Environmental fatigue crack growth in titanium aluminides and hydrogen evolution behaviour

Abstract

The influence of environment on fatigue crack growth behaviour was investigated both in nearly lamellar and in duplex titanium aluminides, and the hydrogen evolution kinetics was analysed by thermal desorption spectroscopy. The tensile strength of the duplex material decreases in the order of the extent of the water molecule content in the environment: the strength in vacuum is the highest, and decreases in the order of laboratory air and finally in water. In the case of the lamellar material, the fatigue crack growth rate in dry air is higher in the R–C crack plane orientation than that in the L–C crack plane orientation. The crack growth rate becomes higher when the crack grows as the lamellae tear. However, in the case of the duplex material, the crack growth rate in the R–C crack plane orientation is smaller in the low ΔK (ΔKeff ) region. When cathodic charging is applied, the fatigue crack growth rate becomes higher than in dry air, particularly in the higher stress intensity factor range. The hydrogen evolution rate is increased by cathodic charging, with lower temperature peaks and higher ones. The peaks at lower temperatures are correlated with the decomposition of hydrides and de‐training of hydrogen from microstructural imperfections such as microvoids. As‐received materials also show an evolution peak at a higher temperature, and the evolution rate is almost independent of cathodic charging. In addition, the evolution rate at a high temperature (above 800 °C) is increased by cathodic charging. The hydrogen is considered to have an important role on fatigue crack growth acceleration.

High-temperature environmental embrittlement of thermomechanically processed TiAl-based intermetallic alloys

“…[12][13][14][15][16] In these cases, hydrogen is introduced from test atmospheres, such as in a flowing hydrogen gas or in air, the moisture in which is suggested to be able to react with the alloy and to generate atomic hydrogen, resulting in reduced tensile elongation. Furthermore, similar results have been obtained under fatigue loading [17] or for polysynthetic texture (PST) TiAl crystals. [18,19] Fatigue crack growth rates were enhanced by moisture in the test environment.…”

“…[18,19] Fatigue crack growth rates were enhanced by moisture in the test environment. [17] The tensile elongation of PST crystals with soft orientation deformed at room temperature was greatly dependent on the testing atmosphere, and was lower in air than in vacuum. [18,19] For the Ti-49 at.…”

Effect of argon purity on mechanical properties, microstructure and fracture mode of commercially pure (cp) Ti and Ti-6Al-4V alloys for ceramometal dental prostheses

“…Previous studies have demonstrated that contamination from the melting atmosphere might easily cause the deterioration of the mechanical properties in titanium castings since cp Ti and Ti-based alloys are very reactive at high temperatures [3]. The formation of intermetallic compounds, and substitutional and interstitial solid solutions under this condition leads to the formation of a fragile material [6], with low elongation. Among the drawbacks caused by the contamination of cp Ti and titanium-based alloys one can list the reduction in the fatigue limit and the elongation of clasps of removable partial denture frameworks [4].…”

“…On the other hand, the casting of commercially pure titanium (cp Ti) or titanium-based alloys for dental purposes, such as partial denture frameworks or certain single units can be very difficult [3,4]; indeed these alloys are considered one of the most difficult metals to process, mainly because they require high temperatures for melting, low castability, high chemical reactivity at high temperature with silica-based investments and the high affinity of titanium for some elements, such as oxygen, nitrogen and hydrogen as well as carbon. The latter dissolves interstitially [5], leading to increase in hardness and alloy embrittlement [6], and the high reactivity, particularly with the investment, induces the formation of the α-case layer [7]. This high chemical reactivity demands the use of specially formulated investment materials containing alumina, magnesia and calcia instead of silica-based investments.…”

Propriedades mecânicas do titânio comercialmente puro e da liga Ti-6Al-4V fundidos em diferentes ambientes

“…O risco de realizar uma fundição de titânio em ambiente rico de impurezas, principalmente oxigênio, nitrogênio e hidrogênio, é o de poder provocar uma grande alteração nas propriedades e na composição do material, já que este metal apresenta alta reatividade em temperaturas acima de 600°C (VOITIK, 1991). Esta contaminação leva à obtenção de um material frágil, quebradiço e obviamente, de baixo alongamento, devido à formação de composto intermetálicos, soluções substitucionais e intersticiais (MINOSHIMA et al, 2001). Dentre os diversos transtornos causados por esta contaminação está a redução no limite de fadiga e alongamento de grampos e armações em peças de prótese parcial removível (VALLITTU;KOKKONEN, 1995).…”