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.     

An investigation of the fracture and fatigue crack growth behavior of forged damage-tolerant niobium aluminide intermetallics

The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class of forged damage-tolerant niobium aluminide (Nb₃Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti (10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa√m at room temperature. Fracture in these alloys occurs by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed at room-temperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa√m), and fatigue crack growth resistance (comparable to Ti-6Al-4V, monolithic Nb, and inconnel 718). The implications of the results are discussed for potential structural applications of the 40Ti alloy in the intermediate-temperature (∼700 °C to 750 °C) regime.     

Fatigue and fracture toughness of a Nb-Ti-Cr-Al-X single-phase alloy at ambient temperature

The fatigue and fracture resistance of a commercially made, single-phase Nb-base alloy with 35 at. pct Ti, 5 at. pct Cr, 6 at. pct Al, and several elements to increase solid solution strengthening have been investigated. The threshold for fatigue crack growth was determined to be ≈7 MPa√m and fracture toughness ≈35 MPa√m. Crack growth was intermittent and sporadic; the fracture path was tortuous, crystallographic, and appeared to favor the {100} and {112} planes. Fatigue crack closure was measured directly at the crack tip. The fatigue and fracture properties of the commercial alloy are compared against those of Nb-Cr-Ti and Nb-Cr-Ti-Al alloys. The comparison indicated that Ti addition is beneficial for, but Al addition is detrimental to, both fracture toughness and fatigue crack resistance.     

An investigation of the effects of loading rate on resistance-curve behavior and toughening in cast lamellar gamma-based titanium aluminides

This article presents the results of a combined experimental and theoretical study of the effects of loading rate (1, 10, and 100 MPa√m · s⁻¹) on the resistance-curve behavior and toughening in cast lamellar gamma-based titanium aluminides (Ti-48Al-2Cr-2Nb, Ti-45Al-2Mn-2Nb + 0.8 vol pct TiB₂, and Ti-47Al-2Mn-2Nb + 0.8 vol pct TiB₂). Note that compositions are quoted in at. pct unless stated otherwise. The fracture-initiation toughness and resistance-curve behavior in Ti-48Al-2Cr-2Nb are shown to be similar at the three loading rates examined. In the case of the Mn-containing alloys, stronger resistance-curve behavior is observed as the loading rate increases from 1 to 10 MPa√m · s⁻¹. However, the fracture-initiation toughness and resistance-curve behavior of the Mn-containing alloys are similar at loading rates of 10 and 100 MPa√m · s⁻¹. The observed resistance-curve behavior is attributed largely to the role of ligament bridging and, to a lesser extent, to the effects of cracktip plasticity. Small- and large-scale bridging models are also shown to predict the measured resistance curves when the observed/measured bridging parameters and material properties are used in the micromechanical modeling of crack bridging. The implications of the results are also discussed for the design of damage-tolerant gamma alloys and microstructures.     

The effects on fracture toughness of ductile-phase composition and morphology in Nb-Cr-Ti and Nb-Siin situ composites

Niobium-chromium alloys, both single and two phase, were alloyed with titanium in order to enhance fracture toughness and fatigue crack growth resistance. The selection of titanium as an alloying element and the relationship of electronic bonding to toughness are examined. The results indicated that toughness increased with a decreasing number of D +s electrons. Titanium was found to increase the toughness of solid-solution Nb-Cr alloys from ≈8 to 87 MPa√m, while for the twophase “insitu composites,” toughness was increased from ≈5 to 20 MPa√m, although this is less than expected. Fracture toughness of the composites correlated nonlinearly with the volume fraction of the phases. The evidence suggests that the toughness of the composites is decreased due to fracture of the intermetallic particles and constraint on matrix deformation imposed by the intermetallic. Fracture characteristics of the Nb-Cr-Ti materials are compared to those of Nb-Cr and Nb-Si materials.     

Comparison of the fatigue and fracture of α+β and β titanium alloys

The present study compares the fatigue and fracture properties of the high-strength β titanium alloy β-Cez with the conventional α+β titanium alloy Ti-6Al-4V, because of increasing interest in replacing α+β titanium alloys with β titanium alloys for highly stressed airframe and jet engine components. This comparison study includes the Ti-6Al-4V alloy in an α+ β-processed condition (for a typical turbine blade application) and the β-Cez alloy in two distinctly different α+β-processed and β-processed conditions (optimized for a combination of superior strength, ductility, and fracture toughness). The comparison principally showed a much lower yield stress for Ti-6Al-4V (915 MPa) than for both β-Cez conditions (1200 MPa). The Ti-6Al-4V material also showed the significantly lower high-cycle fatigue strength (resistance against crack initiation) of 375 MPa (R=−1) as compared to the β-Cez alloy (∼600 MPa, R=−1). Particularly in the presence of large cracks (>5 mm), the fatigue crack growth resistance and fracture toughness of the Ti-6Al-4V material is superior when compared to both β-Cez conditions. However, for small crack sizes, the conditions of both the alloys under study show equivalent resistance against fatigue crack growth. For the β-Cez material, where microstructures were optimized for high fracture toughness (conventional large crack sizes) by thermomechanical processing, maximum K _Ic-values of 68 MPa√m of the β-processed β-Cez condition (tested in the longitudinal direction) decreased by ∼50 pct in the presence of small cracks (1 mm). A similar decrease in fracture toughness was obtained by loading the β-processed β-Cez condition perpendicular to the flat surfaces of the pancake-shaped β grain structure (tested in the short transverse direction). These results were discussed in terms of the effectiveness of the crack front geometry in hindering crack propagation. Further, the results of this study were considered for alloy selection and optimized microstructures for fatigue and fracture critical applications. Finally, the advantage of the α+β-processed β-Cez condition in highly stressed engineering components is pointed out because of its overall superior combination of fatigue crack initiation and propagation resistance (especially against small fatigue cracks).     

Effect of microstructure,strength, and oxygen content on fatigue crack growth rate of Ti-4.5AI-5.0Mo-1.5Cr (CORONA 5)

Fatigue crack growth rate behavior in CORONA 5, an alloy developed for applications requiring high fracture toughness, has been examined for eight material conditions. These conditions were designed to give differences in microstructure, strength level (825 to 1100 MPa [120 to 160 ksi]), and oxygen content (0.100 to 0.174 wt pct), in such a manner that the separate effects of these variables could be defined. For all eight conditions, fatigue crack growth rates (da/dN) are virtually indistinguishable over the full spectrum of stress-intensity range (ΔK) examined,viz., 8 to 40 MPa√m (7 to 36 ksi√in). Concomitantly, it is noted that over the sizable solution annealing range studied (830° to 915 °C [1525° to 1675 °F]), the primary α-phase morphology was substantially invariant. Eachda/dN curve exhibits a bilinear form with a transition point (ΔKT) between 16 and 19 MPa√m (15 and 17 ksi√in). A change in microfractographic appearance occurs at ΔKT, as extensive secondary cracking along α/β interfaces is observed at all hypertransitional levels ofAK, but not for AK < ΔKT. For each material condition, the mean length of primary α platelets is approximately the same as the cyclic plastic zone size at ΔKT. Accordingly, locations ofAKT (and their similarity for the different material conditions) are rationalized in conformance with a cyclic plastic zone model of fatigue crack growth. Finally, the difference in behavior of CORONA 5, as compared to conventional α/β alloys such as Ti-6A1-4V, is rationalized in terms of crack path behavior.     

Effects of ductile laminate thickness,volume fraction,and orientation on fatigue-crack propagation in Ti-Al<Subscript>3</Subscript>Ti metal-intermetallic laminate composites

Fatigue crack propagation (FCP) has been studied in a new class of materials termed metal-intermetallic laminate (MIL) composites (Ti-Al₃Ti). Due to ease of fabrication and control over layer makeup, these MIL composites can be tailored to optimize the constituent properties for structural and higher performance aerospace applications. Effects of ductile reinforcement (titanium alloy) type, thickness, and volume fraction were systematically investigated in both arrester and divider orientations. Stress intensity (K _max) values as large as 40 MPa√m were observed in the higher crack growth regime, indicating that the fracture toughness of the MIL composites is comparable to common structural metals. In both divider and arrester orientations, the overall fatigue crack growth rate showed an improvement with increasing Ti volume fraction and with increasing Ti thickness (at constant ductile-phase volume fraction). It is noted that the fatigue resistance of monolithic Al₃Ti was improved by an order of magnitude by incorporating just 20 vol pct ductile Ti. In the divider orientation, toughening is obtained through plastically stretching the intact ductile Ti ligaments that bridge the crack wake, thus reducing the crack driving force. By virtue of its morphology, the arrester orientation provides toughening by trapping the crack front entirely at the metallic-intermetallic interfaces, thus requiring the crack to renucleate at each interface. Results are compared with specific crack growth rates of conventional monolithic alloys and other composite systems such as TiNb/γ-TiAl and Nb/Nb₃Al. Owing to their low density (∼3.8 g/cc), Ti-Al MIL composites exhibited specific crack growth rates (da/dN vs ΔK/ρ) on par with tougher, but relatively denser, ductile metals such as Ti alloys and high-strength steels.     

An investigation of the fatigue and fracture behavior of a Nb-12Al-44Ti-1.5Mo intermetallic alloy

This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordered B2 + orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature ductility (11 to 14 pct), fracture toughness (60 to 92 MPa√m), and comparable fatigue crack growth resistance to IN718, Ti-6Al-4V, and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized plastic deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (∼9.6 pct of the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the “unzipping” of cracks along slip bands that form ahead of the dominant crack tip. The proposed mechanism of fatigue crack growth is analogous to the unzipping crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold fatigue crack growth rates at 750 °C are attributed to the shielding effects of oxide-induced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics.     

A comparison study of microstructure and mechanical properties of Ti-24Al-14Nb-3V-0.5Mo with and without Si

A comparative study has been made of the room- and elevated-temperature properties, room-temperature fracture toughness, fatigue-crack propagation rates, and 650 °C creep properties of Ti-24Al-14Nb-3V-0.5Mo with and without 0.9 at. pct Si. Both alloys have microstructures consisting of the α ₂, B2, and the orthorhombic O phase, with different proportions of the α ₂ phase relative to the (O + B2) mixtures, depending on solution-treatment temperature. The alloy with a Si addition contains additional primary ζ-Ti₅Si₃ particles distributed in the (O + B2) matrix. Tests of mechanical properties showed that the incorporation of a small fraction (about 0.03 by volume) of the Ti₅Si₃ phase leads to greater room-temperature and elevated-temperature strengths, but lower room-temperature elongations and fracture toughness as compared with the base alloy. Alloys containing greater volume fractions of the α ₂ phase exhibited better tensile ductility, and this was attributed to the concurrent stabilization of the B2 phase. Examination of tensile-tested and fatigued specimens indicates that the primary failure mode of the alloys, regardless of Si addition, was due to the brittleness of the α ₂ phase; the silicide particles that debonded from the matrix also contribute to cracking in the monotonic loading mode. Up to a 20 pct improvement in creep-rupture life was observed in the Si-containing alloys, and this was interpreted in terms of the solute-strengthening effect of Si. While the incorporated Ti₅Si₃ phase has an unfavorable effect on ductility and room-temperature fracture toughness, the difference in fatigue-crack propagation rates between the alloys with and without Si is minimal. It is concluded that the controlling factor for the fatigue failure in orthorhombic alloys is related to the (α ₂ + O + B2) microstructure, instead of the Ti₅Si₃ particles.     

Optimization of Mo-Si-B intermetallic alloys

Mo-Si-B intermetallics consisting of the phases Mo₃Si and Mo₅SiB₂, and a molybdenum solid solution (“α-Mo”), have melting points on the order of 2000 °C. These alloys have potential as oxidation-resistant ultra-high-temperature structural materials. They can be designed with microstructures containing either individual α-Mo particles or a continuous α-Mo phase. A compilation of existing data shows that an increase in the volume fraction of the α-Mo phase increases the room-temperature fracture toughness at the expense of the oxidation resistance and the creep strength. If the α-Mo phase could be further ductilized, less α-Mo would be needed to achieve an adequate value of the fracture toughness, and the oxidation resistance would be improved. It is shown that microalloying of Mo-Si-B intermetallics with Zr and the addition of MgAl₂O₄ spinel particles to Mo both hold promise in this regard. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 15–17, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

The effects of prolonged thermal

Aluminum-lithium alloys are currently being considered for applications at moderately elevated temperatures; accordingly, a study has been made on the effects of prolonged (100 and 1000 hours overaging) thermal exposure at 149 °C and 260 °C on the mechanical properties of a peakaged Al-Li-Cu-Mg-Zr alloy 8090-T8771. In the as-received T8771 temper, the alloy exhibits an excellent combination of strength (˜500 MPa) and toughness (35 MPa√m) with moderate tensile elongation (4 pct). Overaging at 149 °C results in a ˜50 pct reduction in ductility and toughness, primarily associated with the growth of equilibrium phases along grain/subgrain boundaries, resulting in formation of solute-depleted precipitate-free zones and coarsening of matrix8' andS precipitates; strength levels and fatigue-crack growth rates, however, remain largely unchanged. Thermal exposures at 260 °C, conversely, lead to dramatic reductions in strength (by ˜50 to 80 pct), toughness (by ˜30 pct) and fatigue-crack propagation resistance; crack-growth rates at all ΔK levels above ~5 MPa√m are 2 to 3 orders of magnitude faster. Microstructurally, this was associated with complete dissolution of δ′, severe coarsening ofS andT ₂ precipitates in the matrix, and formation of equilibrium Cu- and Mg-rich intermetallic phases in the matrix and along grain boundaries. The resulting lack of planar-slip deformation and low yield strength of 8090 following overaging exposures at 260 °C increase the cumulative crack-tip damage per cycle and reduce the tendency for crack-path deflection, thereby accelerating fatigue-crack growth rates. Despite this degradation in properties, the 8090-T8771 alloy has better strength retention and generally superior fatigue-crack growth properties compared to similarly overaged Al-Li-Cu-Zr 2090 and Al-Cu-Zn-Mg 7150 alloys. formerly with the University of California, formerly with the University of California,     

Toughness and strength characteristics of Nb-W-Si ternary alloys prepared by Arc melting

This article describes the room-temperature and high-temperature mechanical properties and failure modes of series Nb-W-Si alloys—Nb-10W, Nb-10Si, Nb-10Si-5W, Nb-10W-5Si, and Nb-10W-10Si—prepared by arc melting. For the Nb-10W alloy, the microstructure was a monolithic Nb solid solution (Nb _ss ) with a grain size up to a few hundred microns, while the other four alloys consisted of primary Nb _ss and a eutectic of Nb _ss /Nb₅Si₃ (5-3 silicide) as a result of replacing Nb with Si. Among all alloys, the Nb-10W showed the highest fracture toughness of about 15.3 MPa√m^1/2 and the lowest 0.2 pct yield compressive strength of 90 MPa at 1670 K. Conversely, the Nb-10Si-10W had the highest 0.2 pct yield strength of about 330 MPa at 1670 K and the lowest fracture toughness of 8.2 MPa√m^1/2. It is suggested that toughness is supplied by the metallic Nb _ss phase, while high-temperature strength is mainly provided by the brittle silicide phase. For the Nb-10W alloy with the monolithic Nb _ss , intergranular cleavagelike crack propagation is the fracture mode at room temperature, and dislocation movement within the grains and grain-boundary sliding are the dominant modes of high-temperature failure. With two-phase Nb _ss /Nb₅Si₃ microstructures, the compressive damage of all four alloys at high temperature was dominated by debonding of the interfaces between the Nb _ss and the silicide; however, the fracture mode at room temperature is transgranular, controlled by the primary Nb _ss cleavage.     

Dynamic fracture initiation behavior of an HY-100 steel

An investigation was conducted into the effects of test temperature and loading rate on the initiation of plane strain fracture of an HY-100 steel. Fracture toughness tests were conducted using fatigue precracked round bars loaded in tension to produce a quasi-static stress intensity rate of ·K₁ = 1 MPa√m/s and a dynamic rate of ·K₁ = 2 × 10⁶ MPa√m/s. Testing temperatures covered the range from -150 °C to 200 °C, which encompasses fracture initiation modes involving quasi-cleavage to fully ductile fracture. The results of toughness tests show that the lower-shelf values of fracture toughness were substantially independent of loading rate, while the dynamic values exceeded the quasi-static values by about 50 pct on the upper shelf. In analyzing these results, phenomenological fracture initiation models were adopted based on the requirement that, for fracture to occur, a critical strain or stress must be achieved over a critical distance. In separate tests, the observation of microfracture processes was investigated using fractography and anin situ scanning electron microscope (SEM) fracture technique. The layered ppearance of the fracture surfaces was found to be associated with a banded structure which generally contains many MnS inclusions, probably resulting in a reduction of the fracture toughness values.     

The fracture toughness and toughening mechanisms of wrought low carbon arc cast,oxide dispersion strengthened,and molybdenum-0.5 pct titanium-0.1 pct zirconium molybdenum plate stock

The high-temperature strength and creep resistance of low carbon arc cast (LCAC) unalloyed molybdenum, oxide dispersion strengthened (ODS) molybdenum, and molybdenum-0.5 pct titanium-0.1 pct zirconium (TZM) molybdenum have attracted interest in these alloys for various high-temperature structural applications. Fracture toughness testing of wrought plate stock over a temperature range of −150 °C to 1000 °C using bend, flexure, and compact tension (CT) specimens has shown that consistent fracture toughness results and transition temperatures are obtained using subsized 0.5T bend and 0.18T disc-CT specimens. Although the fracture toughness values are not strictly valid in accordance with all ASTM requirements, these values are considered to be a reasonable measure of fracture toughness. Ductile-to-brittle transition temperature (DBTT) values were determined in the transverse and longitudinal orientations for LCAC (200 °C and 150 °C, respectively), ODS (<room temperature and −150 °C), and TZM (150 °C and 100 °C). At test temperatures > DBTT, the fracture toughness values for LCAC ranged from 45 to 175 MPa√m, TZM ranged from 74 to 215 MPa√m, and the values for ODS ranged from 56 to 149 MPa√m. No temperature dependence was resolved within the data scatter for fracture toughness values between the DBTT and 1000 °C. Thin sheet toughening is shown to be the dominant toughening mechanism, where crack initiation/propagation along grain boundaries leaves ligaments of sheetlike grains that are pulled to failure by plastic necking. Specimen-to-specimen variation in the fraction of the microstructure that splits into thin sheets is proposed to be responsible for the large scatter in toughness values at test temperatures > DBTT. A finer grain size is shown to result in a higher fraction of thin sheet ligament features at the fracture surface. As a result finer grain size materials such as ODS molybdenum have a lower DBTT.     

Fatigue crack growth rates and fracture toughness of rapidly solidified Al-8.5 pct Fe-1.2 pct V-1.7 pct Si alloys

The room-temperature fatigue crack growth rates (FCGR) and fracture toughness were evaluated for different crack plane orientations of an Al-8.5 Pct Fe-1.2 Pct V-1.7 Pct Si alloy produced by planar flow casting (PFC) and atomized melt deposition (AMD) processes. For the alloy produced by the PFC process, properties were determined in six different orientations, including the short transverse directions S-T and S-L. Diffusion bonding and adhesive bonding methods were used to prepare specimens for determining FCGR and fracture toughness in the short transverse direction. Interparticle boundaries control fracture properties in the alloy produced by PFC. Fracture toughness of the PFC alloy varies from 13.4 MPa√m to 30.8 MPa√m, depending on the orientation of the crack plane relative to the interparticle boundaries. Fatigue crack growth resistance and fracture toughness are greater in the L-T, L-S, and T-S directions than in the T-L, S-T, and S-L orientations. The alloy produced by AMD does not exhibit anisotropy in fracture toughness and fatigue crack growth resistance in the as-deposited condition or in the extruded condition. The fracture toughness varies from 17.2 MPa√m to 18.5 MPa√m for the as-deposited condition and from 19.8 MPa√m to 21.0 MPa√m for the extruded condition. Fracture properties are controlled by intrinsic factors in the alloy produced by AMD. Fatigue crack growth rates of the AMD alloy are comparable to those of the PFC alloy in the L-T orientation. The crack propagation modes were studied by optical metallographic examination of crack-microstructure interactions and scanning electron microscopy of the fracture surfaces.     

Modified heat treatment for lower temperature improvement of the mechanical properties of two ultrahigh strength low alloy steels

In the previous papers, a new heat treatment for improving the lower temperature mechanical propertise of the ultrahigh strength low alloy steels was suggested by the authors which produces a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite through isothermal transformation at 593 K for a short time followed by water quenching (after austenitization at 1133 K). In this paper, two commercial Japanese ultrahigh strength steels, 0.40 pct C-Ni-Cr-Mo (AISI 4340 type) and 0.40 pct C-Cr-Mo (AISI 4140 type), have been studied to determine the effect of the modified heat treatment, coupled above new heat treatment withγ ⇆ α′ repctitive heat treatment, on the mechanical properties from ambient temperature (287 K) to 123 K. The results obtained for various test temperatures have been compared with those for the new heat treatment reported previously and the conventional 1133 K direct water quenching treatment. The incorporation of intermediate four cyclicγ ⇆ α′ repctitive heat treatment steps (after the initial austenitization at 1133 K and oil quenching) into the new heat treatment reported previously, as compared with the conventional 1133 K direct water quenching treatment, significantly improved 0.2 pct proof stress as well as notch toughness of the 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel at similar fracture ductility levels from 287 to 123 K. Also, this heat treatment, as compared with the conventional 1133 K direct water quenching treatment, significantly improved both 0.2 pct proof stress and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel with increased fracture ductility at 203 K and above. The microstructure consists of mixed areas of ultrafine grained martensite, within which is the refined blocky, highly dislocated structure, and the second phase lower bainite (about 15 vol pct), which appears in acicular form and partitions prior austenite grains. This newly developed heat treatment makes it possible to modify the new heat treatment reported previously so as to raise 0.2 pct proof stress to a higher level and keep notch toughness at the same level. The improvement in the mechanical properties is discussed in terms of metallographic observations and the modified law of mixtures and so forth.     

Room-temperature deformation behavior of directionally solidified multiphase Ni-Fe-Al alloys

Directionally solidified (DS) β + (γ + γ′) Ni-Fe-Al alloys have been used to investigate the effect of a ductile second phase on the room-temperature mechanical behavior of a brittle 〈001〉-oriented β (B2) phase. The ductile phase in the composite consisted of a fine distribution of ordered γ′ precipitates in a γ (fcc) matrix. Three microstructures were studied: 100 pct lamellar/rod, lamellar + proeutectic β, and discontinuous γ. The β matrix in the latter two microstructures contained fine-scale bcc precipitates formed due to spinodal decomposition. Room-temperature tensile ductilities as high as 12 pct and fracture toughness (K _Q ) of 30.4 MPa √m were observed in the 100 pct lamellar/rod microstructure. Observations of slip traces and dislocation substructures indicated that a substantial portion of the ductility was a result of slip transfer from the ductile phase to the brittle matrix. This slip transfer was facilitated by the Kurdjumov-Sachs (KS) orientation relationship between the two phases and the strong interphase interface which showed no decohesion during deformation. In microstructures which show higher values of tensile ductility and fracture toughness, 〈100〉 slip was seen in the β phase, whereas 〈111〉 slip was seen in the β phase in the microstructure which showed limited ductility. The high ductility and toughness are explained in terms of increased mobile dislocation density afforded by interface constraint. The effect of extrinsic toughening mechanisms on enhancing the ductility or toughness is secondary to that of slip transfer.     

Heat treatment for improvement in lower temperature mechanical properties of 0.40 pct C-Cr-Mo ultrahigh strength steel

In the previous paper, it was reported that isothermal heat treatment of a commercial Japanese 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel (AISI 4340 type) at 593 K for a short time followed by water quenching, in which a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite is produced, results in the improvement of low temperature mechanical properties (287 to 123 K). The purpose of this paper is to study whether above new heat treatment will still be effective in commercial practice for improving low temperature mechanical properties of the ultrahigh strength steel when applied to a commercial Japanese 0.40 pct C-Cr-Mo ultrahigh strength steel which is economical because it lacks the expensive nickel component (AISI 4140 type). At and above 203 K this new heat treatment, as compared with the conventional 1133 K direct water quenching treatment, significantly improved the strength, tensile ductility, and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel. At and above 203 K the new heat treatment also produced superior fracture ductility and notch toughness results at similar strength levels as compared to those obtained by usingγ α′ repetitive heat treatment for the same steel. However, the new heat treatment remarkably decreased fracture ductility and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel below 203 K, and thus no significant improvement in the mechanical properties was noticeable as compared with the properties produced by the conventional 1133 K direct water quenching treatment and theγ α′ repetitive heat treatment. This contrasts with the fact that the new heat treatment, as compared with the conventional 1133 K direct water quenching treatment and theγ α′ repetitive heat treatment, dramatically improved the notch toughness of the 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel, providing a better combination of strength and ductility throughout the 287 to 123 K temperature range. The difference in the observed mechanical properties between the above two ultrahigh strength steels is discussed on the basis of the effect of nickel content, fracture profile, and so forth.     

The thermal activation energy for fatigue of Fe- 1 pct Cr- 0.5 pct Mo

The temperature dependence of fatigue crack propagation is considered in an Fe-1 pct Cr-0.5 pct Mo alloy steel. This material was tested at temperatures between 425 and 550 °C, a frequency of 1 Hz, and anR-ratio of 0.1. It is shown that the effect of temperature can be explained in terms of a thermal activation energy for fatigue. The magnitude of this activation energy is a function of ΔK and varies from more than 150 kJ/mole at 15 MPa√m to 30 kJ/mole above 30 MPa√m. The magnitude of these activation energies supports the idea that oxidation, and not creep, is the rate-controlling time-dependent process for the test conditions studied.