M. Dollar

Influence of deformation substructure on flow and fracture of fully pearlitic steel

Abstract Tensile stress-strain data over the whole strain range were obtained for a range of pearlites from very coarse to relatively fine (interlamellar spacings 0.53 and 0.13 μm, respectively). Transmission electron microscopy (TEM) for pearlite subjected to various amounts of strain was performed. Coupling mechanical data with TEM examination provided a detailed picture of how pearlite yields, deforms, work hardens, and fails under uniaxial tension. It is shown that yielding and work hardening of pearlite are largely controlled by processes occurring in ferrite. The role of a cementite plate at low stresses is mainly to limit the slip distance in ferrite. It is found that the tensile fracture is determined by processes in the colonies with lamellae parallel to the tensile axis and that the stress necessary to break a cementite plate corresponds to the true U.T.S. The influence of interlamellar spacing on the yield strength, flow stress, and the true U.T.S. is quantitatively explained.

Deformation mechanisms and ductility of mechanically alloyed NiAl

Abstract An NiAl-based alloy has been produced by mechanical alloying and hot extrusion, resulting in material which is fully dense, with a homogeneous distribution of oxide particles and with a fine grain size of less than 1 μm. Mechanical properties of the mechanically alloyed (MA) NiAl were studied by compression testing from room temperature to 1300 K. At room temperature, the alloy exhibited high yield strength (1380 MPa) and considerable compressive ductility (greater than 11.5%). Transmission electron microscopy of the compressed specimens was carried out. In order to determine the Burgers vectors of slip dislocations a rigorous procedure was followed. The 〈100〉 slip was found to be predominant but strong evidence of 〈110〉 slip was also gathered. The occurrence of the slip vectors satisfies the general requirement for plasticity and contributes to the notable compressive ductility. Cast and hot extruded NiAl has been also investigated for comparison with the MA material. At room temperature, it exhibited a poor ductility (2.3%), low yield strength (400 MPa) and only 〈100〉 slip dislocations were observed. The 〈100〉 slip provides three independent slip systems, an insufficient number for general plasticity. The different behavior of cast and MA NiAl is believed to be a result of distinct textures, 〈111〉 and 〈110〉 respectively, exhibited by these differently processed materials.

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.

TEM investigation of age-hardenable Al 2519 alloy subjected to stress corrosion cracking tests

The influence of changes in chemical composition and pre-aging deformation on the resistance to stress corrosion cracking in the age-hardenable aluminum alloy 2519 was investigated by transmission electron microscopy. The improvement of this resistance may be accomplished by keeping the Cu concentration on the lower side of the allowed limit for the 2519 alloy. Also, plastic deformation prior to aging, comprising both cold rolling and stretching, seems to be beneficial since it promotes a more homogeneous distribution of the precipitates and reduces the number of precipitates on the grain boundaries and thus shrinks the total volume of precipitation-free zones at grain and subgrain boundaries.

Nanocrystalline NiAl-processing, characterization and mechanical properties

Abstract Nanocrystalline ordered NiAl (n-NiAl) was successfully synthesized by an electron beam gas condensation and compacted in-situ at various temperatures. As-compacted material exhibits grain sizes between 2 and 4 nm and densities between 78 and 94% of the theoretical density, increasing with increasing compaction temperature above all as a result of reduced porosity. The nanocrystalline structure of NiAl is stable up to about 1000°C. Microhardness of as compacted n-NiAl increases with increasing density, above all as a result of reduced porosity. For the reasons not fully understood yet, microhardness of n-NiAl increases also with increasing grain size following annealing, a response different from that in the conventional, coarse-grained NiAl. The present material is significantly stronger than its conventional counterpart but not as strong as predicted by Hall–Petch-type modeling. Also, in the nanocrystalline form, NiAl exhibits room temperature ductility, unlike its coarse-grained counterpart. The present study provides probably the first unequivocal experimental evidence of the room temperature ductility of a nanocrystalline intermetallic material. The mechanical behavior of n-NiAl can be rationalized assuming that diffusional, rather than dislocation, mechanisms control strength and ductility of n-materials.

Mechanical properties of NiAl–AlN–Al2O3 composites

Abstract The mechanical properties of NiAl-based composites containing a dispersion of AlN particles and Al2O3 fibers were studied. NiAl matrix powder containing a dispersion of about 5 vol.% AlN was synthesized by mechanical alloying (MA) of elemental nickel and aluminum under a nitrogen gas atmosphere. Nextel 610 short fibers were used as the second reinforcement in the composite. Composites containing 5, 15 and 30 vol.% of the Al2O3 fibers in addition to the AlN dispersion particles were fabricated by hot pressing a dry blend of the MA NiAl powder and the fibers. The as-fabricated microstructures revealed that the matrix is fully dense and bonded well with the randomly distributed Al2O3 fibers. Neither chemical reaction nor cracks were observed at the fiber/matrix interface. Compressive behavior of the composites and a monolithic counterpart was studied at 1300 K and strain rates between 8.5×10−4 and 2.8×10−6 s−1. The 0.2% yield stress of the composites increases with fiber volume fraction at all strain rates. The strain rate–flow stress behavior at 1300 K indicates that the strength of NiAl–5%AlN–30%Al2O3 approaches that of NASAIR 100, a first generation Ni-base single crystal superalloy.

The effect of hydrogen on deformation substructure, flow and fracture in a nickel-base single crystal superalloy

Abstract In this paper the room temperature flow and fracture of a nickel-base single crystal γ/gg′ superalloy in the presence and absence of hydrogen is explored. The procedure of hydrogen-charging employed in this study provides a very high and uniform hydrogen concentration of the order of 5000 appm in the material. It is shown that the most compelling hydrogen-induced changes in deformation behavior are enhanced dislocation accumulation in the γ matrix and extensive cross-slip of super-dislocations. The explanation of these changes is proposed. Both effects contribute to the increase of flow stress and the notable work hardening that occurs prior to fracture. Hydrogen enhanced strain localization in the γ matrix leads to the dramatic loss of ductility and premature cracking, which manifests as failure macroscopically parallel to the {100} faces of γ′ precipitates. On the microscale, cracking, while limited to the γ matrix, occurs parallel to multiple {111} slip systems.

Influence of plastic deformation and prolonged ageing time on microstructure of a Haynes 242 alloy

The material used in this study was a commercial HAYNES® alloy 242™ with a nominal composition of Ni‐25% Mo‐8% Cr (in wt.%). In the standard heat treatment, the 242 alloy is annealed at a temperature between 1065 and 1095 °C and then water quenched. The ageing treatment is carried out at 650 °C for 24 h in order to develop the long‐range‐order strengthening. The alloy in the conventionally aged condition was additionally cold rolled to 50% reduction in thickness and subsequently subjected to prolonged ageing at 650 °C for 4000 h. The enhanced diffusion resulted in the decomposition of the Ni2(Mo,Cr) metastable phase into the stable Ni3Mo‐based phase. The presence of the new stable phase increased the yield and tensile strengths but deteriorated the ductility of the alloy at both room and 650 °C temperatures.

Synthesis and characterization of mechanically alloyed Nb3Al-base alloys

Abstract Two Nb–Al alloys containing 18 and 20 at.%Al were successfully processed by mechanical alloying followed by hot pressing. Analysis of X-ray spectra as well as transmission electron microscopy studies revealed the presence of four phases: niobium solid solution (Nb ss ), Nb 3 Al, Nb 2 Al and dispersoids of Al 2 O 3 . The grain size was estimated to be approximately 1 μm. Nb ss grains contained a very high dislocation density while a high density of planar stacking faults was observed in the intermetallic phases. The alloys exhibited only limited compressive ductility at room temperature but were ductile at 1000°C. The mechanism for creep deformation in the examined Nb–Al alloys is postulated to be dislocation creep through diffusion controlled climb. The compressive strength was higher and creep rates were lower in the present materials compared to reference NiAl-based materials processed and consolidated using the same techniques.

Hydrogen effects on low-cycle fatigue of the single-crystal nickel-base superalloy CMSX-2

The effects of hydrogen on the low-cycle fatigue behavior of CMSX-2 [001]-oriented single crystals were examined. Fatigue tests were conducted under constant plastic strain amplitude control. Cyclic stress-strain curves and fatigue life data at different plastic strain amplitudes were determined for hydrogen-free and hydrogen-charged specimens. Two charging procedures, leading to different hydrogen concentrations, were applied. Hydrogen was found to decrease significantly the number of cycles to failure under the various experimental conditions. The increasing hydrogen concentration and ratio of the hydrogen to nonhydrogen-containing volume were found to shorten fatigue life in hydrogen-charged specimens. Based on the analysis of cyclic stress-strain curves and optical and transmission electron microscopy (TEM), it was established that hydrogen enhanced strain localization and promoted crystallographic, stage I cracking, leading to embrittlement. The overall fracture mechanism is discussed in conjunction with Duquette and Gell’s stage I fracture model.[16]