Unidirectional solidification of Co−Cr−C monovariant eutectic alloys

Compositions on the eutectic liquidus line between Co-25 wt pct Cr-3.5 wt pct C and Co-45 wt pct Cr-2.2 wt pct C wherein the simultaneous freezing of a metal matrix and a carbide occur have been solidified unidirectionally. The composite structures formed consist of a cobalt matrix containing substantial amounts of soluble chromium with the carbide phase, (Cr, Co)₇C₃, as an aligned fibrous dispersion. The structure of one monovariant eutectic composition has been examined as a function of solidification velocity. Cellular growth was observed at rates above approximately 4 cm per hr. However, the cell cusps only gradually deepen with increasing growth rate and the carbide phase remains essentially aligned even at 50 cm per hr. The concentration of carbide fibers in the transverse section was measured at various growth rates and was found to be approximately a linear function of rate. The structure of the alloy was also examined by transmission electron microscopy. These results indicate that the preferred growth direction of the carbide is more important than the solid-solid surface energy in controlling the growth.     

The structure of rapidly solidified Al- Fe- Cr alloys

Four aluminum alloys, designed for use at elevated temperatures, were studied. The alloys were supersaturated with iron and chromium, and one of them contained small amounts of Ti, V, and Zr. The starting materials were alloy powders made by the RSR (Rapid Solidification Rate) centrifugal atomization process. Extrusion bars were made from the four powders. The as-extruded microstructure and the microstructure of the alloys after annealing at 482 °C were investigated by optical and transmission electron microscopy and by X-ray diffraction. The microstructure consists of equiaxed grains of aluminum matrix and two types of precipitates, namely, Al₃(Fe ,Cr) and a metastable phase, Al6(Fe,Cr). The precipitates were different in their shape, size, distribution, and location within the grains.     

Crystallization of amorphous Fe- P- C alloys

The transformation of amorphous Fe-P-C alloys from the amorphous state to crystalline state was studied by differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Compositions included iron and phosphorus rich alloys and alloys with compositions near the eutectic composition. The crystallization of Fe-P-C alloys proceeded by the following processes: for the iron rich alloy, bcc α Fe precipitated first; for the phosphorus rich alloy, Fe₂P and iron carbide precipitated first; for the alloys near the eutectic line, the eutectic reactions dominated, although some α Fe or Fe₂P plus iron carbide crystals were observed. The composition dependence of the transformation can be explained successfully by a free energy model with the help of the phase diagram. From DSC and TEM results, the nucleation rate of the α Fe precipitate decreased with time in the Fe rich alloys according to the recrystallization theory of Avrami. Formerly Research Fellow in the Department of Metallurgy and Materials Science, University of Pennsylvania This paper is based on a presentation made at a symposium on “Recovery Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

Aging behavior of Fe-30 Ni alloys containing niobium

A study has been made of the precipitation reactions in Fe-30 wt pct alloys containing up to 5 wt pct Nb. The as-quenched structures of these alloys consist, of austenite, martensite in twinned as well as in massive form, and Ni₃Nb and Fe₂Nb precipitates. On aging at 700° and 800°C the main precipitation reaction results in the formation of hexagonal Laves phase Fe₂Nb, but Ni₃Nb in both bct and orthorhombic structures also precipitates. The precipitation of Fe₂Nb is a heterogeneous process and results in a considerable increase in the hardness of the alloy.     

Influence of Ti and C on the Solidification Microstructure of Fe-10Al-5Cr Alloys

The influence of Ti and C on the solidification microstructure of Fe-10Al-5Cr (all composition values in weight percent) alloys was examined with solidification modeling and a variety of experimental techniques. Several Fe-10Al-5Cr-Ti-C alloys were fabricated using the arc button melting process and characterized using quantitative image analysis and electron microscopy techniques. The experimental alloys exhibited primary ferrite dendrites with an interdendritic ferrite/TiC eutectic constituent, and the amount of eutectic was affected by the Ti and C concentrations. A liquidus projection and primary solidification paths were calculated for the Fe-10Al-5Cr-Ti-C system in order to estimate the amount of TiC that is expected to form during solidification. The range in the calculated amount of TiC-containing eutectic matched the experimentally measured values reasonably well. The ability to control the amount of TiC that forms during solidification of an Fe-10Al-5Cr-Ti-C-based alloy shows promise for developing corrosion-resistant weld overlay claddings with resistance to hydrogen cracking.     

Influence of post weld heat treatment on impact toughness properties of 1Cr0.5Mo and 2.25Cr1Mo steels

The influence of post weld heat treatment on hardness and impact toughness properties of weldments of 1Cr0.5Mo and 2.25Cr1Mo steels has been investigated. Post weld heat treatment significantly reduced the hardness and the impact transition temperature and increased the upper shelf energy. The decrease in hardness was essentially larger for 2.25Cr1Mo than for the other steel, probably due to the presence of martensite in the as-welded condition in the former material. The decrease in the impact transition temperature was largest for 1Cr0.5Mo.     

Molecular dynamics simulation of atomic displacement cascades in Fe-9 at % Cr and Fe-9 at % Cr-0.1 at % C alloys

The atomic displacement cascades in Fe-9 at % Cr and Fe-9 at % Cr-0.1 at % C alloys are studied by molecular dynamics simulation at an initial temperature of 600 K. The average fractions of defects that “survive” in a displacement cascade are calculated for primary knocked-on atom (PKA) energies of 0.1–20 keV, and the number and size of the vacancy and interstitial-atom clusters that form in a displacement cascade are determined. Carbon at the concentration under study is found not to affect the number of survived defects and the cascade efficiency. At PKA energies of 15 and 20 keV, the presence of carbon slightly decreases the fraction of point defects forming clusters.     

Secondary hardening and fracture behavior in alloy steels containing Mo, W, and Cr

In 4Mo, 6W, 2Mo3W, 2Mo2Cr, and 3W2Cr alloy steels, which contain alloying elements, such as Mo, W and Cr, contributing to the secondary hardening by forming M₂C-type carbide, the secondary hardening and fracture behavior were studied. Molybdenum had a strong effect on secondary hardening, while W had a very weak effect on it but delayed the overaging. The MoW steel exhibited both moderately strong hardening and considerable resistance to overaging. On the other hand, the secondary hardening effect was diminished by the Cr addition, because the cementite of M₃C type was stabilized at higher temperatures and the formation of M₂C carbides was thus inhibited. Although the Cr addition had no merit in the secondary hardening itself, it eliminated the secondary hardening embrittlement (SHE). This was observed as a severe intergranular embrittlement due to the impurity segregation for the Mo and MoW steels and as a decrease in upper shelf energy for W steel, even in the overaged condition.     

Secondary hardening and fracture behavior in alloy steels containing Mo, W, and Cr

In 4Mo, 6W, 2Mo3W, 2Mo2Cr, and 3W2Cr alloy steels, which cointain alloying elements, such as Mo, W and Cr, contributing to the secondary hardening by forming M₂C type carbide, the secondary hardening and fracture behavior were studied. Molybdenum had a strong effect on secondary hardening, while W had a very weak effect on it but delayed the overaging. The MoW steel exhibited both moderately strong hardening and considerable resistance to overaging. On the other hand, the secondary hardening effect was diminished by the Cr addition, because the cementite of M₃C type was stabilized at higher temperatures and the formation of M₂C carbides was thus inhibited. Although the Cr addition had no merit in the secondary hardening itself, it eliminated the secondary hardening embrittlement (SHE). This was observed as a severe intergranular embrittlement due to the impurity segregation for the Mo and MoW steels and as a decrease in upper shelf energy for W steel, even in the overaged condition.     

Fracture toughness of 12Cr2Mo1R steel at elevated temperature

The microstructure,tensile properties,and fracture toughness of 12Cr2Mo1R steel were studied. The results indicate that this steel is characterized by a bainite microstructure,in which several types of carbides precipitate along the ferrite laths. As the temperature increases from room temperature to 375 ℃,the strength of the steel increases slightly and the fracture toughness clearly decreases. However,when the temperature continues to increase up to 500 ℃,the strength decreases and the fracture toughness increases. At all the temperatures investigated,the strength and toughness of the developed 12Cr2Mo1R steel were capable of meeting the design requirements of a high-temperature gas-cooled reactor. The fracture of 12Cr2Mo1R steel at high temperature typically occurs in the ductile mode.     

Effects of simultaneous boron and nitrogen implantation on microhardness and fatigue properties of Fe-13cr-15ni alloys

Eight complex austenitic stainless steel alloys based on the composition Fe-13Cr-15Ni-2Mo-2Mn-0.2Ti-0.8Si-0.06C were implanted simultaneously with 400-keV B+ and 550-keV N⁺ ions and were investigated for changes in fatigue properties and surface microhardness. The nearsurface hardness of all eight alloys improved, but the fatigue life of each decreased. These findings were contrary to those obtained in an earlier study using four simple Fe-13Cr-15Ni alloys, where the dual implantation improved fatigue life by up to 250 pct. While unimplanted specimens failed by slip-band crack initiation, it was hypothesized that the dual implantation suppressed slip to the extent that fewer slip-band cracks were initiated and these were subjected to accelerated crack propagation. In addition, grain-boundary cracking was promoted, yielding a lower fatigue life. Support for this hypothesis was obtained by a study of single crystals of Fe-15Cr-15Ni, which were also implanted with B⁺ and N⁺. The dual implantation caused a lower fatigue life due to concentration of slip along a few slip bands to relieve applied stress. Evidence of grain-boundary cracking was obtained using the four simple alloys, which were subjected to triple ion implantation with B⁺, N⁺, and C⁺. The triple implantation decreased the fatigue life of the alloys and caused accelerated growth of fewer slip bands and grain-boundary cracking due to suppression of surface slip bands. This study thus shows the existence of an optimum level of strengthening when multiple ion implantation is used to improve the fatigue properties of alloys.     

The role of yttrium on the oxide adherence of Fe-24Cr base alloys

A study on isothermal oxidation of Fe-24Cr base alloys containing 0.063 and 0.87 wt pct yttrium has been made to examine the role of yttrium on the improvement of oxide adherence. The isothermal oxidation resistance of the 0.063Y alloy was nearly equal to that of the 0.87Y alloy at 1000 °C and 1100 °C. The oxide scales of the former spalled partially during cooling to room temperature, while the adherence of the oxide on the latter was extremely strong. A morphological observation of the substrate-oxide interface by a scanning electron microscope revealed that cavities developed at the substrate-oxide interface of the 0.063Y alloy caused the partial spallation. No cavities were observed at the substrate-oxide interface of the 0.87Y alloy and voids centered on Y₂O₃ particles were observed in the substrate of the alloy. Those voids indicated that internal oxide particles had acted as preferential sinks for excess vacancies which would otherwise have condensed to form voids at the substrate-oxide interface. A model is proposed for a diffusional process based on excess vacancy sinks at the boundaries of internal oxide particles dispersed in the matrix of the Fe-24Cr-O.87Y alloy, and the effect of yttrium on the surface oxide adherence of the alloy.     

Characterization of high-temperature oxide films on dysprosium-doped Fe-20Cr alloys by electrochemical techniques

The oxidation properties of Fe-20Cr,Fe-20Cr-0.2Dy and Fe-20Cr-1Dy alloys were studied using gravimetric and electrochemical techniques.The high-temperature oxide films of Dy-doped Fe-20Cr alloys were prepared in air at 900 oC for 24,48 and 100 h,respectively.The electrochemical experiment was performed by a three-electrode electrochemical cell and in 0.1 mol/L Na2SO4 aqueous solution.Proper models were built for describing electrochemical impedance spectroscopy of the different oxide layers and the spectra were interpreted in terms of a two-layer model of the films.The results revealed that the oxide films of Dy-doped Fe-20Cr alloys became compacter than that of undoped alloys and retained their good protective ability for a relatively long time.With increasing content of Dy,the protection of the oxide films slightly decreased.Mott-Schottky curves indicated that all the oxides were n-type semi-conductors,and the Nd value of oxide film on Fe-20Cr was much larger than that of Dy-doped Fe-20Cr alloys.The results of kinetic curves and SEM were in agreement with electro-chemical impedance spectroscopy and Mott-Schottky data.     

The relationship between austenite strength and the transformation to martensite in Fe-10 pct Ni-0.6 pct C alloys

The effect of austenite yield strength on the transformation to martensite was investigated in Fe-10 pct Ni-0.6 pct C alloys. The strength of the austenite was varied by 1) additions of yttrium oxide particles to the base alloy and 2) changing the austenitizing temperature. The austenite strength was measured at three temperatures above theM _s temperature and the data extrapolated to the experimentally determinedM _s temperature. It is shown that the austenite yield strength is determined primarily by the austenite grain size and that the yttrium oxide additions influence the effect of austenitizing temperature on grain size. As the austenite yield strength increases, both theM _s temperature and the amount of transformation product at room temperature decrease. The effect of austenitizing temperature on the transformation is to determine the austenite grain size. The results are consistent with the proposal¹ that the energy required to overcome the resistance of the austenite to plastic deformation is a substantial portion of the non-chemical free energy or restraining force opposing the transformation to martensite.     

低温超音速火焰喷涂纳米WC-10Co4Cr涂层的显微结构和性能

以纳米和微米WC-10Co4Cr粉末为热喷涂粉末,采用低温超音速火焰喷涂（LT-HVOF）和超音速火焰喷涂（HVOF）技术制备了WC-10Co4Cr涂层,采用SEM、XRD、和显微硬度仪等对LT-HVOF WC涂层显微结构和性能进行了表征.结果表明：n-WC涂层、lm-WC涂层的显微结构与普通超音速火焰喷涂WC涂层没有明显的区别,其主晶相为WC; m-WC涂层呈明显的层状结构,涂层中WC颗粒尖端发生了钝化和部分熔化,粒径变小,并形成了WC/的核壳结构;其主晶相为.n-WC涂层显微硬度较lm-WC涂层低,但其韧度高而使涂层的磨损失重最低;m-WC涂层的显微硬度和韧度最低,磨损失重最大.     

Mechanical properties of spinodally decomposed Fe-30 wt% Cr alloys: Yield strength and aging embrittlement

The age-hardening due to spinodal decomposition in Fe-30 wt% Cr alloys was studied. The yield stress was measured using monotonic tensile tests over a temperature range from 77 to 473 K. The observed incremental yield stress was found to be essentially test temperature independent, increasing substantially with increasing aging time as long as deformation occurs by a slip dominant mode. TEM observation of these test samples showed many dislocation pile-ups for aged materials while as-quenched materials exhibited a homogeneous dislocation distribution. The composition profile and consequent internal stress field due to isotropic decomposition were simulated using Cahn's multi-wave method. The results of this model were then used to estimate the yield stress for such an isotropic decomposition. Theory and experiment were compared using estimates of the extent of decomposition derived from detailed analyses of small angle neutron scattering (SANS). The results indicated that the misfit effect due to the coherent internal stress field is the dominant mechanism responsible for the observed age hardening. Experimental observations are consistent with a model for aging embrittlement which attributes the origin of twinning deformation to a Cr-rich second phase with Cr concentration greater than some critical value.     

高速钢轧辊中W、Mn、Mo等7元素的联合快速测定

用磷酸、高氯酸统一溶解处理方法制备试液,用同一母液分别以光度法和滴定法测定W、Mn、Mo、Co、Cu、V、Cr元素的含量。通过控制测定的酸度,各元素线性良好,W、Mn、Mo、Co、Cu、V、Cr的RSD分别≤1.42%、≤1.26%、≤2.12%、≤6.72%、≤5.73%、≤3.53%、≤1.53%。该方法缩短了分析周期,精密度和准确度高,能够满足快速测定的要求。