The relationship between toughness and microstructure in Fe-high Mn binary alloys

Metallurgical and Materials Transactions A - The influence of Mn content on the ductile-brittle transition in 16 to 36 wt pct Mn steels was investigated and interpreted in light of the evolving...     

The relationship between toughness and microstructure in Fe-high Mn binary alloys

The influence of Mn content on the ductile-brittle transition in 16 to 36 wt pct Mn steels was investigated and interpreted in light of the evolving microstructure. It was found that when hcp ε martensite is present in the as-quenched condition or forms during deformation, it lowers the toughness. In 25Mn steel, the stress concentrations at e plate intersections result in the formation of planar void sheets along the {111}_γ planes. The deformation-induced α’ martensite in 16 to 20 pct Mn alloys enhances the toughness, but leads to a ductile-to-brittle transition at low temperatures that is due to the intrusion of an intergranular fracture mode. Binary alloys with greater than 31 pct Mn also fracture in an intergranular mode at 77 K although the impact energy remains quite high. Auger spectroscopy of the fracture surfaces shows no evidence of significant impurity segregation, which suggests the importance of slip heterogeneity in controlling intergranular fracture in these alloys.     

The relation between microstructure and toughness in 7075 aluminum alloy

Electron microscopy, fractography, and notched tear tests have been used to investigate the effects of heat treatment upon the fracture behavior of aged 7075 aluminum alloy sheet. Toughness, as measured by crack propagation energy, decreases as the yield stress increases; the toughness of an overaged structure is inferior to that of an underaged structure at the same yield stress. The decrease of toughness with increased aging time is accompanied by a change in fracture mode from predominantly transgranular to intergranular. Transgranular fracture proceeds by dimple rupture and is facilitated by chromium-rich particles which are dispersed throughout the microstructure. Intergranular fracture proceeds by the fracture of grain boundary precipitate particles. The variation of fracture mode with aging time is attributed to a steady decrease of the intergranular fracture stress relative to the transgranular fracture stress, due to increasing grain boundary particle size. A possible explanation of this effect is discussed using the stress concentration due to colinear crack arrays as an analogy. The effects of quenching variations and two-step aging are discussed. It is shown that, in aged 7075, microstructural variables such as the width of precipitate-free zones and the nature of the matrix precipitate do not have a controlling effect on toughness.     

Correlation of microstructure and fracture toughness in high-chromium white iron hardfacing alloys

A correlation is made of microstructure and fracture toughness in hypereutectic high-chromium white iron hardfacing alloys. In order to investigate the matrix effect of these alloys, in particular, four different matrices such as pearlite, austenite, and a mixture of pearlite and austenite were employed by changing the ratio of Mn/Si, while the total volume fraction of carbides was fixed. The hardfacing alloys were deposited twice on a mild steel plate by the self-shielding flux-cored arc-welding method. Fracture toughness was increased by increasing the volume fraction of austenite in the matrix, whereas hardness and abrasion resistance were nearly constant.In situ observation of the fracture process showed that cracks initiated at large primary carbides tended to be blocked at the austenitic matrix. This suggested that fracture toughness was controlled mainly by the amount of austenite in the matrix, thereby yielding the better toughness in the hardfacing alloy having the austenitic matrix. Considering both abrasion resistance and fracture toughness, therefore, the austenitic matrix was preferred for the high-chromium white iron hardfacing alloys.     

Mn含量对Cu-Mn合金结构与性能的影响

基于Cu-Mn中间层可改善Cu与CuCrZr合金的连接性能,采用高能球磨法制备Cu-Mn合金粉末,研究烧结温度为650~850℃、Mn含量(质量分数)为10%~50%的Cu-Mn合金的烧结性能。结果表明:随Mn含量增加,Cu-Mn合金的相对密度和抗拉强度先增大后减小,但Mn含量过高时生成较多的Mn O,导致合金性能下降;随烧结温度升高,合金的相对密度和抗拉强度增大。850℃下烧结的67Cu-33Mn合金孔隙较小、组织均匀,其相对密度和抗拉强度均达到最大值,分别为91.62%和610.91 MPa,表明67Cu-33Mn合金具有最佳的烧结活性,宜作为Cu与CuCrZr合金连接的中间层。     

The relationship between microstructure and the J-R curve

A model is presented that establishes a quantitative relationship between the J _IC and the J-R curve and the microstructural parameters, such as inclusion size and spacing, and the plastic deformation properties of ductile materials, such as yield strength and strain hardening exponents. The model assumes that ductile crack growth occurs by void nucleation, growth, and coalescence. Each of these processes is modeled in the crack tip environment to complete the model. The proposed model is evaluated using tests performed on 303 stainless steel (SS) at room temperature and on 1Cr-Mo-0.25V steel at 565 °C. Satisfactory agreements are obtained between the experimentally obtained J-R curves and those predicted from the model. Limitations of the model are also discussed.     

Effect of transformation substructure on the strength and toughness of Fe−Mn alloys

Phase transformations in Fe?Mn alloys containing up to 9 pct Mn were studied by optical and electron transmission microscopy. Either equiaxed ferrite, massive ferrite, or massive martensite can form on cooling from austenite. The particular type of transformation product formed was found to depend on the alloy content, austenite grain size, and cooling rate. The mechanical properties of all the transformation products were evaluated using tensile and impact testing and are discussed in terms of the observed microstructural features. Yield strength and impact transition temperature were found to be relatively insensitive to manganese content but were strongly influenced by the transformation substructure and grain size of the transformed phase. In martensite it has been shown that the structural unit analogous to grain size in ferrite is the martensite packet size, which in turn is controlled by the prior austenite grain size. The fracture surface of broken impact specimens and the fracture profile were examined by means of electron and optical microscopy techniques. These fractographic observations were correlated with impact test data and microstructural observations of the various transformation products.     

Interrelations between fracture toughness and other mechanical properties in titanium alloys

Critical stress intensity factors and general tensile properties were measured for a class of metastable beta and alpha-beta titanium alloys with a limited number of compositions but processed to have a wide variety of strength levels and microstructures. These data were used to test thirteen theoretical relations between the fracture toughness and tensile parameters. Many of the theoretical relations had been proposed previously, but several new forms were derived in the present work. The correlation showed that for alloys exhibiting a limited range of microstructures, the simpler correlations gave the better fits withK _Ic α γ⁻¹specifically being the best one. For correlations of alloys with a wide range of microstructures, more complex correlations which included microstructural parameters were found to be superior.     

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 relationship of microstructure to monotonic and cyclic straining of two age hardening aluminum alloys

The effect of microstructure on the monotonic and low cycle fatigue properties of a high purity, large grain, ternary aluminum-zinc, magnesium (Al-Zn-Mg) alloy and a high strength 7050 aluminum alloy was investigated. The best combination of fatigue life, strength, and ductility for the ternary alloy resulted when aged to produce a microstructure containing predominately η′ having a Guinier radius of approximately 65? and a small amount of incoherent η (MgZn₂). Superior fatigue life, strength, and ductility were found when the 7050 alloy was aged to produce the maximum number of partially coherent η′ precipitates having a Guinier radius of approximately 35?. Aging the 7050 alloy to produce particles larger than 50? gave a microstructure that had lower fatigue properties at the low plastic strain amplitudes, δε_p/2 <1.0 pct. The empirical CoffinManson relationship was found to hold for a given deformation process, however changes in deformation character resulted in changes in the Coffin-Manson parameters. This research was supported by the U.S. Air Force Office of Scientific Research (AFSC) under Grant No. AFOSR-74-2615, Dr. Alan H. Rosenstein, Contract Monitor.     

The relationship of microstructure to monotonic and cyclic straining of two age hardening aluminum alloys

The effect of microstructure on the monotonic and low cycle fatigue properties of a high purity, large grain, ternary aluminum-zinc, magnesium (Al-Zn-Mg) alloy and a high strength 7050 aluminum alloy was investigated. The best combination of fatigue life, strength, and ductility for the ternary alloy resulted when aged to produce a microstructure containing predominately η′ having a Guinier radius of approximately 65å and a small amount of incoherent η (MgZn₂). Superior fatigue life, strength, and ductility were found when the 7050 alloy was aged to produce the maximum number of partially coherent η′ precipitates having a Guinier radius of approximately 35å. Aging the 7050 alloy to produce particles larger than 50å gave a microstructure that had lower fatigue properties at the low plastic strain amplitudes, δε_p/2 <1.0 pct. The empirical CoffinManson relationship was found to hold for a given deformation process, however changes in deformation character resulted in changes in the Coffin-Manson parameters.     

The microstructure of electrodeposited titanium-aluminum alloys

The microstructure of electrodeposited titanium-aluminide alloys containing 3.6 to 24.1 at. pct Ti was studied by transmission electron microscopy. The surface morphology of the deposits showed that they contained nodular and faceted grains which tended to be less faceted at higher Ti contents. Extensive 111 twinning was observed in all deposits, and growth striations parallel to were observed in the low Ti deposits. The growth of nodules was linked to the presence of these twins; it was hypothesized that the twin boundaries act as easy atomic attachment points and, therefore, enhance the growth rate. The presence of twins and striations was used to pro-pose a growth mechanism. The 5.3, 15.8, and 24.1 at. pct Ti deposits were single-phase grains of the Ll₂ crystal structure, as opposed to the expected equilibrium two-phase mixture of face-centered cubic (fcc) Al (saturated with Ti) and D0₂₂ Al₃Ti. Calculated electron diffraction in-tensity data were used to demonstrate that the decrease in intensity of the superlattice reflections in the substoichiometric deposits is due to a reduction in the difference in atomic scattering factors between the two lattice site types. formerly NIST/NRC Postdoctoral Research Associate, National Institute of Standards and Technology.     

The relation between the Portevin-Le chatelier effect and the solid solubility in some binary alloys

Effects of composition and temperature on the occurrence of the Portevin-Le Chatelier effect were investigated in the literature and the experiments in some alloys. These investigations showed that the alloy systems were divided into two groups according to the minimum solute concentration below which the P-L effect did not occur. The first group consisted of alloy systems in which the P-L effects occurred at the solute content less than 1 at.%, and their common feature was that they had small solubilities of solute. In the second group the P-L effects did not occur at the solute content less than 5 at.% and the solute solubilities were large. Based on these results a new theory for the P-L effect that the precipitates or the short range ordered regions interacted with the dislocations, was proposed and compared with the Cottrell theory.     

Effect of solidification microstructure on corrosion behavior of the chill zone in aluminum binary alloys

The corrosion behavior in NaCl solutions of as-cast Al-Cu, Al-Mg and Al-Zn alloys has been studied in relation to the substructures obtained during the solidification process on the surface of the chill zone. Metallographic, microprobe and X-ray techniques have been used to identify the equilibrium precipitates and to estimate the solute distribution in the alloys. The attack produced on predendritic, dendritic and interdendritic zones has been studied by optical microscopy and by scanning electron microscopy. By using potentiostatic techniques, it has been found that localized corrosion occurred above certain potentials corresponding to the pitting potentials of the phases present in the alloys.     

Effect of solidification microstructure on corrosion behavior of the chill zone in aluminum binary alloys

The corrosion behavior in NaCl solutions of as-cast Al-Cu, Al-Mg and Al-Zn alloys has been studied in relation to the substructures obtained during the solidification process on the surface of the chill zone. Metallographic, microprobe and X-ray techniques have been used to identify the equilibrium precipitates and to estimate the solute distribution in the alloys. The attack produced on predendritic, dendritic and interdendritic zones has been studied by optical microscopy and by scanning electron microscopy. By using potentiostatic techniques, it has been found that localized corrosion occurred above certain potentials corresponding to the pitting potentials of the phases present in the alloys.     

Correlation of microstructure with the wear resistance and fracture toughness of hardfacing alloys reinforced with complex carbides

A correlation was made of the microstructure, wear resistance, and fracture toughness of hardfacing alloys reinforced with complex carbides. The hardfacing alloys were deposited twice on a low-carbon steel substrate by a submerged arc welding (SAW) method. In order to investigate the effect of complex carbides, different fractions of complex carbide powders included inside hardfacing electrodes were employed. Microstructural analysis of the hardfaced layer showed that cuboidal carbides, in which a TiC carbide core was encircled by a WC carbide, and rod-type carbides, in which W and Ti were mixed, were homogeneously distributed in the bainitic matrix. In the surface layer hardfaced with FeWTiC powders, more complex carbides were formed, because of the efficient melting and solidification during hardfacing, than in the case of hardfacing with WTiC powders. As the volume fraction of complex carbides, particularly that of cuboidal carbides, increased, the hardness and wear resistance increased. In-situ observation of the fracture process showed that microcracks were initiated at complex carbides and that shear bands were formed between them, leading to ductile fracture. The hardness, wear resistance, and fracture toughness of the hardfacing alloys reinforced with complex carbides were improved in comparison with high-chromium white-iron hardfacing alloys, because of the homogeneous distribution of hard and fine complex carbides in the bainitic matrix.     

The relationship between microstructure and stress-strain behavior in tension in high strength pipeline steel

In this paper the relationship between microstructure and stress-strain behavior in tensile test of high strength pipeline steel was investigated.The steel with polygonal ferrite-bainite(PF + B) microstructure has a round-house type tensile stress-strain curve with low Y/T ratio,highly uniform elongation and high n-value,which means PF + B microstructure has the best deformability(i.e.Ideal stress-strain behavior) among the four microstructures.The steel with acicular ferrite-martensite&austenite(AF + MA) microstructure has a continuous-yielding type tensile stress-strain curve,whose deformability is worse than that of PF + B microstructure.Both the steels of polygonal ferrite-acicular ferrite(PF + AF) and polygonal ferrite-pearlite (PF+P) microstructure have luders elongation type tensile stress-strain curve with high Y/T ratio,low uniform elongation and low n-value,which means PF + AF and PF + P microstructures have the worst deformability among the four microstructures.     

Dendritic solidification in binary alloys

Alloys generally solidify dendritically, and associated with that is the microsegregation of impurities. Pure metals also solidify in dendritic form as “thermal” dendrites, which actually segregate the system’s enthalpy. In this investigation, small additions of solute to succinonitrile have been studied and dendritic growth observed in a supercooled melt. This free dendritic growth-mode is similar to that experienced by equiaxed dendrites found in alloy castings. Observations of these free dendrites include measurement of velocity and tip radius of the dendrites at different supercoolings and solute concentrations.