The effects of molybdenum and aluminum on the thermal expansion coefficients of nickel-base alloys

The effects of molybdenum and aluminum on the mean linear thermal expansion coefficients from room temperature to 1050°C were determined for two types of nickel-base alloys. The Solid Solution Alloys were cast and homogenized Ni-Co-Cr-Mo alloys with 0, 312, and 612 nominal wt pct molybdenum concentrations. The Gamma Prime Alloys were wrought and heat-treated Ni-Cr-Mo-Al(Ti) alloys with 0, 2, 5, and 8 nominal wt pct molybdenum in each of four aluminum plus titanium levels (3 pct Al, 412 pct Al, 6 pct Al, or 1 pct Al + 312 pct Ti nominal wt pct). Thermal expansion coefficients were determined on at least two specimens from each alloy. It was found that molybdenum lowers the thermal expansion coefficients of both the cast Ni-Co-Cr solid solutions and the wrought Ni-Cr-Al(Ti) two-phase alloys. Both aluminum and titanium were also observed to decrease expansion coefficients in the two-phase, γ + γ, alloys. Results are discussed in terms of relative melting point effects between solute and solvent elements, and in terms of the volume fraction of the γ phase present.     

The effects of molybdenum and aluminum on the hot corrosion (sulfidation) behavior of experimental nickel-base superalloys

The hot corrosion behavior of a series of wrought nickel-base superalloys containing approximately 13 wt pct Cr was studied as a function of molybdenum content from 0 to 8 wt pct Mo in each of four Al + Ti levels (3 pct Al, 41/2 pet Al, 6 pct Al, and 1 pct Al-31/2 pct Ti). Specimens were tested in a burner rig with a 5 ppm sea salt concentration in a 1 pct sulfur diesel fuel burned at a 30:1 air-to-fuel ratio and were cycled between 1600°F (870°C) or 1800°F (980°C) and room temperature every 50 h during a 1000-h test. It was found that molybdenum significantly reduced the amount of hot corrosion attack at 1600°F (870°C) for the three aluminum-containing series of alloys. For the titanium-containing series of alloys tested at 1600°F (870°C) and all alloys exposed at 1800°F (980°C), molybdenum exerted little influence on hot corrosion behavior. Aluminum was found to markedly increase sulfidation rates at both 1600°F (870°C) and 1800°F (980°C) for all molybdenum levels. Titanium appeared to be beneficial to the hot corrosion resistance of these alloys at 1600°F (870°C) and detrimental at 1800°F (980°C). It was further noted that 1600°F (870°C) represented a more severe sulfidation condition than 1800°F (980°C) under these test conditions.     

The effects of microstructure on elevated temperature crack growth in nickel-base alloys

The effects of microstructure on the fatigue and creep crack growth of Waspaloy and P/M Astroloy were evaluated at 650°C. In Waspaloy, changes in γ′ size and distribution did not markedly affect fatigue crack growth. An increase in fatigue crack growth rate occurred at low test frequencies and was associated with a transition to intergranular crack propagation. In P/M Astroloy, a coarser grain size lowered the fatigue crack growth rate. Serrated grain boundaries, though beneficial under creep loading, have no effect in fatigue.     

The influence of sulfides on the oxidation behavior of nickel-base alloys

The oxidation of presulfidized chromium, Ni?Cr, and Ni?Al alloys, and complex nickel base alloys was studied at 1000°C in 1.0 atm of oxygen. Sulfur-rich surface layers were produced in the pretreatment by using H₂S?H₂ mixtures. Presulfidized chromium oxidized at a rate similar to that of sulfur-free chromium. The oxidation rate of presulfidized Ni?Cr alloys was affected by sulfur only when liquid nickel sulfide was present which accelerated the oxidation rate by creating rapid diffusion paths through the Cr₂O₃ scale. The oxidation behavior of presulfidized Ni?Al alloys, with aluminum contents sufficient for the formation of a protective Al₂O₃ layer in the sulfur-free condition, was influenced by sulfur only when aluminum sulfide was formed in the presulfidation treatment which caused the Al₂O₃ scale to be porous. The oxidation behavior of nickel-base alloys containing both chromium and aluminum was insensitive to the presence of sulfides when the concentration of aluminum in the alloy was such that a protective Al₂O₃ scale was formed during oxidation of the sulfur-free alloy and aluminum sulfide was not formed in the presulfidizing treatment.     

The effects of molybdenum additions to nickel-chromium alloys on their sulfidation properties

The effect of molybdenum additions to γ-phase Ni-Cr on the reaction of the alloys with H₂/H₂S atmospheres at 700°C has been investigated. The effect is to reduce the rate of internal sulfidation of the alloys, diminish greatly the extent of liquid sulfide formation and slow the growth rate of external solid sulfide scale. The first two effects are thermodynamic in origin whilst the last effect is due to favorable doping of Cr₂S₃ and to the formation within a Cr₃S₄ scale of MoS₂ precipitates which partially block outward metal diffusion.     

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.     

The effects of molybdenum additions to nickel-chromium alloys on their sulfidation properties

The effect of molybdenum additions to γ-phase Ni-Cr on the reaction of the alloys with H₂/H₂S atmospheres at 700°C has been investigated. The effect is to reduce the rate of internal sulfidation of the alloys, diminish greatly the extent of liquid sulfide formation and slow the growth rate of external solid sulfide scale. The first two effects are thermodynamic in origin whilst the last effect is due to favorable doping of Cr₂S₃ and to the formation within a Cr₃S₄ scale of MoS₂ precipitates which partially block outward metal diffusion.     

Si-Al复合材料热膨胀系数的有限元计算方法研究

针对低膨胀系数的Si-Al复合材料,基于数字图像处理技术,建立与其显微组织照片相一致的有限元模型。利用有限元方法进行热载荷作用下的稳态应变场分析,根据热应变方程计算Si-Al复合材料微区的横、纵向热膨胀系数;统计Si-Al复合材料不同微区的横、纵向热膨胀系数,得到不同温度条件下的Al-50%Si(质量分数)热膨胀系数的有限元预测结果。有限元预测结果与实验测试结果吻合良好,揭示了Si组元分布对Si-Al复合材料热性能的影响,为Si-Al复合材料的热性能预测提供了新的研究手段。     

Heavy alloys with a low thermal expansion coefficient

The possibility of reducing the linear thermal expansion coefficient (LTEC) of heavy tungsten-base alloys by replacing the binder by one possessing Invar properties is studied. Use of Invar type alloys of the Fe-Ni and Fe-Co-Cr types as binders makes it possible to prepare heavy alloys with lower LTEC than for the base (tungsten). Tungsten dissolution in the binder is the main factor which restricts the LTEC of heavy alloys with an Invar binder.     

The influence of molybdenum on the γ/’phase in experimental nickel-base superalloys

The influence of 1, 3, and 5 at. pct Mo on the γ’precipitate has been studied in experimental wrought nickel-base superalloys containing about 14 at. pct Cr and 6-1/2, 9, or 12 at. pct Al, or 2 at. pct Al plus 4 at. pct Ti. Concentrations of all other elements were quite low to limit the observed effects to those of molybdenum alone. Molybdenum markedly increases the γ’ solvus temperature, as determined by the sensitive and relatively simple technique of differential thermal analysis; correspondingly, the weight fraction of γ’ increases with molybdenum additions for a given aging treatment. Molybdenum dissolves extensively in the γ’of the titanium-free alloys, but it dissolves to a considerably smaller extent in the γ’of the titanium-bearing alloys. Molybdenum substitutes for chromium in y’, but does not alter the aluminum or titanium contents of this phase. Lattice parameters of both the matrix and the γ’are increased markedly by molybdenum, in proportion to the molybdenum contents of these phases. The resulting effects on lattice-parameter mismatch correlate rather well with observed γ’morphology, which tends to change from spheroidal to cuboidal in titanium-free alloys, and from cuboidal to spheroidal in 2 at. pct Al-4 at. pct Ti alloys, as molybdenum is added to these alloys. J. W. Freeman was formerly Professor of Metallurgical Engineering, University of Michigan, Ann Arbor, Mich. (Deceased, November, 1970). This paper is based upon a thesis submitted by W. T. Loomis in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Horace H. Rackham School of Graduate Studies, The University of Michigan.     

Unloading yield effects in aluminum alloys

A yield effect produced by unloading and reloading during tensile deformation has been observed for several heat treated aluminum alloys. The effect depends on the composition and heat treatment of the material, as well as on the strain (or stress) at which the unloading takes place. Although the effect is independent of the unloading time at room temperature, it is found to be time dependent at 200 K where diffusion of solute elements in aluminum is sluggish. A qualitative model based on the shearing of coherent precipitates during deformation and subsequent healing of the precipitates by rapid short range diffusion in the unloaded state is proposed to explain the effect.     

The low cycle fatigue of three wrought nickel-base alloys

The low cycle fatigue behavior of three wrought nickel-base alloys, alloy 901, Waspaloy and alloy 718, has been examined at both room temperature and 811 K (1000°F). The fatigue properties can be rationalized in terms of the Manson-Coffin equation relating total strain range with the number of cycles to failure. For a given total strain range the fatigue life of the alloys at 811 K (1000°F) is substantially less than at room temperature. Microstructural examination revealed that crack nucleation and initial propagation was of classical stage I type. The cracks develop at the surface of test specimens and propagate along slip planes. Evidence is presented to relate the observed fatigue softening with shear and dissolution of γ precipitates.     

钼及钼合金烧结技术研究及发展

从粉末冶金烧结理论、烧结设备、烧结工艺参数及新型烧结技术等方面概述了钼及钼合金烧结技术的研究现状及进展。结论指出, 钼及钼合金烧结理论仍集中在传统粉末冶金理论体系; 烧结技术发展方向是获得全致密化、细晶、均质化的烧结坯体; 发展趋势是烧结设备及工装与粉末冶金新技术结合更为紧密, 出现更多交叉研究; 研究热点和难点是大型钼及钼合金坯件烧结致密化、微观组织均质化、细晶化及复杂烧结态异型坯件烧结过程中形状精确控制等。     

Influence of molybdenum on the creep properties of nickel-base superalloy single crystals

The influence of Mo on the creep properties of single crystals of a model nickel-base superalloy has been investigated. The Mo content was systematically varied from 9.8 to 14.6 wt pet in an alloy series based on Ni-6 wt pet Al-6 wt pet Ta. The optimum initial γ-γ′ microstructure for raft development and creep strength was produced in each alloy prior to testing. The creep lives at 982 °C and 234 MPa exhibited a steep peak as a function of Mo content, with the maximum in life occurring at about 14.0 wt pet Mo. Deviations of less than 1 wt pet Mo from the optimum composition resulted in an order of magnitude drop in properties. As the Mo content was increased from 9.8 to 14.0 wt pct, the magnitude of lattice mismatch significantly increased, which was believed to be beneficial because of stronger γ-γ′ interfaces. As the Mo content was increased further from 14.0 to 14.6 w/o, the mechanical properties degraded because of the precipitation of a deleterious third phase. The results suggest that small variations in refractory metal content and initial gg′ size can have profound effects on mechanical properties. Hence, composition ranges and microstructures for the attainment of optimum mechanical properties may be somewhat limited and require close process control.     

Mechanisms for the hot corrosion of nickel-base alloys

The Na₂SO₄-induced accelerated oxidation of nickel-base alloys containing elements such as Cr, Al, Mo, W, and V has been studied in 1.0 atm O₂ in the temperature range of 650° to 1000°C. It has been found that the hot corrosion behavior of these alloys can usually be characterized according to one of two types of attack: 1) Na₂SO₄-induced accelerated oxidation; 2) Na₂SO₄-induced catastrophic oxidation. In both types of hot corrosion, accelerated oxidation occurs as a result of the formation of a liquid flux based on Na2SC>4 which dissolves the normally protective oxide scales. Catastrophic, or self-sustaining rapid oxidation can occur in alloys which contain Mo, W, or V, because solutions of oxides of these elements with Na₂SO₄ decrease the oxide ion activity of the molten salts, producing melts which are acidic fluxes for oxide scales. The accelerated oxidation type of attack which was observed with most alloys which did not contain Mo, W, or V, was more severe than for normal oxidation, but much less severe than catastrophic oxidation. Na₂SO₄-induced accelerated oxidation occurs because the oxide ion activity of the Na₂SO₄ increases to the point where oxide scales can partially dissolve in the basic melt. Generally, this basic fluxing results from the diffusion of sulfur from the Na₂SO₄ into the alloy. In some alloys, the formation of sulfides during basic fluxing is a sufficient condition to cause accelerated oxidation. In other alloys, changes in the oxidation mechanism occur because of depletion of the alloy surface, concomitant with basic fluxing, of those elements needed for protective oxide scales, such as aluminum and chromium.     

The thermodynamic behavior of sulfur in molten nickel and nickel-base alloys

The thermodynamic properties of dilute solutions of sulfur in pure liquid nickel were investigated at 1500, 1550, and 1575°C for sulfur concentrations up to 0.7 wt pct. Based on the infinitely dilute, wt pct standard state, the equilibrium data obtained for the reaction: H₂(g) + S = H₂S(g) were fitted by the equations: logK = − 1489/T − 1.772, and ΔG° = 6812 + 8.11T, cal/mole. For the solution ofS ₂(g) in pure Ni according to the reaction: 1/2S ₂(g) = S (in Ni), the standard free energy of solution is found to be: ΔG° = - 28,342 + 3.62T, cal/mole. For the very dilute solutions of sulfur normally encountered in nickel-base melting, the activity coefficient of sulfur in pure Ni at 1575°C is given by: log f_S= -0.035 (pct S). The effects of alloying elements normally used in nickel-base alloys on the activity coefficient of sulfur in molten nickel were investigated. The activity coefficient of sulfur is increased by all of the alloying elements studied, as evidenced by the interaction parameters: e_S ^fe = +0.005, e_S ^Cr = +0.030, e_S ^Mo = +0.053, e_S ^Ti = +0.160, and e_S ^A1 = +0.133. Measured values of the activity coefficient of sulfur in the quaternary system Ni-S-Cr-Fe agreed reasonably well with those predicted from binary and ternary data. This work constitutes a portion of the work performed by W. F. VENAL for the Ph.D. degree from the University of Illinois at Chicago Circle. Formerly Professor of Metallurgical Engineering at UICC.     

The effect of thermal exposure on the microstructure and mechanical properties of nickel-base superalloys

The three nickel-base superalloys B-1900, TRW-NASA VIA, and René 80 were studied utilizing metallographic and residue analysis techniques in conjuction with mechanical property tests to determine the effect of thermal exposure on the microstructure and mechanical properties. Exposure times of 10, 100, 1000, and 5000 h at temperatures from 1400 to 2000°F (760 to 1093°C) were evaluated. Four minor phases-MC, M₆C, M₂₃C₆, and M₃B₂-plus gamma-prime were observed in the gamma matrix of these alloys. Significant variations in the mechanical properties were observed to occur with thermal exposure. Microstructural evaluation indicated that these variations in properties were due primarily to gamma-prime agglomeration or ripening. Perturbations noted in a number of the mechanical property vs exposure temperature curves in the 1500 to 1900°F (816 to 1038°C) temperature range appeared to be due to the precipitation and growth of M₆C and/or M₂₃C₆ carbides.