The effect of dispersed oxides on the oxidation behavior of Al-and Ti-Containing Ni-20 pct Cr alloys

The effect of dispersed Y₂O₃ and La₂O₃ on the isothermal and cyclic oxidation behavior of Al- and Ti-containing Ni-20Cr alloys has been investigated. It has been found that the dispersolids improve the oxidation resistance of these alloys, but this improvement is less than that observed in aluminum- and titanium-free Ni-20Cr alloys. The oxidation resistance imparted by the presence of the dispersoids was found to increase as the aluminum and titanium contents were decreased. It is proposed that these changes in oxidation resistance arise from differences in the extent of subscale attack, oxidation induced alloy porosity and internal oxidation.     

The effect of dispersed reactive metal oxides on the oxidation resistance of nickel-20 Wt pct chromium alloys

Nickel-20 wt pet chromium alloys containing ThO₂, Y₂O₃, La₂O₃, Al₂O₃ and Li₂O, as prepared by the mechanical alloying technique, were examined for isothermal and cyclic oxidation resistance in dry air at 1000, 1100 and 1200°C. TDNiCr, a commercial electrical heating element alloy (Com Ni-20Cr) and a laboratory melted alloy⁹Lab Ni-20Cr) were also tested. It was found that Y₂O₃, La₂O₃, Al₂O₃ and ThO₂ dispersoids markedly increased both isothermal and cyclic oxidation resistance compared to Lab Ni-20Cr at all temperatures; in contrast Li₂O additions gave no improvement in protection. Com Ni-20Cr was in between Lab Ni-20Cr and the Y₂O₃, A1₂O₃ and ThO₂ containing alloys in both cyclic and isothermal oxidation performance. A mechanism based on alterations in the defect structure of Cr₂O₃ is proposed to explain these dispersed oxide effects on isothermal oxidation behavior. It is based on a reduction in cation transport rates which in turn alter the rate of oxide growth. ThO₂-containing alloys fabricated by the mechanical alloying technique were found to have oxidation resistance fully equal to commercial TDNiCr. Com Ni-20Cr performed better than Lab Ni-20Cr, but not as well as TDNiCr.     

Influence of the precipitate-free zone width on the tensile properties of an Al-6 Wt pct Zn-1.2 Wt pct Mg alloy

The mechanical properties of an Al-6 wt pct Zn-1.2 wt pct Mg alloy with various width of precipitate-free zones have been investigated. The width of the precipitate-free zone (PFZ) has been changed by the quench interruption technique without any appreciable change in the size and distribution of precipitates. An important relationship has been observed between the width of the PFZ and the quench-interruption period;i.e., the width of the PFZ increases in proportion to the square root of the holding time at 200°C. From the analysis of stress-strain curves as well as the observation of dislocation arrangements in slightly deformed specimens, the plastic deformation has been found to occur preferentially in the PFZ. The initial stage of deformation is much affected by the change in the width of the PFZ, but in the later stage, the work-hardening rate seems to be almost independent of the PFZ width. Tensile tests show that the ultimate tensile strength and the 0.2 pct proof stress decrease very little with increasing width of the PFZ, while the uniform elongation is practically constant regardless of the reduction in the nonuniform elongation. The work-hardening rate at the initial stage of deformation is found to decrease in proportion to the reciprocal of the PFZ width. This relationship can be explained from the dislocation model for work hardening.     

The effect of dispersed reactive metal oxides on the oxidation resistance of nickel-20 Wt pct chromium alloys

Nickel-20 wt pet chromium alloys containing ThO₂, Y₂O₃, La₂O₃, Al₂O₃ and Li₂O, as prepared by the mechanical alloying technique, were examined for isothermal and cyclic oxidation resistance in dry air at 1000, 1100 and 1200°C. TDNiCr, a commercial electrical heating element alloy (Com Ni-20Cr) and a laboratory melted alloy⁹Lab Ni-20Cr) were also tested. It was found that Y₂O₃, La₂O₃, Al₂O₃ and ThO₂ dispersoids markedly increased both isothermal and cyclic oxidation resistance compared to Lab Ni-20Cr at all temperatures; in contrast Li₂O additions gave no improvement in protection. Com Ni-20Cr was in between Lab Ni-20Cr and the Y₂O₃, A1₂O₃ and ThO₂ containing alloys in both cyclic and isothermal oxidation performance. A mechanism based on alterations in the defect structure of Cr₂O₃ is proposed to explain these dispersed oxide effects on isothermal oxidation behavior. It is based on a reduction in cation transport rates which in turn alter the rate of oxide growth. ThO₂-containing alloys fabricated by the mechanical alloying technique were found to have oxidation resistance fully equal to commercial TDNiCr. Com Ni-20Cr performed better than Lab Ni-20Cr, but not as well as TDNiCr.     

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.     

The effect of oxygen on the structure and mechanical behavior of Aged Ti-8 Wt pct Al

It is shown that for a Ti-8 wt pct Al alloy aged at a temperature high in the two-phase region (695°C) to precipitate the ordered α₂ phase, an increase in oxygen content from 600 ppm to 1200 ppm decreases the fracture strain from 20 to 1 pct elongation at room temperature and slightly increases the yield strength. The fracture mode is changed from dimpled rupture to predominantly cleavage. Further increase in oxygen content to 3000 ppm does not produce significant additional changes in ductility or yield strength. It is demonstrated that oxygen additions alter the position of the α/α + α₂ coherent solvus, resulting in formation of coherent α₂ in specimens containing ⪞ 1000 ppm oxygen aged at 968 K (695°C). For a given aging time the volume fraction of α₂ increases with increasing oxygen up to 1300 wt ppm and then levels off. The changes in mechanical behavior are attributed to the presence of α₂. The experimental evidence suggests that oxygen partitions preferentially into α₂. Formerly a Graduate Student in the Department of Metallurgy and Materials Science, University of Pennsylvania     

Effect of Mg and Sr additions on the formation of intermetallics in Al-6 Wt pct Si-3.5 Wt pct Cu-(0.45) to (0.8) Wt pct Fe 319-type alloys

Al-Si alloys are materials that have been developed over the years to meet the increasing demands of the automotive industry for smaller, lighter-weight, high-performance components. An important alloy in this respect is the 319 alloy, wherein silicon and copper are the main alloying elements, and magnesium is often added in automotive versions of the alloy for strengthening purposes. The mechanical properties are also ameliorated by modifying the eutectic silicon structure (strontium being commonly employed) and by reducing the harmful effect of the β-Al₅FeSi iron intermetallic present in the cast structure. Magnesium is also found to refine the silicon structure. The present study was undertaken to investigate the individual and combined roles of Mg and Sr on the morphologies of Si, Mg₂Si, and the iron and copper intermetallics likely to form during the solidification of 319-type alloys at very slow (close to equilibrium) cooling rates. The results show that magnesium leads to the precipitation of Al₈Mg₃FeSi₆, Mg₂Si, and Al₅Mg₈Cu₂Si₆ intermetallics. With a strontium addition, dissolution of a large proportion of the needle-like β-Al₅FeSi intermetallic in the aluminum matrix takes place; no transformation of this phase into any other intermetallics (including the Al₁₅(Fe,Mn)₃Si₂ phase) is observed. When both Mg and Sr are added, the diminution of the β-Al₅FeSi phase is enhanced, through both its dissolution in the aluminum matrix as well as its transformation into Al₈Mg₃FeSi₆. The reactions and phases obtained have been analyzed using thermal analysis, optical microscopy, image analysis, and electron microprobe analysis (EMPA) coupled with energydispersive X-ray (EDX) analysis.     

The effect of molybdenum on high-temperature properties of 9 pct Cr steels

Heat-resistant 9 Cr steels with 1, 2, and 3 pct Mo were tested for mechanical properties, weldability, and creep-rupture properties. The elevated-temperature and rupture strengths increase with increasing molybdenum content. While the 9 Cr-1 Mo steel is martensitic and is precipitation strengthened with carbides, the 9 Cr-2 Mo and 9 Cr-3 Mo steels receive added benefits from precipitation of Laves phase and solid-solution strengthening. The latter cause little decrease in ductility and impact resistance. The 9 Cr-2 Mo and 9 Cr-3 Mo steels are characterized by a duplex microstructure which aids weldability. Weld cracking tests show no need for preheating the latter steels, although the martensitic 9 Cr-1 Mo steel is known to be susceptible to weld cracking if not preheated. Both duplex-structure steels have good resistance to stress-relief cracking. Anisotropy of mechanical properties, due to the orientation of the duplex structure in the rolling direction, is less than that observed in the fully martensitic 9 Cr-1 Mo steel.     

On the improvement of creep strength and ductility of Ni-20 pct Cr by small zirconium additions

The creep and fracture properties of high-purity Ni-20 pct Cr and Ni-20 pct Cr-0.11 pct Zr alloys are compared at 1073 K in vacuum. The Ni-20 pct Cr alloy cavitates at the grain boundaries and fractures intergranularly after strains of typically 20 pct. The observed cavity growth rates are in keeping with those predicted. Alloying with zirconium substantially increases the creep strength and ductility. Creep rupture associated with dynamic recrystallization occurs, and voids are observed only in heavily necked parts of the samples. In addition to Ni₅Zr and ZrO₂ inclusions, a Zr₄C₂S₂ carbo-sulfide was identified. Thus, the sulfur-gettering effect of zirconium even at very low residual sulfur levels (20 wt ppm) was confirmed. The zirconium-induced increase in the creep strength is discussed, and the inhibition of creep cavitation by zirconium is examined within the framework of thermal cavity nucleation. Lowering of the grain boundary diffusivity and the grain boundary free energy as well as dynamic recrystallization are likely to reduce cavity nucleation and growth rates in Ni-Cr-Zr and will thus increase its ductility. Finally, the results are used to illustrate the critical importance of minor alloying additions in constructing and using fracture mechanism maps.     

Effect of stress during aging on the precipitation of θ′ in Al-4 Wt pct Cu

Experiments have been made on Al-4 wt pct Cu single crystals to determine the influence of stress, applied during aging, on precipitation. Without stress, θ platelets precipitate on all (100) planes. Transmission electron microscopy showed a marked effect of aging stress on the microstructure. A compressive stress of approximately 48 MPa (～ 7,000 psi) parallel to [001] favored precipitation on (010) and (100) and inhibited precipitation on (001). When a tensile stress of similar magnitude was applied parallel to [001], precipitation was favored on (001) and inhibited on (010) and (100).     

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.     

Effect of undercooling on the microstructure of Ni-35 at. pct Mo (eutectic) and Ni-38 at. pct Mo (hypereutectic) alloys

Ni-35 at. pct Mo (eutectic) and Ni-38 at. pct Mo (hypereutectic) alloy specimens have been solidified from various levels of undercooling in the differential thermal analysis (DTA) and the electromagnetic levitation (EML) units in a pyrex/vycor bed. The evolution of the microstructure in the solidified specimens has been examined in terms of the degree of undercooling, the nature of the first phase to nucleate from the melt, and the specimen cooling rate. The melt has been observed to undercool more in the presence of intermetallic NiMo (β) phase as compared to that in the presence of nickel-rich solid solution (γ). The “anomalous eutectic” type of microstructure has been shown to result from the initial formation of the dendritic skeleton of either of the two phases, its segmentation due to convection and ripening, and the subsequent nucleation of the other phase in the interdendritic liquid regions. The recalescence behavior has been examined as a function of undercooling and the nature of the phase nucleating first in the melt.     

The effect of nitrogen additions upon the pitting resistance of 18 pct Cr, 18 pct Mn Stainless Steel

The effect of nitrogen additions upon the pitting resistance of 18 pct Cr, 18 pct Mn stainless steel has been investigated by potentiokinetic techniques in a 1000 ppm NaCl solution. Nitrogen additions increased the pitting resistance of the steel irrespective of structure, however, the ferritic steel was less pit resistant than the (duplex) steels containing both austenite and ferrite which, in turn, were less pit resistant than the totally austenitic steels. For steels having a duplex structure, the effect of nitrogen on the pitting resistance was observed to follow a linear function of the relative amount of austenite in these steels due to the area effects of the austenite and ferrite which are galvanically coupled in these steels. The addition of nitrogen was found to increase the amount of austenite at a rate of approximately 200 times the percent nitrogen addition from 36 pct austenite for the 0.02 pct N steel to 100 pct for the 0.40 pct nitrogen steel. The addition of nitrogen to the totally austenitic steels increased the pitting resistance at the rate of approximately 0.31 volts per pct nitrogen added, but no mechanism was found for the increased resistance. This paper is based on a presentation made at a symposium on “New Developments in Ferritic and Duplex Stainless Steels,” held at the Fall Meeting in Cleveland, Ohio, on October 19, 1972, under the sponsorship of the Corrosion Resistant Metals Committee of TMS-IMD and the Corrosion and Oxidation Activity of the ASM.     

回火温度对30Cr15Mo1N微观组织和力学性能影响

 为探索高氮马氏体钢在回火过程中组织性能演变，对30Cr15Mo1N高氮钢进行了不同温度下的回火处理，利用OM、XRD、拉伸、冲击、SEM和TEM等方法研究了高氮钢在回火过程中微观组织和力学性能的变化规律。结果表明，30Cr15Mo1N钢经淬火和低温处理后在150～700 ℃回火，随回火温度升高，显微组织中马氏体基体逐渐发生回复与再结晶，组织中马氏体形态逐渐消失，碳氮化物先在马氏体板条边界呈片状或棒状析出，逐渐演变为颗粒状弥散分布，700 ℃时碳氮化物聚集长大、球化。随着回火温度升高，30Cr15Mo1N钢的基体持续软化，析出强化不断增强，导致其在500 ℃以下回火时强度变化较小，抗拉强度保持在2 000 MPa以上；当回火温度大于500 ℃时，强度随回火温度升高而线性下降。随着回火温度升高，30Cr15Mo1N钢的U型缺口冲击吸收功先基本保持不变再持续升高，在700 ℃回火后冲击韧性达到45 J/cm2。不同回火温度下冲击性能的变化与其强度、塑性变化密切相关，冲击韧性好坏主要由塑性大小决定。     

Effect of phosphorus on the mechanical properties of normalized 0.1 pct C, 1.0 pct Mn steels

The effect of phosphorus, in amounts up to 0.2 pct, on the stress-strain characteristics and strain-rate sensitivity of a series of laboratory-produced 0.1C–1.0Mn steels has been determined on normalized 10 mm-thick (0.38 in.) plate at temperatures from −196 to 400°C (−321 to 752°F). It was found that 0.05 pct phosphorus increased the yield strength of the 0.1C–1.0Mn steel about 24 MPa (3.5 ksi) at 22°C (72°F) and above without significantly affecting the Charpy V-notch shelf energy or fracture-appearance transition temperature. Additions of 0.1 and 0.2 pct phosphorus caused a greater increase in the yield strength (48 and 93 MPa or 7 and 13.5 ksi) but also increased the transition temperature. The strengthening effect of phosphorus in these steels is much the same as that found previously for phosphorus in iron, and analysis of the strain-rate-sensitivity data shows that the same deformation models are applicable. Strong elastic interactions between phosphorus atoms and dislocations are believed to be responsible for the observed deformation behavior. Comparison of the present results with those obtained previously on Fe−P alloys and with data in the literature indicates that the strengthening resulting from phosphorus additions to steel is expected to be additive to other strengthening mechanisms.     

The effect of omega phase on the mechanical properties of titanium alloys

The effect of the precipitation of the metastable ω phase on the tensile properties of β-phase titanium alloys has been studied. The volume fraction of ω phase was varied by controlling the heat treatment temperature and the alloy content. It is shown that provided the volume of ω phase is less than 0.6, significant increases in yield strength can be obtained while retaining reasonable ductility. Higher volume fractions results in complete macroscopic embrittlement, but fracture still occurs by microvoid coalescence. Thin film microscopy of deformed samples shows that dislocations bypass the omega particles. The results are discussed in relation to current theories of deformation and fracture of two-phase alloys.     

The effect of sample size on the steady state creep characteristics of Ni-6 pct W

The effects of sample size and grain morphology on the steady state creep properties of Ni-6 pct W in the temperature range of 0.55T _m to 0.74T _m have been studied. It is shown that a decrease in sample thickness results in a corresponding decrease in the number of grains per thickness and gives rise to a decrease in the measured values of bothQ _creep andn when compared to existing data on thick samples with many grains per thickness. The observed effects of sample configuration on the creep properties are explained with a model for creep deformation which is based on the interaction of free surfaces with grain boundary sliding and grain deformation. Using this model, an expression for the stress and temperature dependence of the total steady state strain rate is obtained as a function of the grain matrix strain rate and the grain boundary sliding strain rate. The results of this model are shown to correlate well with the observed deformation characteristics of the thin samples and to explain the variations ofQ{creep} andn with sample morphology. Formerly Graduate Student, Department of Materials Science and Engineering, Stanford University     

The effect of quench rate on the microstructure,mechanical properties,and corrosion behavior of U-6 wt pct Nb

The effect of cooling rate on the microstructure, mechanical behavior, corrosion resistance, and subsequent age hardenability of U-6 wt pct Nb is described and discussed. Cooling rates in excess of 20 Ks^-1 cause the parent γ-phase to transform martensitically to a niobium supersaturated variant of the α-phase. This martensitic phase exhibits low hardness and strength, high ductility, good corrosion resistance, and substantial age hardenability. As cooling rate decreases from 10 Ks^-1 to 0.2 Ks^-1, fine scale microstructural changes (consistent with spinodal decomposition) occur to an increasing extent. These changes produce large increases in hardness and strength and large decreases in ductility, slight decreases in corrosion resistance, and slight changes in age hardenability. At cooling rates less than 0.2 Ks^-1 the parent phase undergoes cellular decomposition to a coarse two-phase lamellar microstructure. This lamellar microstructure exhibits intermediate strength and ductility, substantially reduced corrosion resistance, and no age hardenability. An analysis of the cooling rates at the centers of water quenched plates indicates that fully martensitic microstructures can be obtained in plates as thick as 50 mm.