solidification structure. A longer delay time between the two temperature recalescence events means less residual liquid, lower recalescence temperature and thus depressed remelting. The formation competition between Ni3B and Ni23B6 phases in the alloy melt is nucleation controlled. The heterogeneous site in Ni75B25 alloy melt is a better nucleation substrate for Ni23B6 phase than for Ni3B phase.
The effect of thermal aging on the ductile-brittle transition behavior has been assessed for a modified 9Cr-1Mo steel (P91) using the reference temperature approach under dynamic loading condition (T0dy). The steel in normalized and tempered (NT) condition and in different levels of subsequent cold work (CW) was subjected to thermal aging at temperatures of 873 K and 923 K (600 °C and 650 °C) for 5000 and 10,000 hours. For the NT and all the cold work conditions of the starting material, a drastic increase in T0dy has been noticed after aging at 923 K (650 °C) for 10,000 h. A moderate increase was observed for the NT steel aged at 873 K (600 °C) for 5000 hours and for the 10 pct CW steel aged at 873 K (600 °C) for 10,000 h. A detailed transmission electron microscope (TEM) study of the embrittled materials aged at 923 K (650 °C)/10,000 hours and 873 K (600 °C)/10,000 hours has indicated presence of hexagonal Laves phase of Fe2(Mo,Nb) type with different size and spatial distributions. The increase in the T0dy is attributed to the embrittling effect of a network of Laves phase precipitates along the grain boundaries. 相似文献
The role of phosphorus (P) in the heterogeneous nucleation of eutectic silicon (Si) and the evolution of eutectic grains in hypoeutectic aluminum-silicon alloys were investigated. Systematic additions of P in the range of 0.5 to 20 ppm to Al-7 wt pct Si alloys of different purities have shown that the morphology of the eutectic Si changes from a fine plate- to a coarse flake-like structure. The growth of eutectic grains was investigated by interrupting the eutectic reaction by quenching experiments. Moreover, the macroscopic growth mode of the eutectic grains was characterized by electron backscatter diffraction. An increase in P concentration from 2 to 3 ppm resulted in a transition of the macroscopic growth mode of the Al-Si eutectic in high purity alloys from growth with a planar front with a strong dependence of the thermal gradient, to nucleation in the vicinity of the primary Al dendrites and subsequent growth of distinct eutectic grains. It is suggested that AlP particles are the key impurities acting as potential nucleation sites for eutectic Si. This is further substantiated as with increasing P concentration nucleation and growth of the Al-Si occurred at higher temperatures close the equilibrium Al-Si eutectic solidification temperature at 850 K (577 °C). In addition, the recalescence undercooling ΔTR,eu was reduced from 4.5 K (0.5 ppm P) to 1.5 K (20 ppm P) in high purity alloys. This was accompanied by a drastic increase of the nucleation rate of the eutectic grains. 相似文献
Phase selection and microstructure evolution in nonequilibrium solidification of ternary eutectic Fe40Ni40B20 alloy have been studied. It is shown that γ-(Fe, Ni) and (Fe, Ni)3B prevail in all the as-solidified samples. No metastable phase has been found in the deeply undercooled samples. This is
explained as resulting from the size effect of undercooled solidification. At small and medium undercoolings, the dendrite
γ-(Fe, Ni) appears as the leading phase. This is ascribed to the existence of the skewed coupled growth zone in FeNiB alloy.
With increasing undercooling, the amount of dendrites first increases and then decreases, accompanied by a transition from
regular eutectic to anomalous eutectic. The formation mechanisms of the anomalous eutectics are discussed. Two kinds of microstructure
refinement are found with increasing undercooling in a natural or water cooling condition. However, for melts with the same
undercooling, the as-solidified microstructure refines first, and then coarsens with an increasing cooling rate. The experimental
results show that the nanostructure eutectic cell has been obtained in the case of Ga-In alloy bath cooling with an initial
melt undercooling of approximately 50 K (50 °C). 相似文献
An accurate measurement of enthalpy increment HT–H298.15 has been made for Ti-5 % Ta alloy in the temperature range of 463–1,257 K using drop calorimetry. This temperature interval covers the low temperature α + β two phase and also the α(hcp) → β(bcc) transformation domain (1,083 K ≤ T ≤ 1,183 K), in which the enthalpy versus temperature variation exhibited a clearly delineated inflection. The drop calorimetry data has been phenomenologically modelled to obtain the transformation enthalpy, Δ°Htrα→β as 66 J g?1. Further, in the α → β diffusive transformation zone the transformation kinetics has been quantitatively modelled in terms of Kolmogorov–Johnson–Mehl–Avrami model of diffusion limited phase transformation, to obtain the effective activation energy as 284 ± 10 kJ mol?1. 相似文献
Both Ni-36 wt pct Sb and Ni-52.8 wt pct Sb eutectic alloys were highly undercooled and rapidly solidified with the glass-fluxing
method and drop-tube technique. Bulk samples of Ni-36 pct Sb and Ni-52.8 pct Sb eutectic alloys were undercooled by up to
225 K (0.16 TE) and 218 K (0.16 TE), respectively, with the glass-fluxing method. A transition from lamellar eutectic to anomalous eutectic was revealed beyond
a critical undercooling ΔT1*, which was complete at an undercooling of ΔT2*. For Ni-36 pct Sb, ΔT1*≈60 K and ΔT2*≈218 K; for Ni-52.8 pct Sb, ΔT1*≈40 K and ΔT2*≈139 K. Under a drop-tube containerless solidification condition, the eutectic microstructures of these two eutectic alloys
also exhibit such a “lamellar eutectic-anomalous eutectic” morphology transition. Meanwhile, a kind of spherical anomalous
eutectic grain was found in a Ni-36 pct Sb eutectic alloy processed by the drop-tube technique, which was ascribed to the
good spatial symmetry of the temperature field and concentration field caused by a reduced gravity condition during free fall.
During the rapid solidification of a Ni-52.8 pct Sb eutectic alloy, surface nucleation dominates the nucleation event, even
when the undercooling is relatively large. Theoretical calculations on the basis of the current eutectic growth and dendritic
growth models reveal that γ-Ni5Sb2 dendritic growth displaces eutectic growth at large undercoolings in these two eutectic alloys. The tendency of independent
nucleation of the two eutectic phases and their cooperative dendrite growth are responsible for the lamellar eutectic-anomalous
eutectic microstructural transition. 相似文献
Heat capacities of melts were measured in the range 400 to 1100 K and 0.48 < NAlCl3 < 0.62, the results being expressed by Cp = 40.96 – 0.0295T + 2.01 × 10?5T2 J K?1 g·atom?1i.e., AlCl3 contains 4 atoms, and so forth). This equation was used in interpreting literature vapor pressure data. Measurements were made of the emf of the concentration cell $$AL\left| {_{AlCl_3 }^{NACl(sat)} } \right.\left| {_{(Na^ + )}^{Pyrex} } \right.\left| {_{AlCl_3 }^{NACl(sat)} } \right.\left| {_{AlCl_3 }^{NACl} } \right.\left| {AL} \right. $$ at temperatures 473 to 623 K, and the results were correlated with the vapor pressure data to yield activities of NaCl and AlCl3. Measurements with a sodium electrode confirmed the accepted values for the free energy of formation of A1C13 within about 1.5 kJ mol?1. The activities were used to analyze the phase diagram. Direct measurement of the eutectic temperature with a concentration-cell technique (which avoids supercooling) gave 386 K; the eutectic composition is 60.0 mol pct A1C13. The standard entropy of NaAlCl4(s) is S298.15° = 199.1 J K?1 mol?1. The free energy for NaAlCl4(l) = NaAlCl4(g) is ΔG° = 82740 ?63.66T J mol?1 at around 950 K. 相似文献
Ther modynamic properties of oxygen in liquid Fe?Co and Fe?Ni binary mixtures have been investigated by equilibration with appropriate mixtures of H2 and H2O. The results show that Henry's law is obeyed by dissolved oxygen at all the possible concentrations of oxygen in each binary mixture. The standard Gibbs energy of solution of oxygen ΔGQ° has been presented as a function of temperature for various binary compositions. The variation of ΔGQ° with composition at 1550°C for each system follows a theoretical correlation based on a new treatment of the first approximation to the regular solutions. 相似文献
The Jackson-Hunt model of eutectic growth at small undercoolings is extended to large undercooling values which are commonly encountered under rapid solidification conditions. The parameters, λ2V and λΔT, are found to deviate from constant values at high velocities, and these deviations depend upon the nature of the metastable phase diagram below the eutectic temperature. A limiting velocity is predicted for the formation of a regular, coupled eutectic structure, and the reason for this limiting velocity is shown to be either the temperature dependent diffusion coefficient or the limit of undercooling. 相似文献
Ni-based alloys with high Cr contents are not only known for their excellent high temperature and hot corrosion resistance, but are also known for poor mechanical properties and difficult workability. Powder metallurgical (PM) manufacturing of alloys may overcome several of the shortcomings encountered in materials manufacturing involving solidification. In the present work, six PM Ni-based alloys containing 35 to 45 wt pct Cr and 3.5 to 6 wt pct Nb were produced and compacted via hot isostatic pressing. Samples were heat treated for up to 1656 hours at either 923 K or 973 K (650 °C or 700 °C), and the microstructures and mechanical properties were quantified and compared to thermodynamic calculations. For the majority of the investigated alloys, the high Cr and Nb contents caused development of primary populations of globular α-Cr and δ (Ni3Nb). Transmission electron microscopy of selected alloys confirmed the additional presence of metastable γ″ (Ni3Nb). A co-dependent growth morphology was found, where the preferred growth direction of γ″, the {001} planes of γ-Ni, caused precipitates of both α-Cr and δ to appear in the form of mutually perpendicular oriented disks or plates. Solution heat treatment at 1373 K (1100 °C) followed by aging at 973 K (700 °C) produced a significant strength increase for all alloys, and an aged yield strength of 990 MPa combined with an elongation of 21 pct is documented for Ni 40 wt pct Cr 3.5 wt pct Nb. 相似文献
The rates of absorption of hydrogen in stagnant liquid Ni, Cu, Ag, and Sn have been measured using 1) an unsteady-state gas-liquid metal diffusion cell technique similar to that used by El-Tayeb and Parlee for iron and 2) a steady-state diffusion cell technique recently developed in this laboratory. The rates of absorption are considered to be controlled by diffusion in the liquid. On this basis the chemical diffusion coefficients of hydrogen (DH) in liquid Ni, Cu, and Ag, calculated from the rate data, can be described by:DHNi=7.47×10?3 exp(?8550±1114/RT) cm2/secDHCu=10.91×10?3 exp(?2148±349/RT) cm2/secDHAg=4.54×10?2 exp(?1359±207/RT) cm2/sec In the above equations, the uncertainty in the activation energy (QH) corresponds to the 90 pct confidence level. No reliable Arrhenius equation could be obtained forDHSn, but theDH values in tin are greater than for the other three metals. The following interesting and possibly significant correlations are observed betweenDH,QH, and the hydrogen solubility (SH):DHNi<DHFe<DHCu<DHAg<DHSn, andQHNi>QHFe>QHCu>QHAg, andSHNi>SHFe>SHCu>SHAg>SHSn. 相似文献
TiC was added to Mo-Si-B alloys using a conventional Ar arc-melting technique, and the phase equilibria, microstructure evolution, and high-temperature strength at 1673 K (1400 °C) were investigated. The primary phase changed to Mo solid solution (Moss), Mo5SiB2 (T2), or TiC depending on the composition. Following the primary phase solidification, a Moss + TiC, Moss + T2, or Moss + T2 + TiC + Mo2C eutectic reaction took place as the secondary solidification step. In some alloys, Moss + T2 + TiC and Moss + T2 + Mo2C eutectic reactions were present as higher-order solidification steps. After annealing at 2073 K (1800 °C) for 24 hours, Moss, T2, TiC, and Mo2C coexisted stably with microstructural coarsening. The coarsening rate was much faster in an alloy with no TiC dispersion, suggesting that TiC has a strong pinning effect on the grain boundary and interface migration. Compression tests conducted at 1673 K (1400 °C) revealed strength properties of almost all the alloys that were better than those of the Mo-Hf-C alloy (MHC). Alloy densities were 9 g/cm3 or less, which is lighter than pure Mo and MHC (≥10 g/cm3) and competitive with Ni-base superalloys. TiC-added Mo-Si-B alloys are promising candidates for ultrahigh-temperature materials beyond Ni-base superalloys. 相似文献
The liquid Ni-16.75 at. pct Zr peritectic alloy was substantially undercooled and containerlessly solidified by an electromagnetic levitator and a drop tube. The dependence of the peritectic solidification mode on undercooling was established based on the results of the solidified microstructures, crystal growth velocity, as well as X-ray diffraction patterns. Below a critical undercooling of 124 K, the primary Ni7Zr2 phase preferentially nucleates and grows from the undercooled liquid, which is followed by a peritectic reaction of Ni7Zr2+L → Ni5Zr. The corresponding microstructure is composed of the Ni7Zr2 dendrites, peritectic Ni5Zr phase, and inter-dendritic eutectic. Nevertheless, once the liquid undercooling exceeds the critical undercooling, the peritectic Ni5Zr phase directly precipitates from this undercooled liquid. However, a negligible amount of residual Ni7Zr2 phase still appears in the microstructure, indicating that nucleation and growth of the Ni7Zr2 phase are not completely suppressed. The micromechanical property of the peritectic Ni5Zr phase in terms of the Vickers microhardness is enhanced, which is ascribed to the transition of the peritectic solidification mode. To suppress the formation of the primary phase completely, this alloy was also containerlessly solidified in free fall experiments. Typical peritectic solidified microstructure forms in large droplets, while only the peritectic Ni5Zr phase appears in smaller droplets, which gives an indication that the peritectic Ni5Zr phase directly precipitates from the undercooled liquid by completely suppressing the growth of the primary Ni7Zr2 phase and the peritectic reaction due to the combined effects of the large undercooling and high cooling rate. 相似文献
The effects of alloying elements on the thermal stability, glass-forming ability (GFA), corrosion resistance, and magnetic and mechanical properties of a soft magnetic Fe75P10C10B5 metallic glass with a low glass transition temperature (Tg) of 723 K (450°C) were investigated. The addition of Mo, Ni, and Co significantly increased the stabilization of supercooled liquid, GFA, and corrosion resistance in the H2SO4 solution. The maximum critical diameter (dc) of 4 mm for glass formation was obtained for the Fe55Co10Ni5Mo5P10C10B5 alloy, which shows the largest supercooled liquid region (ΔTx) of 89 K (89 °C). The substitution of Cr for Mo further enhanced the corrosion resistance of the Fe55Co10Ni5Mo5P10C10B5, while the ΔTx and dc decreased. The (Fe, Ni, Co)70(Mo, Cr)5P10C10B5 bulk metallic glasses showed low Tg of 711 K to 735 K (438 °C to 462 °C), wide ΔTx of 67 K to 89 K, high saturation magnetization of 0.79 to 0.93 T, low coercive force of 2.36 to 6.61 A m?1, high compressive yield strength of 3271 to 3370 MPa, and plastic strain of 0.8 to 2.3 pct. In addition, the mechanism for enhancing stability of supercooled liquid was discussed in terms of the precipitated phases during crystallization. 相似文献
The sublimation of CaMoO4(s) is studied by high-temperature mass spectrometry at 1530–1770 K. CaMoO4(g) and MoO3(g) molecules are found to be present in the vapor; their vapor pressure is determined. The pressure of the CaMoO4(g) molecule vapor is determined to be logp = ? 19 685/T + 5.52. The heat of sublimation ΔHs, 0o (CaMoO4(s)) = 446 ± 40 kJ/mol has been determined using the third law of thermodynamics. The atomization energy of the CaMoO4(g) molecules is calculated to be ΔHat, 0o (CaMoO4(g)) = 2910 ± 40 kJ/mol. 相似文献
Adopting a fluxing purification and cyclic superheating technique, Co-10 wt pct Si and Co-15 wt pct Si alloys had been undercooled to realize rapid solidification in this work. It was investigated that the solidification modes and microstructures of Co-Si alloys were deeply influenced by the undercooling of the melts. Both alloys solidified with a near-equilibrium mode in a low undercooling range; the peritectic reaction occurred between the primary phase and the remnant liquids, and it was followed by the eutectic reaction and eutectoid transformation. With the increase of undercooling, both alloys solidified with a nonequilibrium mode, and the peritectic reaction was restrained. As was analyzed, a metastable Co3Si phase was found in Co-10 wt pct Si alloy when a critical undercooling was achieved. 相似文献