Stress corrosion cracking of high strength HY-180M steel in 3.5 Pct NaCl

Stress corrosion cracking of HY-180M steel was studied at 22°C in an aqueous solution of 3.5 pct NaCl (pH = 6.5). The steel had a nominal weight percentage composition of 10Ni-14Co-2Cr-lMo-0.16C and was heat treated to yield a fracture toughness value ofK _Ic ≃ 160 MPa . m^1/2. The SCC velocity (v) was studied as a function of stress intensity (K _I) and electrochemical potential (E) using precracked compact tension specimens, a Ag/AgCl reference electrode and a 1000 h exposure test. Also, the polarization behavior, microstructure, fractography and corrosion products were studied. The results showed that SCC was markedly dependent uponE, and did not occur whenE =-0.52 V_SHE (-0.72 V_Ag/AgCl), which corresponded closely to the thermodynamically reversible potential of iron. However, SCC occurred at a more noble potential of-0.28 V_SHE (-0.48 V_Ag/AgCl ) and at a less noble potential of-0.80 V_SHE (-1.00 V_Ag/AgCl). The stress intensity below which SCC was not observed was K_ISCC ≃ 5.5 MPa . m^1/2 at -0.28 V_SHE and K_ISCC ≃ 60 MPa . m^1/2 at -0.80 V_SHE . Also, Region I behavior (v dependent uponK ₁) and Region II behavior (v independent ofK ₁) were observed. Cracking was considered to occur solely by hydrogen embrittlement at -0.80 Vshe, whereas anodic dissolution processes played a necessary role, either directly or indirectly, in SCC at -0.28 V_SHE . The indirect effects were discussed in relation to hydrolysis effects in the crack promoting hydrogen embrittlement and/or corrosion product wedging stresses.     

Effect of temperature upon stress corrosion cracking of HY-180M steel in 3.5 Pct NaCI

A fracture mechanics and fractographic study of stress corrosion cracking (SCC) of heat treated HY-180 M steel was undertaken over the temperature range 22 to 95 °C at applied potentials of −0.28 V_SHE (−0.48 V_Ag/AgCl) and −0.80 V_SHE (−1.0 V_Ag/Agcl). Particular attention was directed toward Region II behavior, where crack propagation rates were independent of stress intensity(K _l). Region II rates were always higher at the less noble potential of −0.80 V_SHE than at the more noble potential of — 0.28 V_SHE. However, fractography studies suggested that the basic mechanism of cracking at both potentials was the same, and involved hydrogen embrittlement. An Arrhenius analysis of Region II rates showed that crack propagation was under the control of more than one process. Consequently, the mechanistic details remained obscure. Formerly Research Associate in theDepartment of Metallurgical Engineering, University of BritishColumbia.     

Stress Corrosion Cracking of HY-180 Steel in Aqueous 3.5 Pct NaCI

Stress corrosion cracking of HY-180 steel (Fe-10 Ni-2 Cr-1 Mo-8 Co-0.12 C) was studied in aqueous 3.5 pct NaCI (pH = 6.5) at 22 °C. The alloy was austenitized, water quenched and aged at 510 °C for 5 h. Specimens were of the precracked, double cantilever beam (DCB) variety and exposure times extended up to 1000 h. The crack propagation rates (v) were studied as a function of stress intensity(K,) under both freely corroding potentials(E ≈-0.36 V_SHE) and potentials produced by coupling to Zn(E ≈ -0.82 V_SHE. Crack fractography was studied by scanning electron microscopy and corrosion products were identified by electron diffraction analysis. The stress intensity, K_ISCC, below which SCC could not be detected was ~45 MPa m^1/2 for both freely corroding and Zn-coupled conditions. Analysis of the results showed that cracking was consistent with a hydrogen embrittlement mechanism, irrespective of potential. Furthermore, comparison of the data with previous studies on a similarly heat treated and closely related alloy (HY-180 M), containing 14 Co-0.16 C, showed no significant difference in SCC behavior, provided comparison was made at similar electrochemical potentials.     

Caustic Stress Corrosion Cracking of Mild Steel

The stress corrosion cracking (SCC) behavior of cold worked mild steel in hot, aqueous, 33 pct NaOH solutions was studied with prefatigue cracked double cantilever beam specimens. SCC kinetics were studied under freely corroding potentials (E _corr ≈ −1.00 V_SHE) and potentiostatic potentials of −0.76 V_SHE near the active-passive transition. The pH of the liquid within the crack was determined and fractography was studied by scanning electron microscopy. Cracking was transgranular atE _corr, intergranular at −0.76 V_SHE, and produced no detectable change in crack liquid pH from that of the bulk solution. Crack rates were dependent upon temperature, potential, and stress intensity (K ₁). The apparent activation energy in Region II, where crack growth rate was independent ofK, was ∼ 24kJ/mol for both cracking modes. This was considered to be due to mixed rate control involving activation polarization and mass transport processes. The mechanism of cracking was entirely consistent with metal dissolution at –0.76 V_SHE and may involve hydrogen embrittlement and/or dissolution effects atE _corr. DOUGLAS SINGBEIL, formerly Research Student, University of British Columbia, is Research Scientist, Pulp and Paper Research Institute of Canada, 570-Blvd. St. Jean, Pointe Claire, Quebec, Canada H9R 3J9.     

Evaluation of the reduction of iron oxide from liquid slags using a graphite rotating disk

The rotating disk methodology has been used for examination of the reduction of FeO from CaO-FeO-SiO₂ liquid slags (20 and 60 pct FeO) with a CaO/SiO₂ ratio equal to 0.66 and 1.27, in the temperature range 1350 °C to 1420 °C. It has been found that the reduction proceeds under diffusion control. The calculated diffusion coefficients fall in the range 0.76·10⁻⁷ to 1.6·10⁻⁶ cm²/s. Comparison of these values with those given in the literature suggests that the calculated coefficients are related to the diffusion of oxygen ions in the slag. The calculated thickness of the limiting diffusion layer, δ, ranges from 0.65·10⁻³ to 5.25·10⁻³ cm, depending on the reduction conditions. The largest decrease in the limiting diffusion layer thickness takes place at low rotational speeds, i.e., 100 and 400 rev/min. The maximum value of the mass transfer coefficient is 1.71·10⁻³ cm/s for reduction from slag with a CaO/SiO₂ ratio of 1.27, 60 pct FeO, at 1420 °C and 2000 rev/min, and the minimum value is 0.27·10⁻⁴ cm/s for reduction from slag with a CaO/SiO₂ ratio of 0.66, 20 pct FeO, at 1350 °C and 100 rev/min. Good agreement has been found between experimental and calculated reduction rates at low disk rotations (100 and 400 rev/min).     

Characterization of Ti4AlN3

Bulk samples of Ti₄AIN₃ were fabricated by reactive hot isostatic pressing (hipping) of TiH₂, AlN, and TiN powders at 1275 °C for 24 hours under 70 MPa. Further annealing at 1325 °C for 168 hours under Ar resulted in dense, predominantly single-phase samples, with <1 vol pct of TiN as a secondary phase. This ternary nitride, with a grain size of ≈20 μm on average, is relatively soft (Vickers hardness 2.5 GPa), lightweight (4.6 g/cm³), and machinable. Its Young’s and shear moduli are 310 and 127 GPa, respectively. The compressive and flexural strengths at room temperature are 475 and 350 MPa, respectively. At 1000 °C, the deformation is plastic, with a maximum compressive stress of ≈450 MPa. Ti₄AlN₃ thermal shocks gradually, whereby the largest strength loss (50 pct) is seen at a ΔT of 1000 °C. Further increases in quench temperature, however, increase the retained strength before it ultimately decreases once again. This material is also damage tolerant; a 100 N-load diamond indentation, which produced an ≈0.4 mm defect, reduces the flexural strength by only ≈12 pct. The thermal-expansion coefficient in the 25 °C to 1100 °C temperature range is 9.7±0.2 × 10⁻⁶ °C⁻¹. The room-temperature electrical conductivity is 0.5 × 10⁶ (Θ · m)⁻¹. The resistivity increases linearly with increasing temperature. Ti₄AlN₃ is stable up to 1500 °C in Ar, but decomposes in air to form TiN at ≈1400 °C. graduated from the Department in June of 1999 with an MS thesis.     

Shear ligament phenomena in Fe3Al intermetallics and micromechanics of shear ligament toughening

The environment-assisted cracking behavior of a Fe₃Al intermetallic in an air moisture environment was studied. At room temperature, tensile ductility was found to be increased with strain rate, from 10.1 pct at 1×10⁻⁶ s⁻¹ to 14.3 pct at 2 × 10⁻³ s⁻¹. When tensile tests were done in heat-treated mineral oil on specimens that have been heated in the oil for 4 hours at 200°C, ductility was found to be recovered. These results suggest the existence of hydrogen embrittlement. Shear ligaments, which are ligament-like structures connected between microcracks, were observed on the tensile specimens. They undergo ductile fracture by shearing and enhance fracture toughness. This toughness enhancement (represented byJ _l ) was estimated by a micromechanical model. The values of the unknown parameters, which are the average ligament length , the area fractionV _l , and the work-to-fractureτ ₁ γ ₁, were obtained from scanning electron microscopy (SEM) observation. The total fracture toughnessK _c andJ _l were reduced toward a slower strain rate. The experimental fracture toughness,K _Q , was found to be increased with strain rate, from 35 MPa at 2.54×10⁻⁵ mm·s⁻¹ to 47 MPa at 2.54×10⁻² mm·s⁻¹. The fact that strain rate has a similar effect onK _Q andK _c verifies the importance of shear ligament in determining fracture toughness of the alloy. With the presence of hydrogen, length and work-to-fracture of the shear ligament were reduced. The toughening effect caused by shear ligament was reduced, and the alloy would behave in a brittle manner.     

Thermal gradients,strain rate,and ductility in sheet steel tensile specimens

The effect on ductility of strain rate and thermal gradients arising from deformation is examined in tensile specimens of 1008 AK steel. The total elongatione _tot is taken as the measure of ductility, since it reflects changes in the strain hardeningn and strain-rate sensitivitym. Tensile specimens are pulled to failure in 23 °C air, at initial strain rates from 10⁻³ to 10⁻¹ s⁻¹, with thermocouples recording temperature along the 50.8 mm gauge section. The maximum temperature is ∼110 °C just prior to failure at the highest rate. Thee _tot, however, remains fairly constant with rate at ∼40 pct. When thermal gradients are prevented by immersing the specimens in circulating water at 23 °C,e _tot, increases with rate to a maximum of ∼54 pct at 10⁻¹ s⁻¹. Direct measurements of isothermal values ofm at 23, 60, and 90 °C show thatm increases with rate.e _tot, therefore, would be expected to increase with rate. Since under nonisothermal conditionse _tot does not change, it appears thatm and thermal gradients are competing influences on ductility at higher rates. Enhanced ductility in stampings should be possible by suppressing gradients, either by controlling die temperature or by heat transfer properties of a lubricant.     

The high temperature phase diagrams for zirconium-molybdenum and hafnium-molybdenum

The high temperature regions of the Zr−Mo and Hf−Mo binary phase diagrams have been constructured from temperature-composition data obtained by gravimetric and pyrometric methods. The liquidus curves were obtained directly from the measurements of saturation solubilities of molybdenum (single crystal) in liquid Zr and Hf. The solubility results are supported by electron microprobe analyses which identify the formation of thin (∼10 μm) layers of nearly stoichiometric compounds ZrMo₂ and HfMo₂ on the surface of the single crystal molybdenum below the respective peritectic temperatures 1918±5 and 2206±5°C. These thin layers and the negligible diffusion zones of Zr and Hf in single crystal molybdenum do not significantly affect the measured solubilities. The diffusion coefficient of Hf in Mo-single crystal at 2080°C is ∼5×10⁻¹² m² s⁻¹. The melting, solidus, liquidus, eutectic and peritectic temperatures were directly measured by pyrometrically observing the partial or complete destruction of “black-body” conditions inside an effusion cell with the appearance of a liquid phase that forms a highly reflecting mirror. The melting points of Zr and Hf metals, 1860±3 and 2228±3°C, respectively, are in good agreement with previously assessed values. The respective eutectic temperatures peratures and compositions 1551±2°C, 29.0±0.5 at. pct Mo and 1896±3°C, 40.5 at. pct Mo, are considerably more precise and only in fair agreement with previously measured or estimated values. The liquidus composition at the peritectic temperature for the Zr−Mo binary is precisely fixed at 54.0±1.0 at. pct Mo and that for the Hf−Mo binary is 61 ±3 at. pct Mo. The thermodynamic activities of molybdenum in the liquid Zr−Mo alloy indicate positive deviations from Raoult's Law. temporarily attached to the Chemistry Division, Argonne National Laboratory, Argonne IL 60439 This work was performed at Argonne National Laboratory under the auspices of the U.S. Energy Research and Development Administration     

Influence of tempering on the microstructure and mechanical properties of HSLA-100 steel plates

The influence of tempering on the microstructure and mechanical properties of HSLA-100 steel (with C-0.04, Mn-0.87, Cu-1.77, Cr-0.58, Mo-0.57, Ni-3.54, and Nb-.038 pct) has been studied. The plate samples were tempered from 300 °C to 700 °C for 1 hour after austenitizing and water quenching. The transmission electron microscopy (TEM) studies of the as-quenched steel revealed a predominantly lath martensite structure along with fine precipitates of Cu and Nb(C, N). A very small amount of retained austenite could be seen in the lath boundaries in the quenched condition. Profuse precipitation of Cu could be noticed on tempering at 450 °C, which enhanced the strength of the steel significantly (yield strength (YS)—1168 MPa, and ultimate tensile strength (UTS)—1219 MPa), though at the cost of its notch toughness, which dropped to 37 and 14 J at 25 °C and −85 °C, respectively. The precipitates became considerably coarsened and elongated on tempering at 650 °C, resulting in a phenomenal rise in impact toughness (Charpy V-notch (CVN) of 196 and 149 J, respectively, at 25 °C and −85 °C) at the expense of YS and UTS. The best combination of strength and toughness has been obtained on tempering at 600 °C for 1 hour (YS-1015 MPa and UTS-1068 MPa, with 88 J at −85 °C).     

On the fracture and fatigue properties of Mo-Mo<Subscript>3</Subscript>Si-Mo<Subscript>5</Subscript>SiB<Subscript>2</Subscript> refractory intermetallic alloys at ambient to elevated temperatures (25 °C to 1300 °C)

The need for structural materials with high-temperature strength and oxidation resistance coupled with adequate lower-temperature toughness for potential use at temperatures above ∼1000 °C has remained a persistent challenge in materials science. In this work, one promising class of intermetallic alloys is examined, namely, boron-containing molybdenum silicides, with compositions in the range Mo (bal), 12 to 17 at. pct Si, 8.5 at. pct B, processed using both ingot (I/M) and powder (P/M) metallurgy methods. Specifically, the oxidation (“pesting”), fracture toughness, and fatigue-crack propagation resistance of four such alloys, which consisted of ∼21 to 38 vol. pct α-Mo phase in an intermetallic matrix of Mo₃Si and Mo₅SiB₂ (T₂), were characterized at temperatures between 25 °C and 1300 °C. The boron additions were found to confer improved “pest” resistance (at 400 °C to 900 °C) as compared to unmodified molybdenum silicides, such as Mo₅Si₃. Moreover, although the fracture and fatigue properties of the finer-scale P/M alloys were only marginally better than those of MoSi₂, for the I/M processed microstructures with coarse distributions of the α-Mo phase, fracture toughness properties were far superior, rising from values above 7 MPa √m at ambient temperatures to almost 12 MPa √m at 1300 °C. Similarly, the fatigue-crack propagation resistance was significantly better than that of MoSi₂, with fatigue threshold values roughly 70 pct of the toughness, i.e., rising from over 5 MPa √m at 25 °C to ∼8 MPa √m at 1300 °C. These results, in particular, that the toughness and cyclic crack-growth resistance actually increased with increasing temperature, are discussed in terms of the salient mechanisms of toughening in Mo-Si-B alloys and the specific role of microstructure.     

Solubility of some transition metal oxides in cryolite-alumina melts: Part II. Solubility of TiO2

The solubility of TiO₂ in cryolite-alumina melts at 1020 °C was measured; it decreased with increasing alumina concentration up to ∼3.5 wt pct total oxide and then increased at higher alumina concentrations. The solubility was found to be 3.1 wt pct Ti in cryolite, and 2.7 wt pct Ti in an alumina-saturated melt. Modeling indicated that the most probable titanium species are TiOF₂ and Na₂TiO₃, which coexist in the solution; the former dominates at low alumina concentrations and the latter at high alumina concentrations. Additional unknown amounts of fluoride may also be associated with these species. Determination of the solubility of TiO₂ in alumina-saturated melts as a function of temperature showed that the solubility increased from 1.9 wt pct Ti at 975 °C to 2.8 wt pct Ti at 1035 °C, the apparent partial molar enthalpy of dissolution of TiO₂ being 88±4 kJ mol⁻¹.     

The niobium (columbium)-palladium constitution diagram

The Nb-Pd system was investigated over the entire composition range by metallography and X-ray diffraction analysis. The solubility limits of terminal and intermediate phases and solidus temperatures were determined. α-Nb dissolves ∼36 at. pct Pd at. 1520°C and ∼20 at. pct Pd at 800°C; α-Pd dissolves ∼31 at. pct Nb at 1610°C and ∼18 at. pct Nb at temperatures below 1500°C. The presence of three intermediate phases NbPd₂ (MoPt₂-type), α-NbPd₃ (TiAl₃-type), and β-NbPd₃ (β-NbPd₃-type) was confirmed; NbPd₂ melts at 1610°C and one of the NbPd₃ phases transforms at the same temperature into α-Pd solid solution which melts at 1625°C. In addition, an approximately equiatomic high-temperature phase α-NbPd with a homogeneity range of ∼11 at. pct was found which melts at 1520 to 1565°C and probably is an extension of and isomorphous with the α-Pd solid solution. Five three-phase reactions are described, and crystal chemical relationships are discussed. D. P. PARKER formerly with MIT . R. C. MANUSZEWSKI formerly with the ADAHF Research Unit at NBS.     

The interfacial kinetics of the reaction of CO2 with nickel: Part II. the decarburization of liquid nickel

The kinetics of decarburization of liquid nickel in CO₂-CO mixtures have been studied at 1400 and 1500°C, using the experimental arrangement of the impinging jet. At carbon concentrations above about 1 wt pct, pressures of CO₂ ⪯ 0.1 atm, and for total gas flow-rates above about 40 l/min (STP) impinging on a metal surface of 2.08 cm², it is concluded that the interfacial reaction step controls the rate. Comparison with isotope exchange studies indicates that dissociative chemisorption of CO₂ is the rate determining step. Rate constants, based on the nominal surface area, are 1.2 ×10⁻³ and 1.4 × 10⁻³ mol/cm² · s · atm at 1400 and 1500°C, respectively. on leave of absence from the Homer Research Laboratories of the Bethlehem Steel Co.     

Characterization of superplastic deformation behavior of a fine grain 5083 Al alloy sheet

Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were observed over a strain rate range of 3 × 10⁻⁵ s⁻¹ to 1 × 10⁻² s⁻¹, with a maximum value of 0.65 at 5 × 10⁻⁴ s⁻¹ and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation, which produced a tensile elongation of 671 pct at 525 °C. R. VERMA, formerly Visiting Scientist, Department of Materials Science and Engineering, University of Michigan     

The tantalum-palladium constitution diagram

The Ta-Pd system was investigated over the entire composition range by metallography, X-ray diffraction and electron microprobe analysis. The solubility limits of terminal and intermediate phases and solidus temperatures were determined. α-Ta dissolved ∼20 at. pct Pd at 2550°C and ∼10 at. pct Pd at 1000°C; α-Pd dissolves ∼22 at. pct Ta at 1730°C and ∼18 at. pct Ta at 1000°C. The presence of four intermediate phases a, (β-U type), α-TaPd (TiCu type), TaPd₂ (MoPt₂ type), and TaPd₃ (TiAl₃ type) was confirmed; they melt or decompose (α-TaPd) at about 2550, 1410; 1800, and 1770°C, respectively. In addition, an equiatomic high temperature phase, β-TaPd was found which melts at ∼1720°C and may be an extension of and isomorphous with the α-Pd solution. Seven three-phase reactions are described. Formerly with Massachusetts Institute of Technology     

Kinetic study of the chlorination of gallium oxide

The kinetics of the chlorination of gallium oxide in chlorine atmosphere was studied between 650 °C and 800 °C. The calculations of the Gibbs standard free energy variation with temperature for the reaction Ga₂O_3(S)+3Cl_{2 (g)}→2GaCl_3(g)+1.5O_{2 (g)} show that direct chlorination is favorable above 850 °C. Thermogravimetric experiments were performed under isothermal and nonisothermal conditions. The effect of temperature, gas flow rate, and Cl₂ partial pressure were studied. The solids were characterized by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The nonisothermal results showed that chlorination of Ga₂O₃ starts at approximately 650 °C, with a mass loss of 50 pct at 850 °C. The isothermal results between 650 °C and 800 °C indicated that the reaction rate increased with temperature. The correlation of the experimental data with different solid-gas reaction models showed that the results are adequately represented by the model proposed by Shieh and Lee: X=1−{1−b ₂₂[b ₂₁ t+e ^−b ₂₁ ^t−1]}^1/(1−γ). From this model, it was found that the rate of reaction for the chlorination of Ga₂O₃ is of the order 0.68 with respect to Cl₂ and the activation energy is 113.23 kJ/mol. On the other hand, the order of the activation rate of the interface surface is 0.111 with respect to Cl₂ and its activation energy is 23.81 kJ/mol.     

The 885° f (475° c) embrittlement of ferritic stainless steels

The 885odgF (475°C) embrittlement of seven heats of chromium steels was investigated: four vacuum-melted heats with C + N < 0.008 pct and 14 pct Cr, 14 pet Cr-2 pet Mo, 18 pct Cr, or 18 pet Cr-2 pet Mo, and three air-melted heats with C + N > 0.09 pet and 18 pet Cr, 18 pct Cr-2 pet Mo, or 18 pet Cr-2 pet Mo-0.5 pct Ti. The steels were heated at 600° (316°), 700° (371°), 800° (427°), 900° (482°), and 1000°F (538°C) for various times up to 4800 h and the influence of this aging was investigated by hardness measurements, impact tests, and electron metallography. It was demonstrated that the embrittlement due to 885°F (475°C) exposure was caused by precipitation of a chromium-rich α^’ phase on dislocations. The nucleation rate of α^’ was calculated with the aid of Becker’s theory and the results were used to extrapolate experimental data obtained in this study. After an exposure of about 1000 h at 1000°F (538°C), a decrease in room temperature toughness was observed for all steels investigated. The decrease in toughness was not caused by immobilization of dislocations by α^’, but by precipitation of carbonitrides.     

Influence of strain rate and temperature on the deformation behavior of a metastable high carbon iron-nickel austenite

Austenitic specimens of Fe-15 wt pct Ni-0.8 wt pct C were tested in tension at strain rates of 10⁻⁴ s⁻¹ and 10⁻¹ s⁻¹ over the temperature range −20°C to 60 °C. The influence of strain rate and temperature on the deformation behavior depended on whether stress-assisted or strain-induced martensitic trans-formation occurred during testing. Under conditions of stress-assisted transformation, the ductility was low and independent of strain rate. However, when strain-induced transformation occurred, the duc-tility increased significantly and the higher strain rate resulted in greater ductility and more transfor-mation. Although the ductility increased continuously with temperature, the amount of strain-induced transformation decreased and no martensite was observed above 40 °C. Microstructural examination showed that the martensite was replaced by intense bands and that these bands contained very fine (111) fcc twins. The twinning resulted in enhanced plasticity by providing an additional mode of deformation as slip became more difficult due to dynamic strain aging at the higher temperature. This study confirms that the substructure following deformation will depend on the proximity of the deformation temperature to theM _s ^σ temperature. At temperatures much greater thanM _s ^σ , austenite twinning will occur, while at temperatures close toM _s ^σ , bcc martensite will form.     

High-temperature strength and ductility increases of Ti-Mo-Al alloys by step aging

Step-aging programs, based on principles of particle-dislocation interactions, were developed systematically to obtain increases in the high-temperature strength and ductility properties of Ti-7 at. pct Mo-Al alloys. A triple-step aging program applied to Ti-7 Mo-16 Al produced a yield stress σ_0.2 = 1,500 MN/m², elongation to fracture ε _F = 4 pct at room temperature, and σ_0.2 = 900 MN/m², ε _F = 12 pct at 600°C. A two-step aging program resulted in σ_0.2 = 1,350 MN/m², ε _F = 5 pct at room temperature; σ_0.2 = 800 MN/m², ε _F = 20 pct at 600°C. Formerly Assistant Research Professor, Materials Research Laboratory, Rutgers University