Microstructural dependence of Fe-high Mn tensile behavior

The tensile properties of Fe-high Mn (16 to 36 wt pct Mn) binary alloys were examined in detail at temperatures from 77 to 553 K. The Mn content dependence of the deformation and fracture behavior in this alloy system has been clarified by placing special emphasis on the starting microstructure and its change during deformation. In general, the intrusion of hcp epsilon martensite (ε) into austenite (γ) significantly increases the work hardening rate in these alloys by creating strong barriers to further plastic flow. Due to the resulting high work hardening rates, large amounts of e lead to high flow stresses and low ductility. Alloys of 16 to 20 wt pct Mn are of particular interest. While these alloys are thermally stable with respect to bcc α’ martensite formation, 16 to 20 wt pct Mn alloys undergo a deformation induced ε →α’ transformation. The martensitic transformation plays two contrasting roles. The stress-induced ε→ α’ transformation decreases the initial work hardening rate by reducing locally high internal stress. However, the work hardening rate increases as the accumulated α’ laths become obstacles against succeeding plastic flow. These rather complicated microstructural effects result in a stress-strain curve of anomolous shape. Since both the M_s and M_d temperatures for both the ε and α’-martensite transformations are strongly dependent on the Mn content, characteristic relationships between the tensile behavior and the Mn content of each alloy are observed.     

Plastic deformation and fracture of binary TiAl-base alloys

The mechanical behavior of binary TiAl alloys containing 46 to 60 at. pct Al has been studied in bulk materials preparedvia rapid solidification processing. Bending and tensile tests were carried out at room temperature as a function of Al concentration. A few alloys were also tested from liquid nitrogen temperature to ∼ 1000°C. Deformation substructures were studied by analytical transmission electron microscopy and fracture modes by scanning electron microscopy (SEM). It was found that both microstructure and composition strongly affect the mechanical behavior of TiAl-base alloys. A duplex structure, which contains both primary y grains and transformedγ/α ₂ lamellar grains, is more deformable than a single-phase or a fully transformed structure. The highest plasticities are observed in duplex alloys containing 48–50 at. pct Al after heat treatment in the center of theγ + α phase field. The deformation of these duplex alloys is facilitated by 1/2[110] slip and {111} twinning, but very limited superdislocation slip occurs. The twin deformation is suggested to result from a lowered stacking fault energy due to oxygen depletion or an intrinsic change in chemical bonding. Other factors, such as grain size and grain boundary chemistry and structure, are important from a fracture point of view. The results on the deformation and fracture modes as a function of test temperature are also discussed.     

Processing map for controlling microstructure in hot working of hot isostatically pressed powder metallurgy NIMONIC AP-1 superalloy

The hot deformation behavior of hot isostatically pressed (HIP) NIMONIC AP-1 superalloy is characterized using processing maps in the temperature range 950 °C to 1200 °C and strain rate range 0.001 to 100 s^•1. The dynamic materials model has been used for developing the pro-cessing maps which show the variation of the efficiency of power dissipation given by [2m/ (m + 1)] with temperature and strain rate, withm being the strain rate sensitivity of flow stress. The processing map revealed a domain of dynamic recrystallization with a peak efficiency of 40 pct at 1125 °C and 0.3 s^•1, and these are the optimum parameters for hot working. The microstructure developed under these conditions is free from prior particle boundary (PPB) de-fects, cracks, or localized shear bands. At 100 s^•1 and 1200 °C, the material exhibits inter-crystalline cracking, while at 0.001 s^•1, the material shows wedge cracks at 1200 °C and PPB cracking at 1000 °C. Also at strain rates higher than 10 s^•1, adiabatic shear bands occur; the limiting conditions for this flow instability are accurately predicted by a continuum criterion based on the principles of irreversible thermodynamics of large plastic flow.     

Plastic flow and fracture of metallic glass

The tensile flow and fracture behavior of three Pdo.₈Sio2-based alloys in the glassy, “microcrystalline,” and fully crystalline condition has been studied. The glassy alloys flow plastically to a total strain of approximately 0.5 pct e, and exhibit proportional limit stresses of approximatelyE x 10~² whereE is Young’s modulus. This plastic flow is accompanied by the formation of shear deformation bands on the specimen surfaces. Fully crystalline alloys are extremely brittle and fracture via intergranular cracking. Fracture surfaces of the amorphous and “microcrystalline” alloys are inclined at 45 deg to the tensile axis and exhibit two morphologically distinct zones. One zone is relatively featureless while the other contains a “river” pattern of local necking protrusions. Detailed comparison of opposing surfaces indicates that fracture is preceded by large local plastic shear which produces the smooth zone while the local necking pattern is produced during rupture. These observations form the basis for the hypothesis that plastic flow in the glassy material occurs via localized strain concentrations and that fracture is initiated by catastrophic, “adiabatic” shear. Formerly Postdoctoral Associate, Yeshiva University, New York, N. Y.     

Flow and fracture of bimaterial systems based on aluminum alloys

The effect of property mismatches on constrained plastic flow in aluminum alloys was investigatedvia both finite element modeling (FEM) and experimentation. Double-notched tension tests on monolithic aluminum alloys and notched trilayer laminates, consisting of the aluminum alloy and a discontinuously reinforced aluminum material, were used to experimentally study the degree of constraint developed in aluminum alloys for use in bimaterial systems. Constraint levels in bimaterial systems were found to be affected by mismatches in elastic modulus and strength. The trends observed in the development of constrained plastic flow in these studies were rationalized based upon the effects of stress triaxiality on the flow and fracture behavior of the various aluminum alloys investigated.     

Fracture toughness and deformation of titanium alloys at low temperatures

Experimental data concerning the influence of compositional, microstructural and textural variations on the fracture toughness and deformation kinetics of α and α-β titanium alloys are presented. In particular, the influence of these parameters on the thermal and athermal components of the flow stress is compared to their influence on fracture toughness. The similarities in deformation behavior among alloys are noted and contrasted to the dissimilarities in fracture toughness behavior.     

Processing-property-microstructure relationships in TiAl-based alloys

The influence of thermomechanical processing on the microstructure of a range of TiAl-based alloys has been assessed using optical and electron microscopy, and the room-temperature mechanical properties have been determined. Long-term exposure at high temperatures has been used to assess the thermal stability of some of the structures generated through the different processing routes, and it has been found that the (gamma and alpha 2) lamellar structures, in some of the alloys, are unstable at 700 °C, a likely operating temperature. Addition of boron increases the stability of the lamellar structure. The influence of the difficulty of slip transfer between gamma and alpha 2 has been assessed as one of the factors limiting ductility in samples with this lamellar structure. In addition to the alloys produced via the ingot route, some atomized material has been produced and the microstructure and properties of hot-isostatically pressed “hipped” material assessed. Regions, high in titanium, are present in all atomized powders that have been examined, and these regions are found to initiate fracture at very low strains. These results are briefly discussed in terms of the factors that control the room-temperature strength and fracture behavior of TiAl-based alloys. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Processing-property-microstructure relationships in TiAl-based alloys

The influence of thermomechanical processing on the microstructure of a range of TiAl-based alloys has been assessed using optical and electron microscopy, and the room-temperature mechanical properties have been determined. Long-term exposure at high temperatures has been used to assess the thermal stability of some of the structures generated through the different processing routes, and it has been found that the (gamma and alpha 2) lamellar structures, in some of the alloys, are unstable at 700°C, a likely operating temperature. Addition of boron increases the stability of the lamellar structure. The influence of the difficulty of slip transfer between gamma and alpha 2 has been assessed as one of the factors limiting ductility in samples with this lamellar structure. In addition to the alloys produced via the ingot route, some atomized material has been produced and the microstructure and properties of hot-isostatically pressed “hipped” material assessed. Regions, high in titanium, are present in all atomized powders that have been examined, and these regions are found to initiate fracture at very low strains. These results are briefly discussed in terms of the factors that control the room-temperature strength and fracture behavior of TiAl-based alloys. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Microstructure and mechanical behavior of Cr-Cr2Hfin situ intermetallic composites

A detailed investigation of the effects of microstructural changes on the mechanical behavior of twoin situ intermetallic composites with Cr and Cr₂Hf phases in the Cr-Hf system was performed. The nominal compositions (at. pct) of the alloys were Cr-5.6Hf (hypoeutectic) and Cr-13Hf (eutectic). The study included evaluations of strength, ductility, and fracture toughness as a function of temperature and creep behavior. Two microstructures in each alloy were obtained by heat treatments at 1250 ‡C (fine microstructure) and 1500 ‡C (coarse microstructure). A decrease in elastic strength (stress at the onset of inelastic response in the load-deflection curve) with the coarsening of the microstructures was noted for both alloys below 1000 ‡C. The Cr-13Hf alloy retained strength to a higher test temperature, relative to Cr-5.6Hf alloy, under both microstructural conditions. The alloys showed no evidence of ductility at room temperature. However, in the coarse microstructure of the Cr-5.6Hf alloy, the primary Cr exhibited ductility at and above 200 ‡C; ductility in primary Cr could be seen only at and above 1000 ‡C for the fine microstructure. In other words, the temperature at which ductility was first observed decreased from about 1000 ‡C to about 200 ‡C due to high-temperature heat treatment in this alloy. Both microstructures of Cr-5.6Hf alloy showed a significant increase in fracture toughness with increasing test temperature. However, the increases in fracture toughness with temperature for the Cr-13Hf alloy microstructures were relatively small. Both alloys showed about four orders of magnitude reduction in steady-state creep rates relative to pure Cr at 1200 ‡C. The results are analyzed in the light of deformation characteristics and fracture micromechanisms. The effects of microstructural factors, such as the size and continuity of phases, solubility levels of Hf as well as interstitial elements in Cr, on the observed mechanical behavior are discussed. Formerly Research Scientist, Materials and Processes, UES, Inc.     

An Assessment of the Ductile Fracture Behavior of Hot Isostatically Pressed and Forged 304L Stainless Steel

Type 300 austenitic stainless steel manufactured by hot isostatic pressing (HIP) has recently been shown to exhibit subtly different fracture behavior from that of equivalent graded forged steel, whereby the oxygen remaining in the component after HIP manifests itself in the austenite matrix as nonmetallic oxide inclusions. These inclusions facilitate fracture by acting as nucleation sites for the initiation, growth, and coalescence of microvoids in the plastically deforming austenite matrix. Here, we perform analyses based on the Rice–Tracey (RT) void growth model, supported by instrumented Charpy and J-integral fracture toughness testing at ambient temperature, to characterize the degree of void growth ahead of both a V-notch and crack in 304L stainless steel. We show that the hot isostatically pressed (HIP’d) 304L steel exhibits a lower critical void growth at the onset of fracture than that observed in forged 304L steel, which ultimately results in HIP’d steel exhibiting lower fracture toughness at initiation and impact toughness. Although the reduction in toughness of HIP’d steel is not detrimental to its use, due to the steel’s sufficiently high toughness, the study does indicate that HIP’d and forged 304L steel behave as subtly different materials at a microstructural level with respect to their fracture behavior.     

Effect of Temperature on the Fracture Toughness of Hot Isostatically Pressed 304L Stainless Steel

Herein, we have performed J-Resistance multi-specimen fracture toughness testing of hot isostatically pressed (HIP’d) and forged 304L austenitic stainless steel, tested at elevated (300 °C) and cryogenic (? 140 °C) temperatures. The work highlights that although both materials fail in a pure ductile fashion, stainless steel manufactured by HIP displays a marked reduction in fracture toughness, defined using J_0.2BL, when compared to equivalently graded forged 304L, which is relatively constant across the tested temperature range.     

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.     

A Brief Review of High Entropy Alloys and Serration Behavior and Flow Units

Multicomponent alloys with high entropy of mixing,e.g.,high entropy alloys (HEAs)and/or multiprin-cipal-element alloys (MEAs),are attracting increasing attentions,because the materials with novel properties are being developed,based on the design strategy of the equiatomic ratio,multicomponent,and high entropy of mixing in their liquid or random solution state.Recently,HEAs with the ultrahigh strength and fracture toughness,excel-lent magnetic properties,high fatigue,wear and corrosion resistance,great phase stability/high resistance to heat-softening behavior,sluggish diffusion effects,and potential superconductivity,etc.,were developed.The HEAs can even have very high irradiation resistance and may have some self-healing effects,and can potentially be used as the first wall and nuclear fuel cladding materials.Serration behaviors and flow units are powerful methods to understand the plastic deformation or fracture of materials.The methods have been successfully used to study the plasticity of amorphous alloys (also bulk metallic glasses,BMGs).The flow units are proposed as:free volumes,shear transi-tion zones (STZs),tension-transition zones (TTZs),liquid-like regions,soft regions or soft spots,etc.The flow units in the crystalline alloys are usually dislocations,which may interact with the solute atoms,interstitial types,or sub-stitution types.Moreover,the flow units often change with the testing temperatures and loading strain rates,e.g., at the low temperature and high strain rate,plastic deformation will be carried out by the flow unit of twinning,and at high temperatures,the grain boundary will be the weak area,and play as the flow unit.The serration shapes are related to the types of flow units,and the serration behavior can be analyzed using the power law and modified power law.     

Plastic deformation in polycrystalline Nb3Sn

A study of the high temperature plastic deformation of polycrystalline Nb₃Sn has been undertaken on hot isostatically pressed material having grain sizes in the 12 to 60 (μm range. Through compression testing and load-relaxation testing deformation has been studied over a strain rate range from 10⁻⁶to 10⁻²s and a temperature range from 1150 to 1650 °C. Plastic deformation can be observed in compression at 1400 °C and above and extensive deformation is possible at 1650°C. Except for the lowest strain rates at 1650 °C, load-relaxation stress-strain rate relationships are consistent with “power law creep”. Analysis of stress-strain rate-temperature relationships projects an activation energy for creep of very roughly 500 kJ/mol. Observations on yield point behavior and fracture mode transition are presented. A comparison to monocrystalline V₃Si behavior is made, and the role of the sub-structure during testing is considered.     

The Influence of Slip Character on the Creep and Fatigue Fracture of an α Ti-Al Alloy

The mechanical performance of engineering titanium alloys has long been known to be sensitive to the nature of applied load waveform. Sustained load hold periods imposed during the fatigue loading of two-phase α + β alloys are detrimental to material performance. A number of factors, particularly material texture, phase morphologies, and slip behavior, are thought to affect the dwell fatigue responses of these materials. This study examines the roles of slip character and applied load waveform on an alloy with a simpler microstructure than the two-phase alloys studied previously. It is found that planar slip has significant influence over the dwell, fatigue, and fracture behaviors of equiaxed-grained, single-phase α Ti-7 wt pct Al.     

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.     

The influence of microstructure and strength on the fracture mode and toughness of 7XXX series aluminum alloys

The effects of microstructure and strength on the fracture toughness of ultra high strength aluminum alloys have been investigated. For this study three ultra high purity compositions were chosen and fabricated into 1.60 mm (0.063 inches) sheet in a T6 temper providing a range of yield strengths from 496 MPa (72 ksi) to 614 MPa (89 ksi). These alloys differ only in the volume fraction of the fine matrix strengthening precipitates (G. P. ordered + η′ ). Fracture toughness data were generated using Kahn-type tear tests, as well asR-curve andJ _c analyses performed on data from 102 mm wide center cracked tension panel tests. Consistent with previous studies, it has been demonstrated that the toughness decreases as the yield strength is increased by increasing the solute content. Concomitant with this decrease in toughness, a transition in fracture mode was observed from predominantly transgranular dimpled rupture to predominantly intergranular dimpled rupture. Both quantitative fractography and X-ray microanalysis clearly demonstrate that fracture initiation for the two fracture modes occurred by void formation at the Cr-dispersoids (E-phase). In the case of intergranular fracture, void coalescence was facilitated by the grain boundary η precipitates. The difference in fracture toughness behavior of these alloys has been shown to be dependent on the coarseness of matrix slip and the strength differential between the matrix and precipitate free zone (σ_M-σPFZ). A new fracture mechanism has been proposed to explain the development of the large amounts of intergranular fracture observed in the low toughness alloys. Formerly a Research Assistant at Carnegie-Mellon University     

The effect of solidification time and heat treatment on the fatigue properties of a cast 319 aluminum alloy

Solidification time and heat treatment are known to have a large effect on the microstructure of cast aluminum alloys. This study was conducted to quantify how the fatigue properties of a 319-type aluminum alloy are affected by solidification time and heat treatment. Both porosity-containing (non-hot isostatically pressed (HIP)) and porosity-free (HIP) samples in the T6 (“peak aged”) or T7 (“overaged”) heattreated conditions were tested. As the solidification time increased, the average initiating pore diameter increased and stress-controlled fatigue life decreased. Heat treatment was observed to have a large effect on fatigue properties of the HIP samples. However, in the non-HIP fatigue samples, heat treatment did not significantly change the fatigue life or fatigue strength of the cast 319-type alloy. The absence of an influence of heat treatment on fatigue response is attributed to the predominance of the microporosity in fatigue crack initiation in cast aluminum.     

Microstructural effects on the tensile and fracture behavior of aluminum casting alloys A356/357

The tensile properties and fracture behavior of cast aluminum alloys A356 and A357 strongly depend on secondary dendrite arm spacing (SDAS), Mg content, and, in particular, the size and shape of eutectic silicon particles and Fe-rich intermetallics. In the unmodified alloys, increasing the cooling rate during solidification refines both the dendrites and eutectic particles and increases ductility. Strontium modification reduces the size and aspect ratio of the eutectic silicon particles, leading to a fairly constant particle size and aspect ratio over the range of SDAS studied. In comparison with the unmodified alloys, the Sr-modified alloys show higher ductility, particularly the A356 alloy, but slightly lower yield strength. In the microstructures with large SDAS (>50 μm), the ductility of the Sr-modified alloys does not continuously decrease with SDAS as it does in the unmodified alloy. Increasing Mg content increases both the matrix strength and eutectic particle size. This decreases ductility in both the Sr-modified and unmodified alloys. The A356/357 alloys with large and elongated particles show higher strain hardening and, thus, have a higher damage accumulation rate by particle cracking. Compared to A356, the increased volume fraction and size of the Fe-rich intermetallics (π phase) in the A357 alloy are responsible for the lower ductility, especially in the Sr-modified alloy. In alloys with large SDAS (>50 μm), final fracture occurs along the cell boundaries, and the fracture mode is transgranular. In the small SDAS (<30 μm) alloys, final fracture tends to concentrate along grain boundaries. The transition from transgranular to intergranular fracture mode is accompanied by an increase in the ductility of the alloys.