Solid solution strengthening in Ti-Al alloys

Aluminum was found to strengthen textured polycrystalline α-Ti linearly with concentra-tion over the 78 to 810 K temperature range. Between 300 and 530 K about 90 pct of the strengthening effect is athermal and the athermal component, Δτ_a, follows Δτ_a = ϕμε′c over the 0 to 15 at. pct Al range where ϕ is a dimensionless constant near unity, μ. is the modulus, ε′ is the misfit parameter, andc is concentration. It is proposed that edge kinks in screw dislocations interact with solute atoms and give rise to the observed linear athermal strengthening. Above about 530 K, atmosphere and strain aging effects are associated with aluminum.     

A study of fracture in high purity 7075 aluminum alloys

Ten different alloys based on the 7075 composition were used to study the effect of purity level, dispersoid type, and heat treatment on fracture toughness. Five purity levels ranging from 0.03 to 0.30 wt pct Fe + Si and two dispersoid types were investigated. Each alloy was given two heat treatments: the standard T651 heat treatment or a special thermomechanical treatment (TMT). Fracture toughness was measured using notched round tensile specimens taken from both the longitudinal and long-transverse directions. The notched round tensile test was modified to give the “plastic energy per unit area”. This fracture toughness parameter gave the same ranking for corresponding alloy/heat treatment combinations as the total energy per unit area measured on precracked Charpy specimens. The fracture toughness ranking for the ten alloys was the same in the longitudinal and long-transverse directions. This suggests the elongated distribution of constituent particles in the rolling direction does not change the failure mechanism. Fractographic evidence showed a bimodal distribution of ductile dimple size in all ten alloys. The number of large ductile dimples decreased with increasing purity level while the number of small ductile dimples increased. This is interpreted to mean that the smaller dispersoid and hardening particles become increasingly important in controlling the fracture toughness as the large intermetallic particles are eliminated by increasing the purity of these aluminum alloys. Since thermomechanical processing controls the amount and type of these smaller particles, it is a useful means for increasing fracture toughness in high purity aluminum alloys.     

Nodular graphite formation in vacuum melted high purity Fe-C-Si alloys

This paper describes a study of the cast structure of vacuum melted high purity Fe-C-Si alloys with emphasis on hypoeutectic and eutectic compositions. Nodular graphite was observed to form at high cooling rates and coral graphite at low cooling rates. This result was also confirmed by a limited study on directional solidification of alloys prepared from the same starting materials. The formation of nodular graphite at the high cooling rates was suppressed to near zero by changing the starting iron from 99.94 pct electrolytic iron to an ultra-pure zone refined iron, or by holding the melt at a low super-heat prior to cooling. Chemical analysis showed only that the impurity responsible for nodular formation was present at the low ppm level. An attempt is made to explain the appearance of the various microstructures in terms of the nucleation and growth of nodular graphite, coral graphite and the carbide structure of white iron.     

Ductile two-phase alloys: Prediction of strengthening at high strains

When alloys containing two ductile phases are heavily deformed, composite-like microstructures develop and strengths well in excess of either of the phases in single phase form may be exhibited as a result of microstructure/dislocation density effects. In this paper a previously-published model for such strengthening is reviewed, and its application in a predictive capacity discussed. Flow stressvs fabrication strain data for the two components in single phase form and for one two-phase alloy are necessary for this purpose. The model may then be applied to predict strength for any other two-phase alloy as a function of composition, fabrication strain, and interphase spacing. The approach is illustrated using existing data for several alloy systems. For Ag-Fe and Cu-Nb alloys (with very limited mutual solubility) strengths can be predicted within 15 to 20 pct of the experimental values over the entire range of strains and volume fractions for which data are available. In systems where the potential for precipitation hardening exists (e.g., Cu-Fe) thermal history is important. When such hardening becomes a significant factor, the model cannot be used in its present form due to uncertainty over how to “add” the strengthening from this effect. Such hardening may, however, be useful in further improving the properties of these materials.     

电感耦合等离子体质谱法测定高纯五氧化二铌中25种痕量杂质元素

采用电感耦合等离子体质谱法(ICP-MS)测定高纯五氧化二铌中痕量Mg、K、Ca、Cr、Fe时,因质谱干扰严重,从而导致其背景等效浓度值(BEC)较高进而无法准确测定。实验采用氢氟酸-硝酸体系以微波消解方式消解样品,以标准加入法补偿基体效应,控制基体质量浓度为500μg/mL,建立了ICP-MS测定高纯五氧化二铌中包括Mg、K、Ca、Cr、Fe在内的25种痕量杂质元素的分析方法。研究表明:采用屏蔽矩冷等离子体技术,在500μg/mL的五氧化二铌基体溶液中,Na、Mg、Al、K、Ca、Cr、Fe、Cu、Co、Ni、Mn的BEC得到明显改善,尤其是Mg、K、Ca、Cr、Fe的BEC改善效果最为显著,由常规模式下的56.5~194ng/mL降至冷等离子体模式下的0.012~0.203ng/mL;使用经实验室亚沸蒸馏提纯的电子级氢氟酸及硝酸可有效地降低试剂空白值。各元素校准曲线线性相关系数均大于0.9990;方法中各元素的检出限在0.001~0.010μg/g之间,测定下限在0.003~0.033μg/g之间。采用实验方法对高纯五氧化二铌样品中25种杂质元素进行测定,结果表明,各元素测定结果的相对标准偏差(RSD,n=11)为0.90%~12.7%,回收率为91%~111%。方法应用于两批纯度为99.999%的超高纯五氧化二铌实际样品分析,结果与辉光放电质谱法(GD-MS)基本一致。     

Forced velocity pearlite in high purity Fe-C alloys: Part II. Theoretical

The forced velocity pearlite data of Part I is compared to the Zener-Hillert treatment and shown to fit the volume diffusion model but requires aD value higher than that of the Wells’et al.¹⁶ data by a factor of 3 to 4. It is postulated that the highD value is due to a strain in the austenite phase at the growth front. It is found that inclusion of a mobility term, μ, in the Zener-Hillert analysis requires a quadratic form,V = μΔ², in order to fit the data. Additional experiments have shown that the degenerate pearlite found at high growth velocities results from coarsening of fine pearlite as it cools from the growth front temperatures. Finally, some speculations are offered to explain the various interesting growth features of pearlite presented in Part I. This paper is based on a presentati on made at the symposium “Establishment of Microstructural Spacing during Dendritic and Cooperative Growth” held at the annual meeting of the AIME in Atlanta, Georgia on March 7, 1983 under the joint sponsorship of the ASM-MSD Phase Transformations Committee and the TMS-AIME Solidification Committee.     

Forced velocity pearlite in high purity Fe-C alloys: Part 1. Experimental

An experimental technique was developed to measure the interface temperature of directionally transformed pearlite. It was shown that the growth temperature of such forced velocity pearlite is the same as the growth temperature of isothermally formed pearlite which grows at the same velocities. It was also shown that forced velocity pearlite is kinetically limited to maximum growth rates of around 100 μm per second. It was demonstrated that a small but significant fraction of prior austenite grain boundaries interferes with pearlite growth by halting the growth of the cementite lamellae. Velocityvs spacing data were found to fit an equation of the formV ∝ S ^-n withn = 2.07. Arguments are presented which indicate that previous literature values ofn = 2.4 and 2.7 may be high due to experimental difficulties. This paper is based on a presentation made at the symposium “Establishment of Microstructural Spacing during Dendritic and Cooperative Growth” held at the annual meeting of the AIME in Atlanta, Georgia on March 7, 1983 under the joint sponsorship of the ASM-MSD Phase Transformations Committee and the TMS-AIME Solidification Committee.     

Solid-solution strengthening in Ni-C alloys

“Pure” nickel and Ni-C solid-solution alloys of various grain sizes (ASTM no. 2 to 10), with eight different carbon concentrations in the range 0.008 to 0.304 wt pct, were strained in tension between 4° and 474°K at a strain rate of 8.3 × 10⁻⁵per sec. The critical resolved shear stress (CRSS) was independent of temperature in the range 200° to 474° K (athermal region). Below 200°K, the CRSS increased sharply with decreasing temperature, the increase being larger for alloys of high carbon concentration. Both the temperature-dependent and the athermal alloy hardening were found to be linear functions of carbon concentration. The strain-rate sensitivities of flow stress of alloys did not change with strain and were larger for alloys of higher carbon concentrations. The Hall-Petch relation was used to calculate the CRSS of Ni-C single crystals, Τ₀ ^f, so that the data can be compared with existing alloy hardening theories. The data are compatible with the solid-solution theory of Friedel in which the hardening is attributed to both elastic and electrical interactions between dislocation cores and solute atoms. Formerly with the Edgar C. Bain Laboratory for Fundamental Research, U.S. Steel Corp., Research Center, Monroeville, Pa. Formerly with the Edgar C. Bain Laboratory for Fundamental Research.     

高钼低铌钢中铌的测定

在高钼低铌钢中的铌的测定中,钼对铌的测定存在严重的干扰.本文介绍了使用显色剂氯代磺酚S在强酸条件下,加入掩敝剂消除钼离子的影响后,与铌生成11深蓝色的络合物.在3mol/L盐酸介质中,加入适量新型柠檬酸混合掩蔽剂就能有效地消除钼的干扰.通过多项平行试验筛选出合适的掩蔽剂用量、显色剂用量、给出加快络合反应速度的条件.本方法测定数据稳定,准确度高,速度快,能有效地应用于生产.     

Substructure strengthening in dispersion-strengthened nickel alloys

Pure nickel, 80 pct Ni-20 pct Cr, 98 pct Ni-2 pct ThO₂, and 78 pct Ni-20 pct Cr-2 pct ThO₂ were studied in a wide range of thermomechanical conditions to identify strengthening mechanisms in the dispersion-strengthened materials. An X-ray line profile technique was used to determine the distribution of lattice strain, the crystallite domain size and the incidence of twins and stacking faults. Transmission electron microscopy was carried out, and tensile tests were done at room temperature and at an elevated temperature. It was found that cold deformation of Ni?ThO₂ did not produce lattice strains as large as was the case with pure nickel and Ni?Cr. However, deformation of Ni?Cr?ThO₂ did generate high lattice strains, due it is thought to the influence of chromium on cross-slip. The materials containing high lattice strains recrystallized more readily on annealing or testing at high temperature. It was concluded that room temperature strength was related to domain size without regard to composition in the series investigated. Strengthening by particle-dislocation interaction was not thought to be applicable when the domain size was small compared to the interparticle spacing, or at elevated temperatures. High temperature strength was determined primarily by the presence of a polygonized dislocation substructure which was stabilized by the thoria dispersion.     

A comparison of the effect of neutron irradiation on high purity vanadium and vanadium oxygen alloys

An investigation of the effect of neutron damage on the low temperature deformation characteristics of high purity vanadium (R _300K/R _4.2K=1100) was undertaken for two purposes. One purpose was to determine if reducing the impurity interstitial content to a lower level would result in a large difference in the effective stress between irradiated and non-irradiated samples. The present data along with previously obtained data does indicate that the difference increases as the impurity interstitial content is reduced. The explanation of this observation is based on the rapid increase of the non-irradiated yield stress at 77 K due to small increases in the oxygen content; however, the increase of the yield stress of the irradiated samples is much less with the same increase in oxygen content. A second purpose of this investigation was to determine the size and density of observable neutron produced defects as a function of oxygen content by transmission electron microscopy, and to relate the changes in density with changes in the yield stress. It was found that the density decreases and the size increases as the oxygen content decreases. There is qualitative agreement between the increase in yield stress at 300 K and the observable defect density. However, the change in the yield stress at 77 K due to neutron irradiation cannot be related to defect density and size.     

高铌高磷GH4169C和GH4169合金的组织稳定性

为研究高铌高磷GH4169C高温合金在高温长期时效过程中的组织稳定性,通过场发射扫描电镜和数显布氏硬度计对GH4169和GH4169C两合金分别经600、650、704及720℃时效30~10000 h的显微组织和硬度变化进行对比分析。结果表明：在服役温度（650℃）范围内长期时效,GH4169合金和GH4169C合金均表现优异的稳定性;在服役温度以上长期时效,GH4169合金和GH4169C合金稳定性较差,短时间内,合金组织就出现失稳。对比而言,704℃时GH4169C合金组织稳定性较GH4169合金高,而720℃时GH4169C合金组织稳定性劣于GH4169合金。分析认为,GH4169C合金由于提高Nb含量和P含量使的γ＇相稳定性增加,得以在服役温度以上（704℃）表现比GH4169合金更为优异的组织稳定性,但Nb含量的提高也引起啄相含量的增加,导致组织稳定性下降。在超高温（720℃）下,GH4169C合金稳定性劣于GH4169合金。由此推知,相比GH4169合金,改型GH4169C合金在使用温度上有所提高,但提高有限,在超高温下,其稳定性反而降低。     

Elevated-temperature strengthening of iron alloys by titanium

Previous studies of solid-solution strengthening of alloys of iron revealed that titanium-bearing alloys exhibit unusual strength between 300 and 600°C. The nature and the stability of this strengthening effect of titanium were investigated because of its possible value in creep-resistant steels. Repeated hot-hardness surveys indicated that the hardening remained even after annealing to temperatures as high as 900°C. Creep studies at 500°C revealed that the strengthening was stable for at least 200 h and that it was associated with a high creep-activated energy. On the basis of these results and the results obtained by other workers, we suggest that the high-temperature strengthening results from the presence in the lattice of stable clusters containing titanium, carbon, and/or nitrogen atoms which reorient in the stress field of mobile dislocations and thereby impede their motion.     

High-temperature oxidation-resistant coatings on niobium and its alloys

The formation of high-temperature oxidation-resistant coatings on niobium by multicomponent activated diffusion saturation in vacuum is investigated. Thermodynamic calculations of potential chemical reactions that result in anticorrosive protective coatings are performed. Comparative tests of high-temperature oxidation resistance of niobium with protective coatings are conducted at 1700°C in open air.     

Dispersion strengthening of vanadium alloys with nitrides

Conclusions Additional nitride strengthening of vanadium alloys of the V-Nb-Ti system is a promising means of increasing their high-temperature strength. At temperatures of up to 800°C the V-20% Nb-5% Ti-0.43% N alloy investigated substantially surpasses in strength the majority of existing vandium superalloys. In the press-forged condition VTAN-54 alloy possesses an optimum combination of strength and ductility characteristics. It attains its highest strength in the cold-worked condition after 1-h annealing at 600°C. At service temperatures exceeding 800°C nitride strengthening loses its effectiveness owing to the development of intense strength-loss processes.Translated from Poroshkovaya Metallurgiya, No. 9 (225), pp. 51–55, September, 1981.     

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.     

Solution of niobium in iron during arc surfacing

The solution of niobium in iron during arc surfacing is simulated. The model is based on diffusional dissolution of niobium particles in metals. It includes diffusion equations, initial and boundary conditions, and also the equations of motion of the boundary between the media. The diffusion coefficient in liquid iron is assumed to much exceed that in the solid solution. The solution of the Stefan diffusional problem within the solid is found as a Fourier series in terms of cosines; the solution within the liquid is found in terms of error functions. The time for the dissolution of niobium is determined. It is 10–100 s for 10-μm particles at high temperatures.     

Modeling solid solution strengthening in nickel alloys

The yield stress of multicomponent nickel solid solution alloys has not been modeled in the past with respect to the effects of composition and temperature. There have been investigations of the effect on the yield stress of solutes in binary systems at a fixed temperature, but the effects on the yield stress of multiple solute elements and temperature changes have not been investigated. In this article, two different forms of the trough model are considered for nickel-base alloys to determine the most applicable model for solid solution strengthening in the system. The yield stresses of three binary nickel-chromium and three ternary nickel alloys were determined at a range of temperatures. The yield stress of the alloys was then modeled using the Feltham equation. The constants determined in fitting the Feltham equation to the experimental data were then applied to other experimental solid solution alloys and also to published information on commercial solid solution nickel alloys. It was found that the yield stress of the nickel solid solution alloys could be modeled successfully using the Feltham equation.