Pct Proof Stress Research Articles

Development of Low-Yield Stress Co\u2013Cr\u2013W\u2013Ni Alloy by Adding 6 Mass Pct Mn for Balloon-Expandable Stents

This is the first report presenting the development of a Co–Cr–W–Ni–Mn alloy by adding 6 mass pct Mn to ASTM F90 Co–20Cr–15W–10Ni (CCWN, mass pct) alloy for use as balloon-expandable stents with an excellent balance of mechanical properties and corrosion resistance. The effects of Mn addition on the microstructures as well as the mechanical and corrosion properties were investigated after hot forging, solution treatment, swaging, and static recrystallization. The Mn-added alloy with a grain size of ~ 20 µm (recrystallization condition: 1523 K, 150 seconds) exhibited an ultimate tensile strength of 1131 MPa, 0.2 pct proof stress of 535 MPa, and plastic elongation of 66 pct. Additionally, it exhibited higher ductility and lower yield stress while maintaining high strength compared to the ASTM F90 CCWN alloy. The formation of intersecting stacking faults was suppressed by increasing the stacking fault energy (SFE) with Mn addition, resulting in a lower yield stress. The low-yield stress is effective in suppressing stent recoil. In addition, strain-induced martensitic transformation during plastic deformation was suppressed by increasing the SFE, thereby improving the ductility. The Mn-added alloys also exhibited good corrosion resistance, similar to the ASTM F90 CCWN alloy. Mn-added Co–Cr–W–Ni alloys are suitable for use as balloon-expandable stents.

Grain Size Hardening in Mg and Mg-Zn Solid Solutions

Cast specimens of Mg and of several Mg-Zn binary alloys with a wide range of grain sizes were deformed in tension and compression. The k values calculated from the Hall–Petch (H-P) plots of the tensile 0.2 pct proof stress increased with the Zn content, from 0.24 MPa m1/2 for pure Mg to ~0.66 MPa m1/2 for the 2.3 at. pct Zn alloy; k values measured from compression tests were larger, typically by 0.05 MPa m1/2. When the strength measurements were corrected for the pseudoelastic strain resulting from elastic twinning, the k values generally increased, and the difference between tension and compression was eliminated. This showed that the larger k values obtained in compression using uncorrected data were an artifact of the pseudoelastic effect. The apparent friction stress varied between about 14 MPa for pure Mg to very low or negative values for the most dilute alloy, increasing again to about 8 MPa for the most concentrated alloy. The use of strength data corrected for pseudoelasticity effects is necessary for a consistent analysis of the grain size hardening.

Effect of Cold Rolling on as–ECAP Interstitial Free Steel

Ti-stabilized interstitial free steel subjected to eight passes, route BC room temperature equal channel angular pressing (ECAP) additionally was cold rolled (CR) up to 95 pct thickness reduction. Electron back-scattering diffraction and transmission electron microscopy characterized microstructural refinement and microtexture evolution, whereas the mechanical properties were assessed by uniaxial tensile tests. After 95 pct CR, the average high-angle grain boundary spacing reduces to 0.14 μm, whereas the high-angle boundary fraction increases to ~81 pct. The ECAP negative simple shear texture components rotate by ~15 deg around the transverse direction toward the rolling direction for up to 50 pct CR, with typical rolling textures observed at 95 pct CR. The decrease in boundary spacing produces a ~500 MPa gain in 0.2 pct proof stress, a ~600 MPa increase in ultimate tensile strength (UTS), and a ~4 pct loss in total elongation after 95 pct CR. Similar rates of decrease in work hardening correspond to comparable rates of cross and/or multiple slip events irrespective of the processing regime and substructural refinement. The fracture mode of the tensile samples changes from ductile to brittle type between ECAP and 95 pct CR and is attributed to the reduced work hardening capacity of the latter. The modified Hall–Petch equation shows that the convergence of high-angle boundary spacing values with their low-angle counterparts results in an increased contribution via boundary strengthening to the 0.2 pct proof stress and UTS.

Effect of Friction Stir Processing on Microstructure and Mechanical Properties of a Cast-Magnesium–Rare Earth Alloy

Single-pass friction stir processing (FSP) was used to increase the mechanical properties of a cast Mg-Zn-Zr-rare earth (RE) alloy, Elektron 21. A fine grain size was achieved through intense plastic deformation and the control of heat input during processing. The effects of processing and heat treatment on the mechanical and microstructural properties were evaluated. An aging treatment of 16 hours at 200 °C resulted in a 0.2 pct proof stress of 275 MPa in the FSP material, a 61 pct improvement over the cast + T6 condition.

Rapid Heat Treatment of Aluminum High-Pressure Diecastings

Recently, it has been demonstrated that common high-pressure diecasting (HPDC) alloys, such as those based on the Al-Si-Cu and Al-Si-Mg-(Cu) systems, may be successfully heat treated without causing surface blistering or dimensional instability. In some compositions, the capacity to exploit age hardening may allow the proof stress values to be doubled when compared to the as-cast condition. This heat treatment procedure involves the use of severely truncated solution treatment cycles conducted at lower than normal temperatures, followed by quenching and natural or artificial aging. The potential therefore exists to develop and evaluate secondary HPDC alloys designed specifically for rapid heat treatment, while still displaying high castability. This article reports results of an experimental program in which responses of various alloy compositions to age hardening have been investigated with the primary aim of further reducing the duration and cost of the heat treatment cycle while maintaining high tensile properties. Composition ranges have been established for which values of 0.2 pct proof stress exceeding 300 MPa (i.e., increases of ~100 pct above as-cast values) can be achieved using a procedure that involves a total time for solution treatment plus age hardening of only 30 minutes. This rapid aging behavior is shown to be related to precipitation of the complex Q′ phase, which forms primarily when Mg contents of the alloys are above ~0.2 wt pct.

Heat Treatment of High-Pressure Die Castings

High-pressure die-cast Al alloys cannot normally be heated at high temperatures due to the presence of pores containing entrapped gases, which lead to the formation of surface blisters. It has been found that blistering can be avoided by using considerably shorter solution-treatment times and lower temperatures. Experiments with alloys 360 (Al-9.5Si-0.5Mg) and 380 (Al-8.5Si-3.5Cu) have shown that strong responses to age hardening are still possible following these modified solution treatments.[1, 2, 3] For T6 tempers, increases in 0.2 pct proof stress of 80 pct for 360 and 115 pct for 380 have been obtained compared with as-cast values. Using a T4 temper, ductility is improved without any sacrifice in strength.

Fabrication of ultrahigh nitrogen austenitic steels by nitrogen gas absorption into solid solution

For the purpose of fabricating ultrahigh nitrogen austenitic steels (>1 mass pct N), the phenomenon of nitrogen absorption into solid solution was thermodynamically analyzed and applied to Fe-Cr-Mn system ternary alloy. During the annealing of the steel in a nitrogen gas atmosphere of 0.1 MPa at 1473 K (nitrogen absorption treatment), the nitrogen content of the steel was increased with the absorption of nitrogen gas from the material surface and then saturated when the system reached a state of equilibrium. Effect of the steel composition on an equilibrium nitrogen content was formulated taking account of interactions among Cr, Mn, and N atoms, and the condition for fabrication of ultrahigh nitrogen austenitic steels was clarified. The nitrogen addition to ultrahigh content markedly increased proof stress and tensile stress of the austenitic steels without losing moderate ductility. For example, Fe-24Cr-10Mn-1.43N (mass pct) alloy has 830 MPa in 0.2 pct proof stress, 2.2 GPa in true tensile stress, and 75 pct in total elongation. As a result of tensile tests for various nitrogen-bearing austenitic steels, it was found that the proof stress is increased in proportion to (atomic fraction of nitrogen)2/3.

Effect of microstructure on fracture characteristics of Ti-6Al-2Sn-2Zr-2Mo-2Cr-Si

The effects of intermetallic compounds of Ti3Al (α2) and silicide separately on fracture characteristics of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-62222S) alloy were investigated in this study. The alloys with only Ti3Al and only silicide precipitated were established by aging treatments at temperatures of 913 K followed by air cooling and 1088 K followed by water quenching, respectively. X-ray diffraction analysis results showed that the volume fraction of either Ti3Al or silicide increases with increasing aging time. Tensile properties, namely, yield stress (0.2 pct proof stress), ultimate tensile strenght, and elongation of as-received alloy are much better than those of the aged alloys. The strength of the alloy with only Ti3Al is better than that of the alloy with only silicide, while elongation of the alloy with only silicide is better than that of the alloy with only Ti3Al. Fracture toughness, JIC, of the alloy with only silicide is better than that of the alloy with only Ti3Al. The intergranular fracture appears in the alloy with only Ti3Al. Coarsening of Widmanstatten α structure and increasing ductility of β phase during aging are considered to be effective for increasing fracture toughness.

An investigation of the effect of fatigue deformation on the residual mechanical properties of Ti-6Al-4V ELI

Tensile properties, hardness, and Charpy impact toughness of Ti-6Al-4V extralow interstitial (ELI) with equiaxed α and Widmanstatten α structures at various stages of fatigue were investigated. Fatigue crack initiation characteristics of the same alloy were also investigated in this study. In the equiaxed α structure, fatigue cracks initiated mainly at the interface between primary-α grains, while in the Widmanstatten α structure, they initiated across α plates at an angle of around 45 deg to the stress axis. Specimens with the Widmanstatten α structure fractured before adequate fatigue hardening was achieved because a multitude of microcracks readily formed. Specimens with the equiaxed α structure fractured after adequate fatigue hardening developed. Tensile strength, 0.2 pct proof stress, and hardness increased clearly with increasing stress cycles and fatigue steps, particulary in the low-cycle fatigue (LCF) region, while impact toughness and elongation showed a reverse trend. It is suggested, therefore, that the dislocation density multiplies more rapidly near the specimen surface during the early stages of fatigue, while during the later stages of fatigue, dislocation density increases near the center of the specimen. Also, the dislocation multiplication will continue until saturation of the entire specimen has occurred.

Equal-channel angular pressing of commercial aluminum alloys: Grain refinement, thermal stability and tensile properties

Using equal-channel angular (ECA) pressing at room temperature, the grain sizes of six different commercial aluminum-based alloys (1100, 2024, 3004, 5083, 6061, and 7075) were reduced to within the submicrometer range. These grains were reasonably stable up to annealing temperatures of ∼200 °C and the submicrometer grains were retained in the 2024 and 7075 alloys to annealing temperatures of 300 °C. Tensile testing after ECA pressing through a single pass, equivalent to the introduction of a strain of ∼1, showed there is a significant increase in the values of the 0.2 pct proof stress and the ultimate tensile stress (UTS) for each alloy with a corresponding reduction in the elongations to failure. It is demonstrated that the magnitudes of these stresses scale with the square root of the Mg content in each alloy. Similar values for the proof stresses and the UTS were attained at the same equivalent strains in samples subjected to cold rolling, but the elongations to failure were higher after ECA pressing to equivalent strains >1 because of the introduction of a very small grain size. Detailed results for the 1100 and 3004 alloys show good agreement with the standard Hall-Petch relationship.

Some observations on cyclic deformation structures in the high-strength commercial aluminum alloy AA 7150

Load-controlled fatigue testing of the aluminum alloy AA 7150 has been conducted using four-point bending with an R ratio of +0.1 over a range of maximum stress levels from 60 to 120 pct of the 0.2 pct proof stress. The alloy, in the form of 12.5-mm rolled plate, was investigated in underaged (UA), peak-aged (PA), and overaged (OA) conditions, corresponding to a change in average precipitate sizes from 5 nm in the UA condition to 21 nm in the OA condition. Three orientations of the plate were investigated. Orientation and aging condition influenced the degree of surface topographical development but not fatigue life. Detailed transmission electron microscopy (TEM) of the fatigued surface indicated that deformation in all aging conditions occurred by planar slip. Slip was generally restricted to a single slip system within each grain, and subgrain boundaries offered little resistance to dislocation movement facilitating long slip line lengths (measured up to 310 µm) between adjacent high-angle grain boundaries. Planar slip observed in the OA condition is attributed to shearing of large strengthening precipitates, which is promoted by long slip line lengths. No evidence of surface specific changes in slip character was observed.

Relationship between fracture toughness and crack extension resistance curves (R curves) for Ti-6Al-4V alloys

The effects of microstructure, impurity content, and testing temperature on the fracture toughness (as measured by the crack tip opening displacement (CTOD)) and microcrack extension resistance curves (R curves) of Ti-6Al-4V alloys were examined. At 0 °C, microstructure is the most influential factor in the toughness-strength relationship. Acicular microstructure specimens have a higher CTOD than specimens with equiaxed microstructures, regardless of strength (0.2 pct proof stress) and impurity content. At −196 °C, impurity content becomes a controlling factor in the toughness-strength relationship. Extra-low impurity (ELI) specimens, which have a lower impurity content, show a higher CTOD, irrespective of microstructure. Microcracks extended from the notch tip before the maximum load was reached during testing were investigated, and crack initiation (δi) and extension-resistance properties were evaluated by obtaining exact R curves of the microcracks. At 0 °C, specimens with different microstructures and different impurity contents have almost the same δi. But acicular-microstructure specimens with a higher CTOD at a given strength show a greater crack extension resistance. At −196 °C, ELI specimens, which have a higher CTOD, show a larger crack extension resistance. It is concluded that the crack extension-resistance property of the microcracks extended from the notch tip before the maximum load is a controlling factor for the fracture toughness of Ti-6Al-4V alloys.

Phase-stress partition during uniaxial tensile loading of a TiC-particulate-reinforced Al composite

Using neutron diffraction, we measured during in situ loading the lattice elastic mean phase (LEMP) strains in the matrix and reinforcement of a 15 vol pct TiC-particulate-reinforced 2219 Al composite. From the strain components longitudinal to and transverse to loading, the in situ normal phase stresses (average normal stresses in the constituent phases) were obtained through Hooke’s law. The internal stress partition between the matrix and reinforcement, i.e., load sharing, can then be inferred. Internal stress development was also modeled using the finite-element method (FEM), showing good agreement with the experimental results. Both indicate that the relationship between the LEMP strains/phase stresses and the applied load noticeably deviates from linearity during composite microyielding, long before the nominal 0.2 pct proof stress is reached. The nonlinearity arises (despite the linear elastic relationship between phase stresses and LEMP strains) because the applied traction is not synonymous with the phase stresses, and the ratio of phase stresses may vary during loading. Notably, the morphology of the LEMP strain development with applied load differs in the directions parallel to or perpendicular to the load. The differences are explained by considering the evolution of local matrix plasticity. Thermal residual stresses and inelastic stress relaxation, driven by interfacial diffusion, are also discussed.

An investigation of strain hardening and creep in an AI-6061/AI2O3 metal matrix composite

Experiments were conducted to compare the influence of temperature on the flow and strain-hardening characteristics of an Al-6061 metal matrix composite, reinforced with ∼20 vol pct of Al2O3-based microspheres, with the unreinforced monolithic alloy. At room temperature, the yield stresses and the strain-hardening rates are higher in the composite material in the asquenched condition and after aging at 448 K for periods of time up to 300 hours. The 0.2 pct proof stress and the strain-hardening rate decrease with increasing temperature in both materials, but the rate of decrease is faster in the composite so that the unreinforced monolithic alloy exhibits higher yield stresses and strain-hardening rates at temperatures in the vicinity of 600 K. Under conditions of constant stress at high temperatures, the composite exhibits both a higher creep strength than the monolithic alloy and higher values for the stress exponents for creep.

The age-hardening characteristics of an AI-6061/AI2O3 metal matrix composite

Experiments were conducted to determine the age-hardening behavior of an Al-6061 metal matrix composite reinforced with ∼20 vol pct of A12O3 microspheres and prepared using liquid metallurgy processing. The presence of alumina microspheres increases the dislocation density in the matrix of the composite in the as-quenched state and, by comparison with the monolithic alloy, this leads to a significant increase in the yield stress in the as-quenched and unaged condition. From measurements of the 0.2 pct proof stress, there is no evidence for any significant acceleration in the aging process in the composite material: both materials attain similar peak-aged conditions after essentially the same aging times. The microstructures of the composite and the monolithic alloy are similar in the peak-aged condition, with a high density of fine needlelike β″ precipitates and, in the over-aged condition, with a reasonably homogeneous distribution of the rod-shaped β′ phase. It is proposed that the aging behavior is better quantified by determining the yield stress rather than by taking hardness measurements. Formerly Visiting Scholar, Kyushu University,

Tensile and impact properties changes of HASTELLOY X after exposure in high-temperature helium environment

Experimental results are presented on the changes in mechanical properties of HASTELLOY X* after being used in the liner tube of HENDEL hot gas duct under high temperature helium gas for about 6000 hours. In both room and elevated-temperature tensile tests, 0.2 pct proof stress and total elongation were significantly decreased after exposure in high temperature helium. Room-temperature impact toughness of the exposed specimens exhibited a much lower absorbed energy (about 5.0 × 106 J/m2) than that of unexposed specimens (about 1.5 × 106 J/m2). The fracture modes of tensile test specimens were more closely correlated with the test temperature than with the long-time exposure; however, long-time exposure is more affected by the tensile strength of HASTELLOY X than the test temperature. Moreover, “downstream effect” for the carbide precipitation reaction occurred in the liner tube of the HENDEL hot gas duct.

Effect of quench rate on microstructure and tensile properties of ALSL 4320 and 4340 steels

A study has been made of the effect of quench rate on the microstructure and tensile properties of two commercial AISI 4320 and 4340 steels having fully martensitic structures. The steels were quenched from various temperatures from 1323 to 1473 K, at two different quench rates using iced brine (fast quench treatments) and oil held at 373 K (slow quench treatments). Tensile properties of these steels, after double-tempering at 473 K with intermediate quenching and refrigeration, were determined at ambient temperature (293 K) using an Instron test machine. The microstructural changes accompanying these quench rates were examined by means of optical and thin-foil transmission electron microscopic techniques. In the 4320 steel with a relatively high Ms temperature, the slow quench treatments compared to the fast quench treatments increased both the 0.2 pct proof stress and the ultimate tensile strength at similar total elongation levels, regardless of the prior austenite grain size, while the strength data of the slowly quenched steels exhibited a large scatter as the prior austenite grain size increased. However, in the 4340 steel with a relatively low Ms temperature tensile properties were less sensitive to quench rate, while the slow quench treatments compared to the fast quench treatments increased slightly only the 0.2 pct proof stress. From microstructural results, it is suggested that the beneficial effect on the strength of the slowly-quenched steels is caused by a dispersion-hardening effect due to carbon segregation or fine carbide precipitation in the martensite during the quench(i.e., autotempering).

Effect of microstructure on strength and toughness of heat-treated low alloy structural steels

Several low alloy structural steels with different levels of Ni, Cr, and Mo and carbon contents ranging from 0.12 to 0.42 wt pct have been studied to determine the effect of transformation structures on strength and toughness. The strength and toughness were, respectively, evaluated with the yield stress (0.2 pct proof stress) in tensile tests under ambient temperature and ductile-brittle transition temperature (DBTT) in Charpy impact tests. The significant conclusions are as follows: well-defined packets are observed in martensitic and lower bainitic structures, and in this case the packet diameter is the primary microstructural parameter controlling the yield stress and DBTT. The mechanical properties are also improved to a lesser degree with decreasing width of the lath present within the packet. If the steel has an upper bainitic structure, the packet is composed of well-defined blocks, and the block size controls the yield stress and DBTT.

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.

Cast aluminum alloys containing dispersions of zircon particles

A process for preparing Al-alloy castings containing dispersions of zircon particles is described. Composites were prepared by stirring zircon particles (40 to 200 µm size) in commercially pure Al (99.5 pct)* and Al-11.8 pct Si melts and subsequently casting these melts in permanent molds. It was found to be necessary to alloy the above two melts with 3 pct Mg to disperse substantial amounts of zircon particles (25 to 30 pct). Further, it was possible to disperse up to 60 wt pct zircon by adding up to 5 pct Mg; however, the melts containing above 30 wt pct zircon showed insufficient fluidity for gravity diecasting and had to be pressure diecast. Microstructural studies of cast composites indicated the presence of a reaction zone at the periphery of zircon particles, and electron probe microanalysis showed concentrations of Mg and Si at the particle-matrix interface. Hardness, abrasive wear resistance, elastic modulus, 0.2 pct proof stress, and tensile strength of cast Al-3 pct Mg alloy were found to improve with the dispersions of zircon particles. Scanning electron micrographs of abraded and fractured surfaces did not show any evidence of particle pull-outs or voids at the particle matrix interface, indicating strong continuous bonding.