Structure of dislocation cores in metallic materials and its impact on their plastic behaviour

Publisher Summary This chapter discusses the microscopy and plasticity of the L1 2 γ´ Phase. Single-phase ordered intermetallic alloys exhibit a variety of macroscopic mechanical properties that are dominated by the individual behavior of dislocations and, more precisely, by transformations that occur within the core of superdislocations. Positive temperature dependence of the flow stress has been reported in some metals and ceramics, but it is certainly in intermetallic systems that this behavior is the most frequently encountered. This chapter describes macroscopic mechanical properties, including deformation under constant strain rate, creep, and the analysis of transient tests. From one alloy composition to another, the measured orientation dependence may vary in a significantly inconsistent manner as shown by uncertainties on the position of maximum asymmetry. The actual orientation dependence appears, in fact, to be more complicated than a simple theoretical analysis could predict, unless of course this theory is appropriately parameterized.

Chapter 53 Microscopy and plasticity of the L12 γphase

Melamine is a synthetic chemical used in the manufacture of resins, pigments, and superplasticizers. Human beings can be exposed to melamine through various sources such as migration from related products into foods, pesticide contamination, and illegal addition to foods. Toxicity studies suggest that prolonged consumption of melamine could lead to the formation of kidney stones or even death. Therefore, reliable and accurate detection methods are essential to prevent human exposure to melamine. Sample preparation is of critical importance, since it could directly affect the performance of analytical methods. Some methods for the detection of melamine include instrumental analysis, immunoassays, and sensor methods. In this paper, we have summarized the state-of-the-art methods used for food sample preparation as well as the various detection techniques available for melamine. Combinations of multiple techniques and new materials used in the detection of melamine have also been reviewed. Finally, future perspectives on the applications of microfluidic devices have also been provided.

A Review of Methods for Detecting Melamine in Food Samples.

In the present study a new insert design is presented and validated to enable reliable dynamic mechanical characterization of low strain-to-failure materials using the Split-Hopkinson Pressure Bar (SHPB) apparatus. Finite element-based simulations are conducted to better understand the effects of stress concentrations on the dynamic behavior of LM-1, a Zr-based bulk metallic glass (BMG), using the conventional SHPB setup with cylindrical inserts, and two modified setups—one utilizing conical inserts and the other utilizing a “dogbone” shaped specimen. Based on the results of these computational experiments the ends of the dogbone specimen are replaced with high-strength maraging steel inserts. This new insert-specimen configuration is expected to prevent specimen failure outside the specimen gage section. Simulations are then performed to validate the new insert design. Moreover, high strain-rate uniaxial compression tests are conducted on LM-1 using the modified SHPB with the new inserts. An ultra-high-speed camera is employed to investigate the changes in failure behavior of the specimens. Additional experiments are conducted with strain gages directly attached to the gage section of the specimens to determine accurately their dynamic stress–strain behavior.

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Design of Inserts for Split-Hopkinson Pressure Bar Testing of Low Strain-to-Failure Materials

Temperature-related variations in tensile yield strength and ductility were studied on cast, homogenized and isothermally forged Al67Ti25Cr8. Yield strength dropped discontinuously between 623 K and 773 K and then decreased gradually with increasing temperature. Below 623 K, fracture occurred prior to macroscopic yielding. Ductility decreased from 0.2 percent at 623 K to zero at 773 K, but increased again at higher temperatures. At 1073 K, an elongation of 19 percent was obtainable, and ultimate tensile strength and localized necking were observed. Fracture surfaces and deformed microstructures were examined. The 1073 K tensile specimen that exhibited 19 percent elongation showed grain boundary serrations and some evidence of recrystallization (likely dynamic) although fracture occurred predominantly via an intergranular mode.

Tensile behavior of the L12 compound Al67Ti25Cr8

The microstructure of the ordered intermetallic alloy with a nominal composition of Al66Fe9Ti24 is nearly single-phase L12 structure, with a few second phase agglomerates at some grain corners. Room temperature compression tests showed that this material exhibits a plastic strain of about 11% at fracture. Final fracture of the compression specimens occurred by a shear-off process along a surface oriented about 45 degrees to the compression axis. Fractographic analysis revealed that the fracture is transcrystalline and the fracture mode is mainly quasicleavage plus tearing. Transmission electron microscopy (TEM) was used to explore its deformation mechanisms. The dislocation density was low after homogenization, but is greatly increased during deformation. The deformation mode was found to be 〈110〉 {111} slip instead of twinning as in Al3Ti. The a〈110〉 superdislocations dissociated into two partials of a/3〈211〉-type, bounding a superlattice intrinsic stacking fault (SISF) on the {111} slip plane.

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Plastic deformation and fracture behavior of a Fe-modified Al3Ti-base L12 intermetallic alloy

TEM study on dislocation structures of Al66Fe9Ti24 intermetallic alloy deformed at room temperature

Compression test results of our research on Al 3 Ti-base alloys are reported. It is evident that the specimen length-to-width ratio we used for compression testing can significantly reflect the difference in ductility of different alloys. Thus the tests fulfilled the aim of our present research.

Chen Shipu

Papers

Plastic deformation and fracture behavior of a Fe-modified Al3Ti-base L12 intermetallic alloy

TEM study on dislocation structures of Al66Fe9Ti24 intermetallic alloy deformed at room temperature

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