Chi-Mei Hsiao

In situtem observations of nucleation and bluntness of nanocracks in thin crystals of 310 stainless steel

Nucleation and bluntness of nanocracks were studied through in situ tensile tests for thin crystals of 310 stainless steel by transmission electron microscopy (TEM). A dislocation free zone (DFZ) could form after the dislocation emission had just ceased. The DFZ is an elastic zone so that the local stress near the crack tip in the DFZ is possibly up to the cohesive strength, because of which a nanocrack could initiate in the DFZ or at the crack tip. The nanocrack in the DFZ or at the crack tip would blunt into a void or a notch through the increment and movement of dislocations in the plastic zone even when keeping constant displacement. If constant displacement was kept for a long time, nanovoids could initiate in the DFZ through diffusion and enrichment of supersaturation vacancies. The connection of the nanovoids would result in the initiation of nanocracks.

The effect of anodic polarization on the ambient creep of brass

The effect of anodic and cathodic polarizations on the ambient creep of brass in 3.5% NaCl and CH3COOH + CH3COONa solutions has been investigated. The results showed that the creep rate in both solutions increased linearly with the increase of the anodic current density and was approximately inversely proportional to the square of the radius of the specimen. The de/dt in 3.5% NaCl solution was much higher than that in the CH3COOH + CH3COONa solution at the same anodic current density. The creep rate resulting from dislocation climb induced by vacancy supersaturation during the anodic polarization has been calculated as follows: dϵdt = Kβir2 where K = 8παMϱDsΔtΔr/C0dFZ ln (R/b), and β is a constant related to the degree of de-alloying. The experimental data support the proposed mechanism. Cathodic polarization could also increase the ambient creep of brass by a mechanism related to hydrogen-facilitated local plastic deformation.

In-situ TEM observation of dissolution-enhanced dislocation emission, motion and the nucleation of SCC for Ti24Al11Nb alloy in methanol

Many experiments have showed that anodic polarization can facilitate ambient creep for various metals and alloys. Anodic polarization can decrease the yield strength, and enlarge the plastic zone on the surface ahead of a loaded crack tip. The density of dislocations in a region very close to the fracture surface of SCC was much higher than that far away from the fracture surface. All of these experiments showed that anodic dissolution facilitated the localized plastic deformation. Up to now, however, direct proof of anodic dissolution or corrosion-enhanced dislocation emission, multiplication and motion is lacking. Using a special constant deflection device, the dislocation configuration change ahead of a loaded crack tip before and after anodic dissolution together with the initiation of SCC for Ti-24Al-11Nb alloy in methanol can be in-situ observed in TEM. The results showed that the localized anodic dissolution could facilitate dislocation emission, multiplication and motion and SCC with size of nanometers would initiate in the dislocation-free zone (DFZ) or at the original crack tip after the dissolution-enhanced dislocation emission and motion reached a critical condition.

The effect of pre-creep on stress corrosion cracking of austenitic stainless steel and brass

The effect of pre-creep on the stress corrosion cracking (SCC) of austenitic stainless steel in a 42% MgCl2 solution and brass in an ammonia solution was investigated using constant load and constant deflection samples. For both austenitic stainless steel and brass, pre-creep had no influence on the failure time and the threshold stress intensity of SCC if the constant load sample was used. Since for constant deflection samples, pre-creep could greatly increase the threshold stress intensity of SCC for both austenitic stainless steel and brass, constant deflection specimens are not suitable for investigating the effect of pre-creep on SCC. Pre-cathodic protection at a non-cracking potential for long times could inhibit the SCC of brass during subsequent exposure in cracking potential range. This is, however, attributed to the change of composition and pH value of the solution rather than the effect of pre-creep.

The corrosion fatigue fractography of Ti-24Al-11Nb

Many researchers have studied the fracture behaviors and fractography of the Ti-24Al-11Nb alloy. While hydrogen induced delayed fracture could occur in this titanium aluminide in hydrogen gas at temperatures above 300[degree]C, there was no obvious difference in fractography of the alloy in hydrogen or air. The fractography of stress corrosion cracking in a solution was also similar to that of overloaded fracture in air. The recent work showed that hydrogen induced delayed fracture for a notched sample could occur during dynamic cathodic charging at the temperature. There were a lot of small dimples on the fracture surface near the notch tip when K[sub I] neared the threshold k[sub IH]. This differed from that of an overloaded fracture. The fractography of corrosion fatigue in methanol or during dynamic charging for the Ti-24Al-11Nb alloy was studied in this paper.

Effect of Hydrogen on Cathodic Corrosion of Titanium Aluminide

Abstract Cathodic corrosion of titanium aluminide (TiAl) during hydrogen charging in various acidic aqueous solutions and in molten salt at 160°C was studied. At constant potential, the rate of cat...

Hydrogen induced cracking in Fe3Al+Cr

The room temperature embrittlement in air and the environment sensitivity of the ordered Fe[sub 3]Al alloy severely restrict its use. Recently, Liu et al. reported that room temperature ductility can be improved by the addition of chromium of 2 at%--6 at%. They found that with an additional chromium the fracture mode changed to one of approximately 50% transgranular cleavage and 50% intergranular failure. This experimental phenomenon was explained in terms of a combined effect of an easier cross slip, increasing cleavage strength and solid solution softening. The fracture surfaces of the hydrogen induced delayed fracture of the ordered Fe[sub 3]Al alloy, which cleavage fracture occurred at room temperature, are dependent upon K[sub I] and the hydrogen content. This work will investigate whether the fracture surfaces of hydrogen induced delayed fracture of ordered alloy Fe[sub 3]Al + Cr are dependent upon the K[sub I] and how the microstructure effects the hydrogen induced delayed fracture.

Investigation of fractal dimensions of hydrogen-induced brittle fracture of titanium aluminide

Abstract A relation between the stress intensity factor K I ∗ required for brittle crack initiation and propagation and the fractal dimension DF of the fracture surface was derived, i.e. ln K I ∗ =( 1 2 ) ln 2γE′ + ( 1 2 ) ln (d f /L 0 )(1−D F ) where df is the fracture unit, L0 is the length of the straight projection line of the fracture profile, γ is the real surface energy, and E′ = E (plane stress) or E/(1 − v2) (plane strain).The real surface energy can be calculated on the basis of the measured linear relation on ln K I ∗ vs.Df. The equation is not only suitable for overload fracture but also for delayed fracture, e.g. hydrogen-induced cracking and stress corrosion cracking. The experiment results showed that the hydrogen-induced delayed cracking occured in Ti-24Al-11Nb alloy during dynamic charging, and the threshold stress intensity factor was very low, i.e.KIH/KIC = 0.43. The experimental relation between the stress intensity factor K I ∗ and DF was consistent with the theoretical equation.

Interphase stress corrosion crack

A kind of interphase SCC has been found for the Ti-24Al-11Nb alloy tested in the methanol solution. Only the furnace cooled specimens with lower KISCC initiated and propagated preferentially along the α2β interphase boundary or nucleated simultaneously along the interphase boundary and in the individual phase rather than propagation through the α2β interface.

Energetical analysis of sensitivity of hydrogen embrittlement of ordered alloys

Intermetallic compounds are regarded as highly promising for applications as high temperature structural materials. However, recent studies have demonstrated that a number of ordered alloys exhibit environmental embrittlement when tested in various environments at ambient temperature. When exposed to hydrogen gas or electrolytically charged with hydrogen, a significant ductility loss has been reported. The reaction of water vapor with reactive elements in the crack tip results in the generation of atomic hydrogen which diffuses from the surface to the interior and causes embrittlement. In this paper, the authors try to analyze the sensitivity of hydrogen embrittlement of ordered alloys based upon energetical considerations.