Motohiro Kanno

Intergranular fracture caused by trace impurities in an Al–5.5 mol% Mg alloy

Abstract A coarse-grained Al–5% Mg alloy, which does not show high temperature embrittlement, is successfully prepared using high purity raw materials and a graphite crucible. By preventing the contamination of sodium and hydrogen, it becomes possible to examine separately the effects of various trace elements on hot ductility in the Al–5% Mg alloy. Sodium, calcium, or strontium of 2 mol ppm brings about the high temperature embrittlement based on intergranular fracture, while lithium of 4 mol ppm does not. Sodium is the most highly embrittling among such detrimental elements. The detrimental effect of such impurity is due to its segregation to grain boundaries. Further, the embrittlement caused by sodium or strontium of 2 mol ppm is greatly suppressed by an addition of more than 1000 mol ppm of silicon which scavenges those detrimental elements from grain boundaries.

Effects of a small addition of magnesium and silver on the precipitation of T1 phase in an Al4%Cu1.1%Li0.2%Zr alloy

Abstract A transmission election microscopy study has been carried out on Al 4%Cu-1.1%Li-0.2%Zr alloys with and without 0.4% of magnesium and silver to provide understanding of the role of added magnesium and silver in the precipitation of T 1 phase which is the major strengthening phase. Specimens were solution treated, water quenched and subsequently aged at 180 °C for various times. Single addition of silver had no effect on T 1 precipitation while magnesium addition induced a refinement of the distribution of T 1 plates. In the magnesium-free alloys, T 1 plates nucleated only on the dislocation loops which surrounded incoherent dispersoid particles containing zirconium. In the magnesium-bearing alloys, on the contrary, octahedral voids and monolayer Guinier-Preston (GP) zones on {111} matrix planes were newly revealed at early stages of aging and were found to act as nucleation sites for T 1 phase in addition to the dislocation loops mentioned above. Silver addition to the magnesium-added alloy caused an increase in the distribution density of the GP zones and hence a further increase in the density of T 1 plates. Both elements were shown to be enriched in the GP zones together with copper.

Evidence for the transport of impurity hydrogen with gliding dislocations in aluminum

Environmental embrittlement in most metallic materials is known to be caused by hydrogen penetration from the atmosphere into the material under tensile stress. The authors have shown the location of internal hydrogen atoms in the microstructure of some aluminum base alloys by means of tritium autoradiography in which the location of a hydrogen atom can be visualized as a silver particle produced through the photographic reaction by the {beta}-ray emitted from tritium: radio isotope of hydrogen. They also developed a unique testing machine which, being equipped with an ultra high vacuum chamber and a quadrupole mass spectrometer, can detect a trace amount of hydrogen gas evolved from the specimen on fracture. This paper describes a direct evidence of the correlation between hydrogen atom transport and dislocation glide in aluminum, which has been obtained by means of tritium autoradiography on a deformed sample.

Visualization of hydrogen transport in high strength steels affected by stress fields and hydrogen trapping

Abstract Effects of stress fields and hydrogen trapping on hydrogen transport were studied in high strength steels using hydrogen microprint technique. Hydrogen transport was shown to be promoted by an applied stress field. The approach described here enables direct evaluation of hydrogen transport affected by hydrogen trapping and stress fields.

Visualization of hydrogen diffusion in steels by high sensitivity hydrogen microprint technique

Abstract Hydrogen diffusion in steels was examined by both a high sensitivity hydrogen microprint technique (HMT) and an electrochemical hydrogen permeation method. The main diffusion path in an extremely low carbon steel was lattice within grains; grain boundaries were not accelerated diffusion paths. In the case of a hypo-eutectoid steel, hydrogen diffused through proeutectoid ferrite and ferrite in pearlite under steady-state of hydrogen diffusion. The diffusion paths, however, were carbide/ferrite interfaces when hydrogen charging was interrupted before achievement of the steady state. This is probably ascribable to the reversible trapping effect of the interface. The detection efficiency of the high sensitivity HMT was 75% for the low carbon steel and 40% for the hypo-eutectoid steel.

Quantitative evaluation of detection efficiency of the hydrogen microprint technique applied to steel

The detection efficiency of a hydrogen microprint technique (HMT) applied to steel specimens was examined experimentally. Amounts of hydrogen released from the specimen surface were measured by an electrochemical hydrogen permeation method, and the amounts were also evaluated by means of HMT under the same charging conditions as employed for the permeation test. Detection efficiency of conventional HMT was found to be as low as about 1%, but nickel plating of the steel surface was found to increase this efficiency remarkably. This high efficiency by nickel plating was achieved only when the relative humidity of an experimental atmosphere was controlled to 80% or higher. The thus-modified HMT showed a detection efficiency of about 40%.

Hot ductility of Al-Mg and Al-Mg-Y alloys impaired by trace sodium

Abstract Effect of trace amounts of impurity sodium on hot ductility of Al-5%Mg and Al-5%Mg-0.04%Y alloys was examined at temperatures ranging from 200 °C to 400 °C. The embrittlement appears in the Al-Mg-Y alloy containing 0.60ppm sodium but does not in the Al-Mg-Y alloy containing 0.06ppm sodium. The embrittlement in the ternary alloy is caused only by sodium. On the other hand, the embrittlement appears in the Al-Mg alloy containing 0.61ppm sodium as well as in the Al-Mg alloy containing 0.06ppm sodium. The former is caused by both sodium and hydrogen, and the latter by hydrogen alone. It was found that sodium on the order of 0.1 ppm is enough to cause the embrittlement dependent on the grain size. It is concluded that the effect of both impurity elements must be eliminated to inhibit the embrittlement.

Reply to comments on “Intergranular fracture caused by trace impurities in an Al–5.5 mol% Mg alloy”

Abstract The mechanism of high temperature embrittlement caused by trace sodium (∼2 ppm) in an ultra-high purity Al–5.5 mol% Mg alloy with coarse grains is discussed in comparison with previous reports and other proposed mechanisms.

Intergranular fracture in some precipitation-hardened aluminum alloys at low temperatures

Intergranular fracture at low temperatures from room temperature down to 4.2 K has been studied in some precipitation-hardened aluminum alloys. Microscopic appearance of intergranular facets is revealed to be greatly affected by the microstructure adjacent to the grain boundaries (GBs). When large precipitates on GBs and wide precipitation-free zones (PFZs) are present, coalescence of microvoids initiated at the GB precipitates causes the intergranular fracture with dimples. This fracture process is found to be unaffected by deformation temperature. On the other hand, in the presence of fine precipitates on GBs and narrow PFZs, matrix slip localization exerts significant influence on the fracture behavior. At low temperatures, large stress concentration at GBs leads to intergranular fracture, forming sharp ledges on the fracture surfaces, while at room temperature, the dynamic recovery process is thought to relax such stress concentration, resulting in a transgranular “uctile” rupture.

Analysis of hydrogen evolved from fracture surfaces of a TiAl alloy

Intermetallic compounds are known to have some very attractive properties but to date their drawbacks have prevented their practical use. One drawback is their low ductility at ambient temperature, and extensive attempts have been made to overcome this by, for instance, chemical modification and microstructural control. The ductility of various binary intermetallic compounds is also known to be enhanced by the addition of a third element. Such compounds, however, show lower ductility when tested in hydrogen gas or in air with humidity, a phenomenon called environmental embrittlement. If hydrogen atoms from testing atmospheres of indeed embrittle the alloys, why does impurity hydrogen, which has already been introduced into the alloys during processing, have no influence on their low ductility Little or no attention has been given to the relation between the low ductility of alloys and impurity hydrogen in the alloys correlated with fracture of the alloys, hydrogen atoms are expected to evolve from the fracture surfaces. To check this line of reasoning, gases evolved from specimens of a TiAl alloy were analyzed using a newly developed experimental system.