Effects of helium on the mechanical properties and microstructure of niobium

Mechanical property specimens of niobium (Cb) were doped with helium by the tritium trick to concentrations as high as 500 appm. The tritium decays by the reaction³H →³He +β ⁻ at a rate that produces about 7 appm per day in the host microstructure. Tensile properties were measured from room temperature to 800°C, and creep properties from 700 to 1000°C at stresses from 45 to 75 MPa. Transmission electron microscopy was used to study the microstructure of the helium doped specimens, and the observations were correlated with the mechanical property results. The results of this investigation showed that niobium has a high tolerance to helium trapped in the microstructure. The tensile and creep strengths of niobium increased as helium concentration increased. The ductility decreased significantly as the helium concentration increased, but niobium retained substantial ductility even at a high helium concentration of 500 appm. This amount of helium would be generated by (n,α) reactions in the microstructure of a niobium first wall after a 20 y exposure in a D-T fusion reactor. Thus, niobium and niobium alloys are potential candidates for high temperature structural materials in D-T fusion reactors.     

The effect of alloying elements and microstructure on the strength and fracture resistance of pearlitic steel

The processes of ductile and brittle fracture in fully pearlitic steel and their relation to both the scale of the microstructure and the presence of substitutional alloy elements have been investigated at room temperature using smooth tensile and over a range of temperatures using V-notched Charpy impact specimens. The results show that the early stages of cracking, revealed in both types of specimen, are largely the result of shear cracking of the pearlite lamellae. These cracks grow and can reach a size when they impinge upon the prior austenite boundary; afterward the character of fracture can be either microvoid coalescence or cleavage, depending on test conditions and metallurgical variables. Further, the carbide plates of the pearlite lamellae can act as barriers to the movement of dislocations as is the case normally with grain boundaries. For pearlite an optimum spacing of approximately 0.2 μm resulting from a balance between carbide plate thickness and interlamellar spacing was found to enhance toughness, although such changes are much smaller than corresponding changes due to varying alloy elements. Specific alloy elements used herein strengthened the lamellar ferrite in pearlite, inhibiting the movement of dislocations while also usually decreasing the lamellar cementite plate thickness for the same spacing. This dual behavior results in enhanced resistance to the initiation and propagation of microcracks leading to an improvement in strength, ductility, and toughness. The most effective alloy elements for the composition ranges studied in fully pearlitic steels are Si and Ni for strength improvement, and Ni and Mn for toughness.     

On the effect of additions of niobium and vanadium on strength and formability of cold-rolled sheets

Study of the correlation between manufacturing parameters, microstructure, and ratio of formability to strength of cold-rolled sheets made on experimental scale of niobium- and vanadium-microalloyed steels. Dependence of strength upon grain size and precipitation behavior. Effect of the state of precipitation on recrystallization kinetics, microstructures, and development of the crystallographic texture.     

The effect of niobium on the hardenability of microalloyed austenite

The powerful effect that varying the extent of niobium-carbide dissolution has on the “hardenability” of microalloyed austenite is demonstrated using dilatometric measurement of the critical cooling rate required to from microstructures containing >95 Pct martensite. The results can be rationalized on the hypothesis that the hardenability of austenite is enhanced by niobium in solid solution, possibly by its segregation to austenite grain boundaries, but is decreased by precipitation of niobium-carbide particles. This effect appears analogous to that of boron in steels and is found to be independent of variations in the austenite grain size.     

The role of microstructure on the strength and toughness of fully pearlitic steels

An experimental program was carried out to clarify the structure-property relationships in fully-pearlitic steels of moderately high strength levels, and to identify the critical microstructural features that control the deformation and fracture processes. Specifically, the yield strength was shown to be controlled primarily by the interlamellar pearlite spacing, which itself was a function of the isothermal transformation temperature and to a limited degree the prior-austenite grain size. Charpy tests on standard and fatigue precracked samples revealed that variations in the impact energy and dynamic fracture toughness were dependent primarily on the prior-austenite grain size, increasing with decreasing grain size, and to a lesser extent with decreasing pearlite colony size. These trends were substantiated by a statistical analysis of the data, that identified the relative contribution of each of the dependent variables on the value of the independent variable of interest. The results were examined in terms of the deformation behavior being controlled by the interaction of slip dislocations with the ferrite- cementite interface, and the fracture behavior being controlled by a structural subunit of constant ferrite orientation. Preliminary data suggests that the size of such units are controlled by, but are not identical to, the prior-austenite grain size. Possible origins of this fracture unit are considered.     

The role of microstructure on the strength and toughness of fully pearlitic steels

An experimental program was carried out to clarify the structure-property relationships in fully-pearlitic steels of moderately high strength levels, and to identify the critical microstructural features that control the deformation and fracture processes. Specifically, the yield strength was shown to be controlled primarily by the interlamellar pearlite spacing, which itself was a function of the isothermal transformation temperature and to a limited degree the prior-austenite grain size. Charpy tests on standard and fatigue precracked samples revealed that variations in the impact energy and dynamic fracture toughness were dependent primarily on the prior-austenite grain size, increasing with decreasing grain size, and to a lesser extent with decreasing pearlite colony size. These trends were substantiated by a statistical analysis of the data, that identified the relative contribution of each of the dependent variables on the value of the independent variable of interest. The results were examined in terms of the deformation behavior being controlled by the interaction of slip dislocations with the ferrite- cementite interface, and the fracture behavior being controlled by a structural subunit of constant ferrite orientation. Preliminary data suggests that the size of such units are controlled by, but are not identical to, the prior-austenite grain size. Possible origins of this fracture unit are considered.     

The effect of forging history on the strength and microstructure of TDNiCr (Ni-20Cr-2ThO2)

Forging variables were evaluated to determine their influence on the elevated temperature strength and microstructure of TDNiCr. Grain size was the principal microstrucrural feature related to elevated temperature strength and was controlled primarily by the thermomechanical variables of forging temperature and final annealing condition. Tests at 2000°F (1366 K) revealed a factor of eight increase in tensile strength as grain size increased from 1 to 150 μm, while stress-rupture strength improved by three to five times as grain size increased from 15 to 1500 μm. Forged material of grain size ≥ ∼150 μm displayed a level of elevated temperature strength comparable to that of optimized TDNiCr sheet. The presence of a preponderance of small twins and a strong preferred orientation may have also been factors contributing to the excellent high temperature strength of large grain forged material.     

The effect of neutron irradiation damage on niobium oxygen alloys

An investigation was undertaken to determine the effect of neutron irradiation damage on the low temperature deformation characteristics of niobium oxygen alloys. Alloys containing from 57 to 10,590 at. ppm oxygen interstitial content were tested in compres-sion by strain rate cycling at a base strain rate of 8.33 × 10^-5 s^-1. Tests were conducted between 25 and 500 K on both nonirradiated crystals and crystals irradiated at a fluence of ∼2.0 × 10¹⁹ neutron/cm² (E > 1 MeV). A parametric analysis of dislocation mecha-nisms was performed using the theory of thermally activated dislocation motion. From the results of this investigation the following conclusions were made: The in-trinsic lattice is the rate-controlling barrier at low oxygen concentrations. The oxygen interstitials appear to be part of the rate-controlling barrier at higher oxygen concentra-tions. The irradiation damage has very little effect on the magnitude of effective stress of the irradiated samples irrespective of the oxygen concentration. The differences in effective stress between irradiated and nonirradiated samples are due to the effect of oxygen on the effective stress of the nonirradiated samples and a scavenging mechanism. The transmission electron microscopy results indicate that the oxygen atoms act as nu-cleation sites for the observable defects. There is qualitative agreement between the increase in the internal stress and the observable defect density. However, there is ab-solutely no agreement between the observable defect density and the changes in the dif-ferences in the effective stress between the irradiated and nonirradiated samples, or the almost zero change in the effective stress of the irradiated samples.     

The effect of microstructure on the fatigue crack growth behavior of Age-Hardened high strength steels in a corrosive environment

Fatigue crack growth rates have been measured in laboratory air and in a 3.5 pct NaCl aqueous solution under cathodic polarization of -0.85 V(Ag/AgCl reference electrode) for various agehardened, high strength steels. At low ΔK values in air, the fatigue crack growth resistance of the underaged condition was improved compared to the overaged condition. This improvement can be explained with increased crack closure and slip reversibility. The fatigue crack growth resistance of material containing both coherent precipitates and incoherent precipitates with the matrix was similar to that of the material including only incoherent precipitates because of the restriction of inhomogeneous deformation by the incoherent precipitates. Corrosion fatigue crack growth resistance in the aqueous solution was dependent on aging conditions. The environmental sensitivity was greater in the underaged materials owing to hydrogen embrittlement. The environmental sensitivity of underaged materials containing coherent precipitates with the matrix was improved by the coexistence of incoherent precipitates. The decreased amount of strain to fracture due to hydrogen diffusing into plastic zone led to decreased crack closure.     

Giant low-field reversible magnetocaloric effect at liquid helium temperature of niobium and iron co-substituted EuTiO3 compounds

Giant magnetocaloric effect(MCE) materials in the liquid helium temperature region have attracted a lot of attention in the field of low-temperature magnetic refrigeration(MR).In this study,a series of niobium(Nb) and iron(Fe) co-substituted EuTiO₃ perovskites with cubic structure(space group pm3m) was successfully fabricated,and their magnetic properties as well as cryogenic magnetocaloric effects were investigated in detail.As expected,the introduction of Nb and Fe can significantly...     

氮对含铌20MnSi钢组织演变的影响

 研究了不同冷速条件下氮对铌微合金化20MnSi钢组织演变的影响。试验钢经1 200 ℃全固溶处理后快冷至[Ac3,]然后分别以200、100 ℃/h速度冷却至室温。对试样进行了OM、SEM和TEM观察。结果表明：钢中细小Nb(C,N)粒子在原奥氏体晶内高密度位错区的密集析出导致贫碳区的形成,进而激发针状铁素体的形核长大。铌微合金钢增氮后能有效抑制钢中针状铁素体的生成,促进等轴铁素体的生成和珠光体球团的细小均匀化,同时珠光体退化倾向减弱或消失。     

含铌冷轧DP780钢的连退工艺对组织及性能的影响

 通过成分工艺优化,在传统冷轧铁素体和马氏体双相钢DP780的显微组织上引入了一定体积分数的残余奥氏体,研究了冷轧退火工艺参数对双相钢DP780的显微组织和力学性能的影响。通过调整连续退火工艺来控制显微组织中一次铁素体、二次铁素体、马氏体、残余奥氏体的比例、尺寸、形貌、分布,同时获得了连退工艺参数-显微组织-力学性能的本质关系。结果表明,通过在传统冷轧铁素体和马氏体双相钢的组织上引入了体积分数为5%～7%的残余奥氏体,不仅可以获得[ReL/Rm≤0.5]的超低屈强比型冷轧DP780,也改善了成型性能。     

The role of microstructure on strength and ductility of hot-extruded mechanically alloyed NiAl

Mechanical alloying followed by hot extrusion has been used to produce very fine-grained NiAl-based alloys containing oxide dispersoids. The dispersoids affect the progress of recrystallization during hot extrusion and contribute to the preservation of the 〈110〉 deformation fiber texture. The 〈110〉 texture enables the activation of 〈110〉 〈100〉 and 110 〈110〉 slip systems. The occurrence of 〈100〉 and 〈110〉 slip dislocations satisfies the von Mises criterion for general plasticity and is postulated to contribute to notable room-temperature compressive ductility of the mechanically alloyed (MA) materials. Another factor likely affecting the compressive ductility is the predominant occurrence of low-angle grain boundaries. The attractive dislocation — dispersoid interactions lead to a ductility trough observed at 800 K in the MA materials. The MA NiAl materials are strong at both ambient and elevated temperatures due to fine grain and the presence of dispersoids and interstitial atoms.     

The effect of interdiffusion on the interfacial microstructure of Au-Pd

The interfacial microstructure of {001} twist interphase interfaces in gold-palladium bicrystal thin film couples has been investigated by transmission electron microscopy. Experimental results demonstrate that the character and spacing of the interfacial dislocations change with annealing. In particular the dislocations become progressively screw dominated with increasing annealing time and/or temperature, indicating that the lattice parameters change at the interface. These values approach each other due to interdiffusion with concomitant compositional changes. The structural evolution of the interfaces due to annealing was also studied. In gold-palladium couples, evidence was found for the presence of two intermediate phases. A model for the possible stages of the structural evolution during annealing is proposed, based on detailed considerations of both microstructure and diffraction observations.     

The effect of morphology on the strength of pearlite

The effects of various morphological features on the strength of high-purity pearlite were studied. A continuous-cooling mode of transformation from different austenitizing temperatures was used to produce variations in average nodule diameter and minimum interlamellar spacing. It was found that, for a constant transformation temperature, nodule size was directly related to prior austenite grain size. On the other hand, minimum interlamellar spacing is controlled by transformation temperature, independent of prior austenite grain size and nodule size. Both the yield strength and fracture stress of pearlite was found to be inversely proportional to interlamellar spacing and independent of prior austenite grain size and nodule size.     

The effect of morphology on the strength of pearlite

The effects of various morphological features on the strength of high-purity pearlite were studied. A continuous-cooling mode of transformation from different austenitizing temperatures was used to produce variations in average nodule diameter and minimum interlamellar spacing. It was found that, for a constant transformation temperature, nodule size was directly related to prior austenite grain size. On the other hand, minimum interlamellar spacing is controlled by transformation temperature, independent of prior austenite grain size and nodule size. Both the yield strength and fracture stress of pearlite was found to be inversely proportional to interlamellar spacing and independent of prior austenite grain size and nodule size.     

The effect of zirconium on the strength of thorium

The flow stress of thorium-zirconium alloys containing up to 12 at. pct zirconium was studied over a range of temperatures and strain rates. It was found that the flow stress could be analyzed as the sum of a thermally activated and an athermal component. The thermally activated component was nearly the same in the base metal and the zirconium alloys. The major effect of zirconium was a considerable increase in the athermal component. The magnitude of this increase was directly proportional to the zirconium content. Zirconium also retarded the recovery and recrystallization processes. D. C. ZABEL is a former Graduate Assistant, Ames Laboratory, USAEC, Iowa State University, Ames, Iowa. 50010.     

The effect of substrate on the microstructure and creep of eutectic In-Sn

This study was conducted in order to determine and understand the effect of substrate on the behavior of eutectic In-Sn. Samples for mechanical testing were produced with either bare Cu or Ni on Cu substrates. Both the microstructure and the mechanical behavior are strongly dependent on substrate. When eutectic In-Sn is joined to bare Cu, Cu diffusion into the joint causes the alloy to become off-eutectic, giving a nonuniform and irregular microstructure. The addition of a layer of Ni acts as a diffusion barrier, preventing Cu diffusion sufficiently such that a uniform, normal colony-based eutectic forms. Deformation is more uniform in the In-Sn on Ni, while it is concentrated along the length of the joint in the In-Sn on Cu. This distinction is reflected in the different shapes of shear stress-strain curves between In-Sn on Cu and In-Sn on Ni. The stress exponents and activation energies for creep also vary with substrate. Creep deformation is governed by the In-rich β phase for In-Sn on Cu and by the Sn-rich γ phase for In-Sn on Ni. If In-Sn on Ni samples are aged, the microstructure coarsens and the mechanical behavior changes to resemble that of the as-cast In-Sn on Cu. Formerly with the Department of Materials Science and Mineral Engineering, University of California at Berkeley.     

The effect of intercritical annealing temperature on the structure of niobium microalloyed dualphase steel

The structures produced in a Nb-microalloyed steel by oil quenching after intercritical anneals at 760 and 810 °C have been examined by light and transmission electron microscopy. After both anneals, the periphery of the austenite pool transforms on cooling to ferrite in the same orientation as the ferrite retained during intercritical annealing. Thus the ferrite forms by an epitaxial growth mechanism without the formation of a new interface or grain boundary. The new ferrite is precipitate-free in contrast to the retained ferrite which develops a very dense precipitate dispersion during intercritical annealing. In the carbonenriched interior of the austenite pool beyond the epitaxial ferrite only martensite forms in specimens annealed at 760 °C but various mixtures of ferrite and cementite form in specimens annealed at 810 °C. The latter structures include lamellar pearlite, a degenerate pearlite, and cementite interphase precipitation. All Nb is in solution in the austenite formed at 810 °C, and therefore the low hardenability of the specimens annealed at that temperature is best explained by the effect of low austenite carbon content.