The effects of composition on mechanical properties of W-4Re-Hf-C alloys

Arc melted W-4Re-Hf-C alloys containing up to about 0.8 mol pct HfC were fabricated into rod and sheet for evaluation of compositional effects on mechanical properties in the as-worked condition. The ductile-brittle transition temperatures (DBTT) of electropolished bend and tensile specimens were independent of HfC content in the range studied but dependent on the excess Hf or C above stoichiometric HfC. The lowest DBTT was found at Hf contents slightly in excess of stoichiometric. Tensile and creep strength variations at 0.2 to 0.5 mol pct HfC were also dependent on excess Hf and C. Maximum creep strengthening occurred also where the Hf content was slightly in excess of stoichiometric. Analyses of extracted second-phase particles indicated that they were hafnium carbide with a minor amount of tungsten carbide in solid solution. Creep strength was reduced by increasing tungsten carbide content in the particles.     

Combined matrix/boundary precipitation strengthening in creep of Fe-15 Cr-25 Ni alloys

High temperature creep behavior of carbide precipitation strengthened Fe-15 Cr-25 Ni alloys with different carbon content have been investigated. Grain boundary carbides obstruct dislocation annihilation at the grain boundary and, therefore, increase the dislocation density near the grain boundary. This gives rise to formation of a hard grain boundary region and significantly increase creep resistance of the alloy. The grain boundary precipitation strengthening and combined matrix/boundary strengthening are modeled following the concept of hard-soft composite structure, and a unified creep equation is derived by taking account of back stress from intergranular carbide particles, “boundary obstacle stress”. The models and analysis show that grain boundary precipitation strengthening is predominant for soft matrix but decreases with the increase of matrix strength, indicating the existence of coupled matrix/boundary strengthening.     

Composition effects on mechanical properties of HFC-strengthened molybdenum alloys

The mechanical properties of swaged rod thermomechanically processed from arc-melted Mo-2Re-Hf-C alloys containing as much as 0.9 mol pct HfC have been evaluated. The low temperature ductilities of these alloys were not influenced by the amount of HfC present but by the amount of Hf in excess of stoichiometry. Maximum ductility occurred at 0.2 to 0.3 at. pct excess Hf. At 0.3 to 0.5 mol pct HfC, alloy strength varied directly with the Mo content of extracted carbide particles, both decreasing as the amount of excess Hf increased. Additions of 2 at. pct Re had little effect on strength or ductility. Tensile and creep strengths of Mo-2Re-0.7Hf-0.5C alloy equaled or exceeded those of other high strength Mo alloys.     

Composition effects on mechanical properties of HFC-strengthened molybdenum alloys

The mechanical properties of swaged rod thermomechanically processed from arc-melted Mo-2Re-Hf-C alloys containing as much as 0.9 mol pct HfC have been evaluated. The low temperature ductilities of these alloys were not influenced by the amount of HfC present but by the amount of Hf in excess of stoichiometry. Maximum ductility occurred at 0.2 to 0.3 at. pct excess Hf. At 0.3 to 0.5 mol pct HfC, alloy strength varied directly with the Mo content of extracted carbide particles, both decreasing as the amount of excess Hf increased. Additions of 2 at. pct Re had little effect on strength or ductility. Tensile and creep strengths of Mo-2Re-0.7Hf-0.5C alloy equaled or exceeded those of other high strength Mo alloys.     

Structure and mechanical properties of tempered martensite and lower bainite in Fe−Ni−Mn−C steels

The structure and mechanical properties of tempered martensite and lower bainite were investigated in a series of high purity 0.25 pct C steels with varying amounts of nickel and manganese. The martensites in 0.25 C-5 Ni?Fe and 0.25 C-3 Mn?Fe alloys were mainly untwinned, while those in 0.25 C-5 Ni-7 Mn?Fe and 0.25 C-7 Mn?Fe alloys were heavily twinned. Manganese appears to promote carbide precipitation along the lath boundaries in tempered martensite. At equivalent yield and ultimate tensile strength levels, the tempered martensite of lower manganese steels showed better impact toughness than the tempered martensite of higher manganese steels. The impact toughness (compared at similar strength levels) of untwinned tempered martensite of 0.25 pct C steel with Widmanstatten precipitation of carbide was higher than that of lower bainite, which showed unidirectional carbides. The reasons for the difference in impact toughness between the alloys, and also between the structures are rationalized in terms of internal twinning, grain boundary precipitation and carbide morphology together with other microstructural features.     

Activated sintering of W-HfC composite materials

The features of consolidation of the particles during the activated sintering of tungsten powders with different values of dispersity (d _av = 2–3 and 0.8–1.0 μm) are investigated. Sintering was activated by introducing nickel additives (up to 0.5 wt %), tungsten nanoparticles (up to 30 wt %), and finely dispersed hafnium carbide (5–30 vol %) with subsequent milling in a vibrating mill. The uniaxial compaction of the samples has been performed under pressures from 50 to 1000 MPa, and sintering was performed in vacuum at 1850°C with holding for 1 h. It is shown that the additives of tungsten carbide increase the density of sintered billets and, in combination with dispersed hafnium carbide, tungsten-based composite materials with a grain size up to 2 μm can be obtained.     

HfC含量对钼钛锆合金显微组织和力学性能的影响

在钼钛锆（titanium zirconium molybdenum alloy,TZM）合金粉末中分别添加质量分数为0、0.25%、0.50%、1.00%的HfC粉末颗粒,利用粉末冶金结合轧制变形的方法制备多元复合强化钼合金。通过金相组织观察、扫描电子显微镜形貌表征、能谱分析以及力学性能测试等手段,研究了HfC颗粒对TZM合金显微组织和力学性能的影响。结果表明,添加HfC颗粒可以抑制TZM合金晶粒在烧结过程中的长大,但添加量超过0.50%时,抑制效果减弱。当HfC颗粒质量分数为0.25%时,TZM合金的室温和高温抗拉强度最强,维氏硬度最高,塑性最优。     

Hafnium carbide strengthening in a tungsten-rhenium matrix at ultrahigh temperatures

Tungsten-rhenium-hafnium carbide (W-Re-HfC) alloy is the strongest metallic material at temperatures greater than 2000 K. In the present study, the mechanical properties of tungsten and a W-3.6Re-0.26HfC alloy were determined from 1700 to 2980 K in a vacuum below 10⁻⁵ Pa. HfC particles had an exceptional strengthening effect in the tungsten-rhenium matrix at temperatures up to 2700 K. The strengthening was attributed to the high thermodynamic stability of HfC particles at ultrahigh temperatures. The growth behavior of HfC particles in the tungsten-rhenium matrix was examined. Carbon was found to be the rate-limiting element in the growth process of HfC particles. The strengthening mechanisms in a W-3.6Re-0.26HfC were discussed. It was concluded that the strength of a dispersion-strengthened material was proportional to the square root of the volume fraction of the particles. The calculation of a W-3.6Re-0.26HfC alloy's yield strength, calculated based on the dislocation pinning and the particle statistical distribution, was in good agreement with the experimental data over the entire temperature range.     

Phase stability and mechanical properties of carbide and boride strengthened chromium-base alloys

Phase stability and mechanical properties of five carbide and two boride strengthened chromium-base alloys are presented. Compositions examined were Cr-0.5 TaC (mole pet), Cr-0.5 TiC, Cr-0.5 Cb(Nb)C, Cr-0.5 HfC, Cr-0.5 ZrC, Cr-0.5 CbB, and Cr-0.5 TaB. A transition in stability from the carbide of the principal alloying metal to Cr₂₃C₆, complete at approximately 2800°F, occurs in the Cr-0.5 TaC, Cr-0.5 TiC, and Cr-0.5 CbC alloys. Similarly, a change in phase stability from borides of columbium (niobium) and tantalum to Cr₄B occurs at ∼2800°F in the Cr-0.5 CbB and Cr-0.5 TaB compositions. The compounds HfC and ZrC, respectively, remained stable in the Cr-0.5 HfC and Cr-0.5 ZrC alloys at this temperature. Stress-rupture properties at 2100°F improved for several alloys when aged at this temperature to precipitate the carbide or boride of the principal alloying metal following higher temperature heat treatment to form the Cr₂₃C₆ or Cr₄B phases. Rupture life of the Cr-0.5 TaC alloy, for example, was increased at 15 ksi and 2100°F from 4 hr for as-fabricated material, to 186 hr after heat treatment. Improvement of rupture life for similar material and test conditions from 24 hr to 382 hr was observed in the Cr-0.5 TaB composition.     

The partitioning of refactory metal elements in hafnium-modified cast nickel- base superalloys

Data from the electron microprobe have been used to elucidate the role of hafnium in altering the microstructure of the nickel-base superalloys B-1900, Alloy 713 LC, Udimet 700, and Mar-M246. The addition of about 1.3 to 2.0 pct Hf to these alloys improves their strength and ductility at both room temperature and 760°C. The data indicate that hafnium partitions to the surface of the MC carbides, replacing titanium and some molybdenum and/or tungsten in the carbide. On the basis of the observed morphology and composition of the carbides, it is postulated that, primarily, hafnium modifies the solidification sequence, which a) results in the formation of discrete, uniformly distributed MC carbides, b) retards the formation of secondary carbides, and c) contributes indirectly to solid solution strengthening of the matrix. Subsidiary of American Metal Climax, Inc. AMAX Division.     

The effect of carbide precipitation on the hydrogen-enhanced fracture behavior of alloy 690

Alloy 690 is susceptible to hydrogen embrittlement where hydrogen reduces the ductility and causes the fracture morphology to change to predominantly intergranular. The role of carbide precipitation in the embrittlement behavior is not well defined. The objective of this work is to understand the effect of intergranular carbide precipitation on the hydrogen embrittlement of alloy 690. The work reported herein used tensile and compact-tension specimens in both the solution-annealed condition (minimal grain-boundary carbide precipitation) and in the solution-annealed condition followed by an aging treatment to precipitate grain-boundary carbides. By performing the mechanical tests on materials in both uncharged and hydrogen-charged conditions, it was possible to evaluate the degree of embrittlement as a function of the carbide precipitation. It is shown that the embrittlement due to hydrogen increased as the material was aged to allow grain-boundary carbide precipitation. It is proposed that the increase in embrittlement was caused by increased hydrogen at the carbide/matrix interface due to the trapping and increased stresses at the precipitate interface, which developed from strain incompatibility of the precipitate with the matrix. It is further shown that increasing the hydrostatic stress increased the tendency for intergranular fracture, as is consistent with other nickelbase alloys.     

Preliminary evaluation of tensile and stress-rupture behavior of W + 24 At. Pct Re + 0.4 At. Pct HfC wire

The tensile properties of hafnium carbide-dispersed tungsten-rhenium alloy wire, W + 24 at. pct Re + 0.4 at. pct HfC (W24ReHfC), were studied from liquid nitrogen temperature (LN₂) to 1750 K and its stress-rupture behavior determined from 1144 to 1500 K. These results are compared to previous data on W + 4 at. pct Re + 0.4 at. pct HfC (W4ReHfC) and W + 0.4 at. pct HfC (WHfC) wire.[5] The room-temperature (RT) tensile strength of the W24ReHfC wire was about 3250 MPa and higher than that of the W4ReHfC (3160 MPa) and WHfC (2250 MPa) wires. The RT ductility of the W24ReHfC wire was quite high with a 50 pct reduction of area, whereas the W4ReHfC wire and the WHfC wire had RT ductilities of 28 and 2 pct, respectively. At temperatures of 1144 to 1366 K, the W24ReHfC wire had tensile strengths favorably comparable to the W4ReHfC and WHfC wires. However, above 1366 K, the W4ReHfC wire had both a greater tensile strength and stress-rupture strength than the W24ReHfC wire. The main contributions to the strengthening of the W24ReHfC wire were the fine and elongated fibrous grain microstructures and the dispersion of the HfC particles in the W-Re matrix. These properties suggested that the W24ReHfC wires hold promise as potential fiber reinforcements in composites from RT to about 1350 K.     

控轧控冷工艺条件下Nb—V钢碳氮化物的析出行为

本文通过热模拟实验和电子显微技术等方法,系统地研究了控轧控冷对铌钒钛复合微合金化低碳热轧钢板的铌、钒、钛碳氧化物的析出行为的影响,研究结果对开发高强度船体用钢板具有参考价值。通过研究表明,在奥氏体区和铁素体区都用Ni(C,N）析出,对Nb、V的析出起了诱导作用,并与Nb、V形成复杂的碳氮化物。在铁素体中主要以基体均匀沉淀析出和位错沉淀析出。     

Auger electron spectroscopy study of grain boundary segregation in alloy K-500: Part II. Behavior in slow strain rate tensile test

An auger electron spectroscopy (AES) study was performed to characterize grain boundary segregation behavior and grain boundary precipitation occurring during slow strain rate tensile tests (SSRTTs) of alloy K-500. It was discovered that flake-like precipitates formed on grain boundaries of some test alloys which had a high bulk Cu level. Auger electron (AE) spectra taken from precipitates and AE mapping of grain boundary facets containing precipitates indicated that these precipitates were most likely titanium carbides. The grain boundary segregation that existed in as-processed alloys was also changed in the course of SSRTTs. A significant reduction in C segregation and a noticeable increase in Cu segregation were observed in test alloys after SSRTTs. Graphite precipitation was not detected in any of the test alloys before or after SSRTTs. The carbide precipitation and change in segregation pattern are qualitatively understandable on the basis of available thermodynamic data, but further study is necessary to fully explain the observed phenomena.     

Molybdenum in Nb-C alloys

The effects of molybdenum alloying additions to niobium on the carbide phases and their precipitation behavior were investigated. The experimental alloys included Nb-0.1C, Nb-15Mo-0.1C, and Nb-30Mo-0.1C. After selected heat treatments the microstructural changes were determined by metallography and the carbide phases were extracted and identified by X-ray diffraction and chemical analysis. The results are essentially in agreement with recent phase diagram determinations. Additions of 30 wt pct Mo appears to slightly increase the solubility of carbon in niobium at temperatures around 1650°C. The solubility of molybdenum in Nb₂C is very small. Discontinuous precipitation of β-Nb₂C was found to occur in the Nb-30Mo-0.1C alloy during annealing at 1200°C. The important, overall effect of molybdenum in Nb-C alloys is to decrease the rate of niobium carbide precipitation so that appreciable carbon supersaturation can be achieved even after comparatively slow furnace cooling.     

Effects of Be and Fe additions on the microstructure and mechanical properties of A357.0 alloys

A357 hypoeutectic alloy is a heat-treatable Al-Si-Mg system with a nominal composition of Al-7 pct Si and about 0.6 pct Mg have widespreaded applications, especially in the aerospace and automotive industries. The purpose of this study was to determine the influences of Be and Fe content on the microstructure and mechanical properties of A357.0 alloys. Distinct morphologies were discerned between Be-containing and Be-free alloys. The Be-free alloys contain larger amount of iron-bearing phases with Mg than in Be-containing alloys. The addition of Be can change the plateletlike structure of iron-bearing phases to a comparatively harmless round nodular form. Also, the amounts of iron-rich phases are significantly lower and the silicon particles are smaller and more spherical in the Be-containing alloys. Small amounts of Be in A357.0 caused significant increases in the precipitation kinetics of Mg₂Si. It was found that the addition of Be lowers the ternary and binary eutectic melting point. The amount of Mg available to form the major strengthening phase Mg₂Si is increased promoting the tensile strength of A357.0 casting. The tensile properties were improved with decreasing Fe content and the addition of Be. The effect is more apparent in the higher Fe alloys than that in the lower Fe alloys.     

Deformation properties of sintered tungsten-based heavy alloys

The deformation properties of tungsten-based sintered alloys are studied. Plasticity diagrams are plotted for these alloys at deformation temperatures up to 900°C and various state-of-stress schemes. The properties of the tungsten-based sintered alloys are determined during alternating deformation. The joint action of the temperature (up to 900°C) and the state of stress on the ductility of the tungsten-based sintered alloys is analyzed. The results obtained are used to simulate damage accumulation for various methods of strain hardening of heavy alloys (rotational reduction, hydraulic forging, rolling in multiroll passes). Hydraulic forging is considered, and the limiting reductions are determined using the condition of impossible irreversible metal microfracture.     

Microstructure and mechanical behavior of spray-deposited Al-Cu-Mg(-Ag-Mn) alloys

The effect of alloy composition on the microstructure and mechanical behavior of four spray-deposited Al-Cu-Mg(-Ag-Mn) alloys was investigated. Precipitation kinetics for the alloys was determined using differential scanning calorimetry (DSC) and artificial aging studies coupled with transmission electron microscopy (TEM) analysis. DSC/TEM analysis revealed that the spray-deposited alloys displayed similar precipitation behavior to that found in previously published studies on ingot alloys, with the Ag containing alloys exhibiting the presence of two peaks corresponding to precipitation of both Ω-Al₂Cu and θ′-Al₂Cu and the Ag-free alloy exhibiting only one peak for precipitation of θ′. The TEM analysis of each of the Ag-containing alloys revealed increasing amounts of Al₂₀Mn₃Cu₂ with increasing Mn. In the peak and over-aged conditions, Ag-containing alloys revealed the presence of Ω, with some precipitation of θ′ for alloys 248 and 251. Tensile tests on each of the alloys in the peak-aged and overaged (1000 hours at 160 °C) conditions were performed at both room and elevated temperatures. These tests revealed that the peak-aged alloys exhibited relatively high stability up to 160 °C, with greater reductions in strength being observed at 200 °C (especially for the high Mn, low Cu/Mg ratio (6.7) alloy 251). The greatest stability of tensile strength following extended exposure at 160 °C was exhibited by the high Cu/Mg ratio (14) alloy 248, which revealed reductions in yield strength of about 2.5 pct, with respect to the peak-aged condition, for the alloys tested at both room temperature and 160 °C.     

High-temperature synthesis of tungsten carbide in a methane-hydrogen gas medium

Tungsten carbide synthesis in a methane-hydrogen medium have been studied and the conditions for obtaining WC power with no free-carbon impurity determined. Hard alloys based on a “gas” carbide have been shown to exhibit high ductility. The results of the study have been used to develop a commercial process to produce high-ductile hard alloys for high-pressure equipment, stamping dies, and drilling tools.     

On the precipitation of TiC in liquid iron by reactions between different phases

In order to study the in situ formation of carbide-reinforced metal matrix composites and the inoculation reactions of some low alloy steels, titanium carbide precipitation in liquid iron was investigated. TiC particles were formed directly in liquid iron solution through reactions between different phases. The high-temperature reactions studied in the present work are reactions between (a) solid graphite and liquid Fe-Ti alloys, (b) Fe-Ti and Fe-C liquid alloys, and (c) pure solid titanium and liquid Fe-C alloys. Small samples for these reactions have been processed using a mirror furnace facility in a precisely controlled manner. Samples of Fe-Ti alloys reacted with graphite were also processed by a high frequency induction furnace and other facilities. The precipitation behavior of titanium carbide in various reactions and the distribution of carbide particles in iron melts were investigated. Different growth morphologies of the carbide were observed under various conditions. The effects of convection on the reactions were observed and discussed, and by processing the samples with various orientations relative to gravity, various patterns of carbide distribution were obtained.