High-temperature mechanical behavior of cold-worked stress-relieved Zr-2.5Nb

The mechanical behavior of cold-worked stress-relieved Zr-2.5Nb pressure tube material used in pressurized heavy-water reactors is summarized in terms of ultimate tensile strength (UTS), yield strength (YS), and percentage elongation to fracture, strain-rate sensitivity (m), strain-hardening index (n), and activation energy for the self-diffusion of zirconium. Tensile tests carried out in the temperature range of 298 to 773 K at three different strain rates are used to calculaten andm. At higher temperatures, UTS, YS, percentage elongation, and n are found to be strain rate dependent. The plateaus observed in curves for UTS or YS versus temperature are typical of dynamic strain aging behavior. The drop in strength at high temperature is correlated with the self-diffusion of zirconium in α-phase zirconium by comparing the activation energies of the two phenomena. Transmission electron microstructures of thin foils from samples tested at ambient temperature and at 773 K are used to explain the observed variation ofn andm with test temperature. A similar trend in the variation ofm and percentage elongation with temperature is also explained.     

Modeling of stress ratio effect on Al alloy SAE AMS 7475-T7351: Influence of loading direction

The main goal of this work was to evaluate the effectiveness of Walker’s equation in collapsing the fatigue crack propagation data of a SAE AMS 7475-T7351 aluminum alloy loaded either longitudinally (L-T) or transversely (T-L) to the rolling direction. T-L orientation testpieces presented lower ductility and fracture toughness values than L-T orientation. As a consequence, during the fatigue crack propagation tests, T-L testpieces exhibited a stronger influence of monotonic modes of fracture, resulting in higher Paris exponent values,m. Walker’s model was able to collapse fatigue crack propagation data of L-T test pieces at different applied stress ratios,R. However, for the T-L orientation, due to theR ratio dependency onm andC, simply averaging ofm values for the calculations of Walker’s exponent proved to be inefficient. A simple analytical procedure was proposed by the authors to modify Walker’s model to take into account such effect. For T-L test pieces, when Walker’s model is modified by considering both Paris’s exponent as well the coefficient as a function of theR ratio, the fatigue crack growth data collapses within a narrow band, thus allowing predictions to be made satisfactorily. The collapsed band is even narrower if the empirical relationm=a+blogC is used instead of simple polynomial equations due to a better correlation coefficient.     

Effects of alloying elements on plastic deformation of single crystals of MoSi2

Effects of alloying elements on the compression deformation behavior of single crystals of MoSi₂ have been investigated in the temperature range from room temperature to 1500°C. The alloying elements studied include V, Cr, Nb and Al that form a C40 disilicide with Si and W and Re that form a C11_b disilicide with Si. The addition of Al is found to decrease the yield strength of MoSi₂ at all temperatures while the additions of V, Cr and Nb are found to decrease the yield strength at low temperatures (below 800°C) and to increase the yield strength at high temperatures (above 1300°C). In contrast, the additions of W and Re (C11_b formers) are found to increase the yield strength at all temperatures. Of the ternary elements investigated, Al and Nb seem to be the most effective in improving the low-temperature deformability of MoSi₂ while Re and Nb seem to be the most effective in improving the high-temperature strength, judging from the observed yield strength.     

Néel temperature of high Mn austenitic steels

The predictive equations for the Néel temperature (T_N) of high Mn austenitic steels were reviewed and reevaluated using 116 different measured T_N values. The previous equations gave good predictions for T_N values of high Mn-low C austenitic steels, but not for those of medium and low Mn-high C austenitic steels, including TWIP steels with less than 20 at.% Mn. Therefore, an improved T_N equation for medium and low Mn-high C as well as high Mn-low C was newly suggested. To improve the accuracy of the new equation, especially for high C TWIP steels, the T_N values of three different high C TWIP steels were experimentally measured using a superconducting quantum interference device (SQUID) because there are few measured T_N values of high C TWIP steels in the literature.     

Analysis and estimation of the yield strength of API X70 and X80 linepipe steels by double-cycle simulation tests

In this study, the spiral piping and electric resistance welding piping was conducted on API X70 and X80 linepipe steel sheets having different microstructures, and the yield strengths of the flattened sheets were measured. A double-cycle simulation test with tension-compression-tension or compression-tension-tension for the piping and flattening processes was conducted to estimate the yield strength. The simulation test results indicated that the yield strengths of the outer or inner wall of the pipe could be estimated by combination of Swift’s equation and the Bauschinger stress parameter, and that these estimated yield strengths were well matched within a small error range with the measured yield strengths. Thus, the variations in yield strength before and after the piping could be effectively estimated using the tension/compression properties of the leveled sheets because the strength differential effect was small and the reverse flow curves were expressed by a single curve. These findings suggested that the present estimation method played an important role in controlling microstructural and manufacturing process parameters to minimize the reduction in yield strength of the linepipe steel sheets.     

Densification and compressive strength ofin-situ processed Ti/TiB composites by powder metallurgy

In the present study, the densification of Ti/TiB composites, the growth behavior ofin-situ formed TiB reinforcement, the effects of processing variables — such as reactant powder (TiB₂, B₄C), sintering temperature and time — on the microstructures and the mechanical properties ofin-situ processed Ti/TiB composites have been investigated. Mixtures of TiB₂ or B₄C powder with pure titanium powder were compacted and presintered at 700°C for 1 hr followed by sintering at 900, 1000, 1100, 1200, and 1300°C, respectively, for 3hrs. Some specimens were sintered at 1000°C for various times in order to study the formation behavior of TiB reinforcementin-situ formed within the pure Ti matrix. TiB reinforcements were formed through different mechanisms, such as the formation of fine TiB and the formation of coarse TiB by Ostwald ripening or the coalescence of fine TiB. There was no crystallographic relationship between TiB reinforcement and the matrix. There were voids at the interface between the TiB reinforcement and the Ti matrix due to the preferential growth of coarse TiB without a particular crystallographic relationship with pure Ti matrix and the surface energy between the Ti matrix and TiB reinforcements. Therefore, the densification of Ti/TiB₂ compacts was hindered by the preferential growth of coarse TiB reinforcements. The mechanical properties ofin-situ processed composites were evaluated by measuring the compressive yield strength at ambient and high temperatures. The compressive yield strength of thein situ processed composites was higher than that of the Ti-6A1-4V alloy. It was also found that the compressive yield strength of the composite made from TiB₂ reactant powder was higher than that of the composite made from B₄C at the same volume fraction of reinforcement. A crack path examination suggested that the bonding nature of interface between matrix and reinforcement made from TiB₂ reactant powder was better than that made from B₄C.     

Correlation between microstructure and mechanical properties of severely deformed metals

There is a correlation between the microstructure and the mechanical behavior of ultrafine-grained face centered cubic (fcc) metals processed by equal-channel angular pressing (ECAP). It is shown that the saturation yield strength is related to the maximum dislocation density according to the Taylor equation and, in addition, the value of the parameter α in the Taylor equation is strongly affected by the stacking fault energy because of different geometrical arrangements of dislocations within the grains. It is also demonstrated that the ductility of Cu processed by ECAP decreases with increasing strain but at extremely high strains the ductility is partially restored due to the recovery of the microstructure.     

Effects of Zr Content on the Yield Strength of an Al-Sc Alloy

The effects of Zr content on the yield strength of an Al-Sc alloy are investigated experimentally. It has been shown that the yield strength increases with time at the beginning of annealing for the investigated one Al-Sc alloy and three Al-Sc-Zr alloys. Such an increase of yield strength results from the high nucleation rate for Al₃Sc particles in Al-Sc alloy and Al₃(Sc_1−x ,Zr _x ) particles in Al-Sc-Zr alloys. Throughout the annealing, the yield strength increases with the Zr content, indicating that the alloy with higher Zr content possesses higher yield strength. The high yield strength of the alloy with high Zr content is due to the higher number density and volume fraction of the particles as well as their smaller size and inter-particle spacing. Such a microstructural feature for the particles exhibits a larger Orowan strengthening effect by inhibiting the dislocation movements.     

Thermodynamic analysis of the data on the solubility of nitrogen in gamma iron at a high pressure

An equation is proposed for an isotherm of the solubility of gaseous nitrogen in γ iron at a high gas pressure. This equation takes into account the partial volume of nitrogen in austenite. This equation is used to analyze the experimental data on the solubility of nitrogen in γ iron at a nitrogen pressure of up to 348 MPa and to estimate the experimental value of the Wagner interaction parameter in austenite at a temperature of 1273 K (? _N ^N = 6.3 ± 1.0). This value is compared to the theoretical value (? _N ^N = 9.0) that corresponds to the interaction potential between nitrogen atoms in austenite that was determined earlier using Mössbauer spectroscopy. The possible causes of the discrepancy between the experimental and calculated values of parameter ? _N ^N are discussed.     

The characteristics of the pitting corrosion of stainless steel above breakdown potential

The pitting corrosion of AISI 304 stainless steel is studied in 1 N H₂SO₄|;0.5 N NaCl at 25°C. With the potentiokinetic method, a linear relationship is obtained between the breakdown potential and the cubic root of the scanning rate. Extrapolating to zero scanning rate gives the steady state breakdown potential E_b = 596 mV(SHE). At constant potential above E_b, the pits density first increases with time and tends to a steady state value (S.S.D.P.) which is reached above 10 mA cm⁻². The average deepening rate of pits V_dp increases with E − E_b according to log V_dp = −7.5 + 2.89 log (E − E_b). The time dependance of current density J at constant potential above E_b follows the equation log J = a + b log t. The values of b are different below and above 10 mA cm⁻², confirming the change in mechanism of pit corrosion: no new pit is initiated above that value. It is suggested that it is possible to characterize the quality of passivating layers not only on the basis of E_b, but also in terms of S.S.D.P., V_dp and b at given values of E − E_b.     

Prediction of the limiting drawing ratio and the maximum drawing load in cup-drawing

A new, simple and practically applicable equation, including the normal anisotropy value R and the strain hardening exponent n, for estimating the limiting drawing ratio LDR in cup-drawing of a cylindrical cup with a flat-nosed punch is derived. The normal anisotropy is based on Hill's theory of an anisotropy sheet that is isotropic in its plane. Whiteley's equation for estimating the LDR, and Hill's upper limit value of LDR, are two special cases of the new equation. Compared with the published experimental work, good agreement between the calculation and the experiment is obtained. The new equation shows that the most important parameter for LDR is the normal anisotropy value R, the strain hardening exponent n has also some influence on the LDR, and clearly explains the real interaction between the normal anisotropy value R and the strain hardening exponent n on the LDR. It is different from other equations, which are functions of the normal anisotropy value R only.A new equation, incorporating the value of LDR derived as above and the critical drawing load Pc based on the maximum load principle for localization of plastic flow, for estimating the maximum drawing load Pc at a certain drawing ratio DR in cup-drawing with a flat-nosed punch is developed. This equation is simple and supplies an accurate estimation of the maximum drawing load Pd. A comparison between the calculation and the experiment shows that good agreement is also obtained.It is thereby possible to better understand and control the cup-drawing behavior of sheet metal.     

Effect of Scandium Additions on the Tensile Properties of Cast Al-6Mg alloys

Effect of aging on the mechanical properties of Al-6Mg alloy doped with varying concentration of scandium ranging from 0.2 to 0.6 wt.% is analyzed. As-cast samples were aged isochronally for 60 min at different temperatures ranging from 100 to 500 °C. Evaluation of mechanical properties of the aged Al-6Mg (Sc) alloys was done by employing an Instron testing machine. Various strain rate of testing were used to find out the values of strain-rate sensitivity of the experimental alloys. The influence of scandium is much pronounced on yield strength than on the tensile strength. Alloys with higher scandium content have shown higher yield strength and the values of strain-rate sensitivity ‘m’ at peak-aged condition have been found to be comparatively high at higher scandium concentration. The fracture of the experimental alloys occurs through microvoid coalescence.     

Microstructural design of high-strength aluminum alloys

A summary is presented of recent attempts to model the effects of precipitate shape, orientation, and distribution on yield strength and age-hardening response, using appropriate versions of the Orowan equation and models of precipitation strengthening developed for Al alloys containing a single uniform distribution of rationally oriented plate-or rod-shaped precipitates, which are either shearable or shear resistant. It is demonstrated that these models of particle strengthening are capable of predictions that are in excellent quantitative agreement with experimental observations that high tensile yield strength is associated with microstructures containing a high density of intrinsically strong, plate-shaped precipitates with {111}_α or {100}_α habit planes and large aspect ratio. The authors predict that further improvement in the strength of existing Al alloys might be achieved by increasing the number density and/or aspect ratio of rationally oriented precipitate plates.     

Correlation between interfacial energy and phase diagram in ceramic-metal systems

Estimates of interfacial energies between ceramics (MX) and liquid metals (A) have been arrived at by employing an improved version of the Becker’s model for interfacial energy calculations at liquid metal-liquid metal interfaces to calculate the total energy of interatomic bonds across an interface. The results of such an approximation yield values that are very close to the experimental values; for instance, the estimated value for Al₂O₃/lFe is 2 J · m⁻², which compares well with the experimental data ranging from 1.9 to 2.3 J · m⁻². It is suggested that interfacial energies depend on two terms: (1) the formation energy of MX and (2) the pairwise interaction energies between M and A atoms and X atoms and vacant interstitial sites. It is also found that the calculated interfacial energies in eutectic systems (carbides-metal, etc.) are generally low, while those in the monotectic systems (oxides-metal, etc.) are generally high.     

Tensile and fracture toughness properties of SiC<Subscript>p</Subscript> reinforced Al alloys: Effects of particle size,particle volume fraction,and matrix strength

The goal of this work was to evaluate the effects of particle size, particle volume fraction, and matrix strength on the monotonic fracture properties of two different Al alloys, namely T1-Al2124 and T1-Al6061, reinforced with silicon carbide particles (SiC_p). From the tensile tests, an increase in particle volume fraction and/or matrix strength increased strength and decreased ductility. On the other hand, an increase in particle size reduced strength and increased the composite ductility. In fracture toughness tests, an increase in particle volume fraction reduced the toughness of the composites. An increase in matrix strength reduced both K _crit and δ_crit values. However, in terms of K _Q (5%) values, the Al6061 composite showed a value similar to the corresponding Al2124 composite. This was mainly attributed to premature yielding caused by the high ductility/low strength of the Al6061 matrix and the testpiece dimensions. The effect of particle size on the fracture toughness depends on the type of matrix and toughness parameter used. In general, an increase in particle size decreased the K _Q (5%) value, but simultaneously increased the amount of plastic strain that the matrix is capable of accommodating, increasing both δ_crit and K _crit values.     

Correlation between interfacial energy and phase diagram in ceramic-metal systems

Estimates of interfacial energies between ceramics (MX) and liquid metals (A) have been arrived at by employing an improved version of the Becker’s model for interfacial energy calculations at liquid metal-liquid metal interfaces to calculate the total energy of interatomic bonds across an interface. The results of such an approximation yield values that are very close to the experimental values; for instance, the estimated value for Al₂O₃/lFe is 2 J · m⁻², which compares well with the experimental data ranging from 1.9 to 2.3 J · m⁻². It is suggested that interfacial energies depend on two terms: (1) the formation energy of MX and (2) the pairwise interaction energies between M and A atoms and X atoms and vacant interstitial sites. It is also found that the calculated interfacial energies in eutectic systems (carbides-metal, etc.) are generally low, while those in the monotectic systems (oxides-metal, etc.) are generally high.     

An investigation of the deformation mechanisms in sn5% sb alloy using tensile, creep and abi tests from ambient to 473k

Tensile, creep, and automated ball indentation (ABI) tests have been conducted to study deformation mechanisms in Sn5%Sb alloy between ambient and 473 K. A power law relationship was obtained between minimum creep rate and applied stress, with stress exponent,n=5 and activation energy,Q=12.6±1.1 kCal/ mole. At 473 K, a transition fromn=5 ton=3 was observed at low stresses. ABI tests showed a power law relationship between strain rate and ultimate tensile stress with values ofn=5 andQ=13.0±1.8 kCal/mole. Tensile results were in broad agreement with the creep and ABI data. A new deformation mechanism is proposed for then=5 region involving viscous glide of dislocations assisted by dislocation core diffusion.     

Fabrication of Al alloys reinforced with AlN particles formed byin-situ reaction

The tensile properties and microstructures of Al alloys reinforced with AlN particles formed byin-situ reaction under a nitrogen atmosphere were analyzed. It was found that AlN particle layers formed on the surface of the Al particles in the powder bed, which replaced the Mg₃N₂ coated layers through the reaction, Mg₃N₂+2A1 → 2AlN+3Mg. The tensile strength and 0.2% offset yield strength in the control alloys were significantly greater than those in commercial alloys. This increase was due to the fine AlN particles formed by the abovein-situ reactions of the Mg₃N₂ formation and its decomposition into AlN. This article is based on a presentation made in the symposium “The l^st KIM-JIM Joint Symposium: High Strength Ratio Aluminum Alloys”, held at Inha University, Inchon, Korea, October 22, 1999 under the auspices of The Korean Institute of Metals and Materials and The Japanese Institute of Metals.     

Failure analysis of beta-C titanium alloy high-pressure vessels

An attempt has been made to apply the linear elastic fracture mechanics concept to Beta-C titanium alloy pressure vessels that exhibited brittle fractures during hydrotesting. Based on the results of stress analysis on the real structures and fracture surface examinations, a stress-intensity factor,K _IC, was estimated. TheK _IC value of the material in the cracking direction was measured by a surface semi-elliptical crack method. It was found that theK _IC value of the material is very close to the estimated stress-intensity factorK _I during failure, which places the pressure vessels in a critical condition in that a small variation in flaw size may cause a catastrophic failure. A compromise must be made betweenK _IC and the required yield strength. In this restricted case, the yield strength of the material should be controlled in the range of 1150 to 1200 MPa to avoid brittle fracture and the possible occurrence of yield during hydrotesting. Control of microstructure and other mechanical properties is also discussed in this investigation.     

晶粒尺寸对42CrMoVNb钢超高周疲劳性能的影响

研究了不同热处理制度下得到的3种具有不同晶粒尺寸的42CrMoVNb高强度钢的超高周疲劳性能. 结果表明, 超高周疲劳强度和疲劳强度比并不随晶粒尺寸的减小而单调提高, 中等晶粒尺寸的试样具有最高的疲劳强度和疲劳强度比. SEM断口观察表明, 绝大部分试样的疲劳裂纹起源于夹杂物. 随着疲劳断口裂纹源夹杂物处应力强度因子幅ΔK_inc的减小, 疲劳寿命N_f增加; 而在夹杂物周围的粗糙粒状区域(GBF)的应力强度因子幅ΔK_GBF并不随N_f变化而变化, 基本为一常数, 且粗晶粒试样的ΔK_GBF高于细晶粒试样. 这表明, 细化晶粒对高强度钢的超高周疲劳性能有着复杂的影响,存在一个合理的细化晶粒范围.