L.M. Hsiung

Creep of a heat treated Mg–4Y–3RE alloy

Abstract Creep of WE43-T6 was characterized at temperatures ranging from 423 to 523 K and stresses ranging from to 30 to 300 MPa. Creep stress exponent as well as activation energy were measured. There appears to be a softening in creep strength at temperatures above 473 K. Also, the applied stress level affects the creep mechanism. Microstructures of the alloy both before and after creep were examined. Two kinds of precipitates, metastable β′ and stable β phases, were observed in the microstructure. These precipitates readily transform and coarsen during creep. Dislocation–precipitate interaction was quite extensive. Fracture surface revealed that there is a gradual transition from brittle to ductile mode as testing temperature increases. A comparison of the creep properties among the present alloy and other rare earth-containing alloys were made.

Superplastic behavior of a powder metallurgy TiAl alloy with a metastable microstructure

Superplasticity in a powder-metallurgy TiAl alloy (Ti-47Al-2Cr-2Nb) with a metastable microstructure has been studied. Samples were tested at temperatures ranging from 650 to 1100{degrees}C, and at strain rate ranging from 10{sup -6} to 10{sup -4} s{sup -1}. An elongation value of over 300 obtained at a strain rate of 2 x 10{sup -5} s{sup -1} and at a temperature as lo as 800{degrees}C, which is close to the ductile-to-brittle-transition temperature. This is in contrast to the prior major observations of superplastic behaviors in TiAl in which typical temperatures of 1000{degrees}C have usually been required for superplasticity. It is proposed that the occurrence of superplasticity at 8000{degrees}C in the present alloy is caused by the presence of a B2 phase. During superplastic deformation (grain boundary sliding), the soft P grains accommodate sliding strains to reduce the propensity for cavitation at grain triple junctions and, thus, delays the fracture process. The final microstructure consists of stable, equiaxed y+a{sub 2} grains.

Interfacial Dislocations and Deformation Twinning in Fully Lamellar TiAl

Abstract Deformation twinning, which takes place abnormally within lamellar TiAl subjected to creep deformation at strain rates as low as 10−7 s−1, has been found to be intimately related to the motion, pileup and dissociation of interfacial (Shockley partial) dislocations. Since the interfacial (Shockley partial) dislocations are energetically unfavorable to undergo cross-slip or climb, under normal conditions, they can only move conservatively along interfaces. Consequently, the pileup configuration once generated cannot be easily dissipated and thus remains in place even at elevated temperatures. The dislocation pileup eventually leads to the emission of deformation twins from the interfaces into γ lamellae when a local stress concentration due to the dislocation pileup becomes sufficiently large. Deformation twinning of {111}〈11 2 〉 and {11 2 }〈111〉 types (both generate Σ3 twin boundaries) has been observed. Both types of twinning can be rationalized by dislocation mechanisms involving the core dissociation of interfacial dislocations: 1/6[ 1 2 1 ](111)→1/6[011](100)+1/6[ 1 1 2 ] 1 11 and 1/6[1 2 1](111)→1/2[0 1 0](001)+1/6[111](11 2 ), and the emission of 1/6〈 1 1 2 〉 and 1/6〈111〉 twinning dislocations into γ lamellae to form the ( 1 11)[ 1 1 2 ]- and (11 2 )[111]-type twins, respectively. The critical shear stress for the {111}〈11 2 〉-type twinning is evaluated using the Peach–Koehler formula based upon the pileup configuration of interfacial dislocations.

Shock-Induced Omega Phase in Tantalum

The deformation substructure developed within polycrystalline tantalum under a high shock pressure (45 GPa) with a 1.8 {micro}s duration has been examined using transmission electron microscopy. A shock-induced {beta} (bcc) {yields} {omega} (hexagonal) displacive transformation is observed for the first time within tantalum. Needle- or plate-like {omega} phase is found to form accompanied with the {l_brace}112{r_brace} type deformation twins within shock-recovered tantalum. The orientation relationships between the shock-induced {omega} and parent {beta} phases are determined to be {l_brace}10{bar 1}0{r_brace}{sub {omega}} {parallel} {l_brace}211{r_brace}{sub {beta}}, [0001]{sub {omega}} {parallel} {sub {beta}} and {sub {omega}} {parallel} [0{bar 1}1]{sub {beta}}. The lattice parameters of {omega} phase are a{sub {omega}} = ({radical}2a{sub {beta}}) = 0.468 nm and c{sub {omega}} = ({radical}3/2) a{sub {beta}} = 0.286 nm (c/a = 0.611). Both deformation twins and shock-induced {omega} phase are primarily formed along the {l_brace}211{r_brace}{sub {beta}} slip planes with high resolved shear stresses, and have a common habit plane [i.e. the {l_brace}211{r_brace} plane] with the parent {beta} matrix. It is suggested that shear deformation on the {l_brace}211{r_brace} planes is the major cause for the formation of shock-induced {omega} phase within tantalum.

Shock-induced displacive transformations in tantalum and tantalum-tungsten alloys

A recent investigation on the deformation substructure of shocked tantalum by transmission electron microscopy (TEM) has for the first-time revealed that a displacive omega transformation can also take place in tantalum (a group V transition metal) under a high shock peak pressure (45 GPa). Plate- or lath-like {omega} phase ({omega}{prime} hereafter) has been observed within shocked tantalum, which is considered to be unusual since tantalum has a bcc structure and exhibits no equilibrium phase transformation up to its melting temperature at ambient pressure. The occurrence of displacive omega transformation within shocked tantalum is of great interest because it provides not only an effective strengthening mechanism for tantalum and tantalum alloys but also an opportunity to study and understand the mechanisms of displacive {beta} {yields} {omega}{prime} transition induced by high strain-rate deformation. Results from the investigation of displacive omega transformation in tantalum and tantalum-tungsten alloy are reported and discussed here.

Effect of strain rate on the elevated-temperature tensile properties of an AlPb alloy

Recent studies have demonstrated that superplasticity can be found at strain rates of up to 10{sup 0} to 10{sup {minus}4} s{sup {minus}1}. These strain rates are considerably higher than the typical values of 10{sup {minus}4} to 10{sup {minus}3} s{sup {minus}1} at which conventional superplastic forming is usually performed. Mechanistically, the deformation properties of a material can be altered dramatically, with the presence of even a small amount of liquid phase. The liquid phase acts as a lubricant, which promotes grain and particle sliding, and drastically changes the deformation mechanism. In the present study, the authors present the effect of strain rate on the elevated-temperature tensile properties, particularly ductility, of an Al-0.15Zr-0.75Pb (composition in weight percent) alloy. Pb was tentatively added in order to form liquid phase at grain boundary as the alloys were tested at temperatures above the melting temperature (T{sub m}) of Pb, i.e., 327 C. The tensile properties of an Al-0.15Zr alloy were also characterized for a direct comparison.

HRTEM Study of Oxide Nanoparticles in 16Cr-4Al-2W-0.3Ti-0.3Y2O3 ODS Steel

Crystal and interfacial structures of oxide nanoparticles in 16Cr-4Al-2W-0.3Ti-0.3Y{sub 2}O{sub 3} ODS ferritic steel have been examined using high-resolution transmission electron microscopy (HRTEM) techniques. Oxide nanoparticles with a complex-oxide core and an amorphous shell were frequently observed. The crystal structure of complex-oxide core is identified to be mainly monoclinic Y{sub 4}Al{sub 2}O{sub 9} (YAM) oxide compound. Orientation relationships between the oxide and matrix are found to be dependent on the particle size. Large particles (> 20 nm) tend to be incoherent and have a spherical shape, whereas small particles (< 10 nm) tend to be coherent or semi-coherent and have a faceted interface. The observations of partially amorphous nanoparticles lead us to propose three-stage mechanisms to rationalize the formation of oxide nanoparticles containing core/shell structures in as-fabricated ODS steels.

Low-Temperature Aging and Phase Stabiliy of U6Nb

Aging behavior and phase stability of a water-quenched U-6wt%Nb (U-14at%Nb) alloy artificially aged at 200 C and naturally aged at ambient temperature for 15 years have been investigated using Vickers hardness test, X-ray diffraction analysis, and transmission electron microscopy techniques. Age hardening/softening phenomenon is observed from the artificially aged samples according to microhardness measurement. The age hardening can be rationalized by the occurrence of spinodal decomposition, or fine scale of Nb segregation, which results in the formation of a nano-scale modulated structure within the artificially aged samples. Coarsening of the modulated structure after prolonged aging leads to the age softening. The occurrence of chemical ordering (disorder-order transformation) is found in the naturally aged sample based upon the observations of antiphase domain boundaries (APBs) and superlattice diffraction patterns. A possible superlattice structure is accordingly proposed for the chemically ordered phase observed in the naturally aged alloy sample.

Interfacial Control of Creep Deformation in Ultrafine Lamellar TiAl

Solute effect on the creep resistance of two-phase lamellar TiAl with an ultrafine microstructure creep-deformed in a low-stress (LS) creep regime [where a linear creep behavior was observed] has been investigated. The resulted deformation substructure and in-situ TEM experiment revealed that interface sliding by the motion of pre-existing interfacial dislocations is the predominant deformation mechanism in LS creep regime. Solute segregation at lamellar interfaces and interfacial precipitation caused by the solute segregation result in a beneficial effect on the creep resistance of ultrafine lamellar TiAl in LS creep regime.

Role of interfacial dislocations on creep of a fully lamellar tial

Deformation mechanisms of a fully lamellar TiAl ({gamma} lamellae: 100 {approximately} 300 nm thick, {alpha}{sub 2} lamellae: 10 {approximately} 50 nm thick) crept at 760 C have been investigated. It was found that, as a result of a fine structure, the motion and multiplication of dislocations within both {gamma} and {alpha}{sub 2} lamellae are limited at low creep stresses ( 400 MPa).