Creep crack growth in the absence of grain boundary precipitates in UDIMET 520

Abstract

The effects of grain size and environment on creep crack growth (CCG) in Ni-base superalloy, UDIMET 520, were studied through experiments at 540 ЊC. Specially designed solution and aging treatments were used to produce ␥ Ј strengthened microstructures with different grain sizes but without any M 23 C 6 grain boundary precipitates. Five grain sizes, which fall into three groups (i.e., small, medium, and large), were employed. The creep crack growth rates (CCGRs) in specimens with small grain sizes were approximately 2.5 times lower than those with medium and large grain sizes, as a result of crack branching and the presence of some undissolved primary MC carbides at the grain boundaries. Otherwise, the CCGRs were insensitive to the grain size. Fractographic observations on the fracture surfaces and metallographic examinations on the cross sections of the interrupted CCG specimen revealed intergranular microcracks and a faceted intergranular mode of fracture in both air and argon environments. The test results suggest that the formation and propagation of intergranular cracks by grain boundary sliding (GBS) is the main micromechanism responsible for CCG in both air and argon environments at the relatively low test temperature employed. Grain boundary oxidation attack in the air environment simply accelerates the crack growth process. The present results are in agreement with the theoretical predictions of the GBS-controlled CCG model previously developed by the authors.

Effect of Microstructure on Creep Crack Growth Behavior of a Near-α Titanium Alloy IMI-834

“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The stress intensity factor K for SENT-type of specimen is calculated using the formula [17] …”

“…As a first choice, due to its simple definition, the stress intensity parameter K, widely used for creep-brittle materials such as nickel base superalloys, [11,12] has been chosen to characterize CCG behavior of the present alloy. The variation of the creep crack growth rate (CCGR) da/dt with the stress intensity factor K on log-log scale for the alloy in both a + b and b conditions at 600°C are shown in Figures 4(a) and (b), respectively.…”

“…[1] Under such conditions, path-independent energy integrals C* or C t based on elastic plastic fracture mechanics (EPFM) concepts as recommended by ASTM E-1457 standard [2,3] have been used to characterize creep crack growth (CCG) behavior of materials. Extensive work on CCG behavior of power plant steels [4][5][6][7][8][9][10] and aeroengine materials, such as nickel-based superalloys, [11,12] has been reported in literature. However, similar studies on titanium-based alloys are very limited, [13][14][15] in particular, with regard to microstructural aspects of CCG micromechanisms.…”