A Model for Creep and Creep Damage in the γ-Titanium Aluminide Ti-45Al-2Mn-2Nb

Abstract

Gamma titanium aluminides (γ-TiAl) display significantly improved high temperature mechanical properties over conventional titanium alloys. Due to their low densities, these alloys are increasingly becoming strong candidates to replace nickel-base superalloys in future gas turbine aeroengine components. To determine the safe operating life of such components, a good understanding of their creep properties is essential. Of particular importance to gas turbine component design is the ability to accurately predict the rate of accumulation of creep strain to ensure that excessive deformation does not occur during the component’s service life and to quantify the effects of creep on fatigue life. The theta (θ) projection technique is an illustrative example of a creep curve method which has, in this paper, been utilised to accurately represent the creep behaviour of the γ-TiAl alloy Ti -45Al-2Mn-2Nb. Furthermore, a continuum damage approach based on the θ-projection method has also been used to represent tertiary creep damage and accurately predict creep rupture.

Experimental and numerical investigation of the creep behaviour of Ni‐based superalloy GH4169 under varying loading

“…The creep problem the same as the fatigue problem needs to be solved urgently. The model research of the creep problem can be divided into 3 directions: (1) The creep model [1][2][3] was established on the basis of the creep strain rate, which is the key parameter by considering the creep activation Nomenclature: a i , b i , c i , d i , parameters about η i ; f 1 , f 2 , f 3 , parameters about σ 0.2 (MPa); k M1 , k M2 , adjustable variables; t 1 , varying loading time in (σ 1 , T 1 ) (h); t 2 , varying loading time in (σ 2 , T 2 ) corresponding to t 1 (h); t c, 1 , t c, 2 , creep rupture time (h); t i , creep time at a certain creep strain (h); E, Young modulus (MPa); M1, the creep time satisfies the normal distribution; M2, the creep time satisfies the log-normal distribution; T, T 1 , T 2 , temperature (°C); T m , melting point (°C); S M1, i , S M2, i , the standard deviations of creep time at a certain creep strain (h); X M1;i ; X M2;i , the average creep time at a certain creep strain (h); ε, total strain (%); ε c , creep strain (%); ε p , plastic strain (%); θ, temperature (°C); η 1 / % , η 2 / % , η 3 / % , η 4 , η 5 , parameters in creep model; ζ, relative creep time; σ, σ 1 , σ 2 , net sectional stress (MPa); σ 0.2 , yield strength (MPa) energy. The advantage is with a clear physical meaning, but the disadvantage is the low calculation accuracy leading that it is not easy to be applied.…”

An analysis of small punch creep behaviour in the γ titanium aluminide Ti–45Al–2Mn–2Nb

“…Therefore, it can be deduced that the SP creep behaviour of this γ-TiAl alloy is strongly dependent on the maximum principle stress, σ 1 . Previous studies have shown that tertiary creep in this alloy initiates due to microstructural instabilities [16], such as strain incompatibilities between lamellar grains [23], with uniaxial creep fracture surfaces displaying interlamellar and intergranular cracking [4]. Therefore, the nature of the cracking and the dependence of this alloy on σ 1 indicate that the lamellar microstructure strongly influences the SP creep behaviour, with rupture a result of microstructural degradation such as the consolidation of inter-lamellar cracks.…”

“…To predict SP creep in γ-Ti-45Al-2Mn-2Nb a suitable creep model is required. One such model has been previously described [16] which uses the theta (θ) projection technique to relate creep behaviour to test conditions. This model also predicts creep rupture based on the accumulation of creep damage (W).…”

“…where c and k are material constants [16]. This model is applied to the problem of SP creep in γ-TiAl by reducing element stiffness as W exceeds W crit at each Gauss point in each element.…”

“…where n 1 À 4 and A 1 À 4 are material constants obtained from uniaxial constant stress creep tests [16] the values of which are given in Table 1. The activation energies Q n 2 and Q n 4 are both 330 JK À 1 mol À 1 and Ris the universal gas constant (8.314 JK À 1 mol À 1 ).…”