Small crack growth behavior in selective laser melted TC4 alloy

The three-dimensional small crack growth behavior of selective laser melting (SLM) TC4 alloy was investigated by in-situ observation fatigue test method, and the long crack growth curve was measured by load reduction method under the same experimental conditions. The results show that at the early stage of small crack growth, the fatigue crack growth(FCG) rate fluctuates obviously under the influence of microstructure, and the FCG path is zigzag. With the increase of crack length, the influence of microstructure decreases, and the FCG path is straight, and the FCG rate increases steadily with the crack length. Internal defects can still reduce alloy fatigue life. Considering the small and long crack growth data, it is demonstrated that the small crack can still propagate under threshold value of long crack, and under the same stress intensity factor amplitude ΔK, small crack growth rate is higher than that of long crack. There is a typical "small crack effect", thus the small crack behavior should be considered when fatigue life prediction is carried out

Creep Behavior and Deformation Mechanism of a Third-Generation Single Crystal Ni-Based Superalloy at 980 °C

As the primary choice for aero-engine turbine blades, creep resistance is an important mechanical property for the developing third-generation single crystal Ni-based superalloys. The creep behavior of the superalloy in the [001] orientation was studied at 980 °C under a series of stress levels, accompanied with scanning electron microscope (SEM) and transmission electron microscope (TEM) observation to investigate the microstructure and deformation mechanism. The deformation mechanism of the alloy is found to be dislocation gliding, propagating and forming a dislocation network in the γ/γ′ interface. Dislocation networks could hinder the movement of dislocation and decrease the creep rate to a constant during the steady-creep stage. The formation of dislocation networks was analyzed due to the interaction of {111} dislocations. Then dislocations cut into γ′ phases as individual super-dislocations, anti-phase boundary dislocation pairs, and stacking faults. The super-dislocation in the γ′ phase may cross-slip into the {001} plane from the {111} plane to form Kear–Wilsdorf locks, which could inhibit dislocations from gliding or cross-slipping and then enhance the creep resistance

EXPERIMENTAL STUDY REGARDING ADHESIVE THICKNESS-DEPENDENCE OF STRENGTH FOR LIGHTWEIGHT ALLOY ADHESIVELY BONDED JOINTS

The effect of the adhesive thickness on the overall strength of single lap joints made by lightweight aluminum alloy was experimentally studied. The relation between the overall strength and the adhesive thickness is influenced by some intrinsic characteristics of adhesives,one of which is the adhesive toughness addressed by the present study. The results show that the variation of overall strength corresponding to relatively ductile adhesive is more remarkable compared to that corresponding to relatively brittle adhesive,especially in the comparatively small adhesive thickness range. When the adhesive thickness is larger than the critical thickness,the overall strength of the single lap joints appears equal to the bulk shear strength of the adhesives. The present experimental results agree with the previous model prediction

Low Cycle Fatigue of Single Crystal Nickel-based Superalloy DD6 at 1100℃

The total strain-controlled low cycle fatigue(LCF) behaviors of a single crystal superalloy DD6 at 1100℃ for R=-1 and 0.05 were investigated. The results of LCF tests indicated that the cyclic hardening/softening behavior of the alloy not only has the relationship with the microstructure of the material, but also the loading status. The mean stress relaxation occurred under asymmetric straining. The rate of mean stress relaxation increased with the increasing of strain amplitude; when R=-1, the alloy shows tension-compression asymmetry behavior. All the LCF data obtain under various ratios were well correlated by three models for lifetime prediction, the precision rates predicted are fallen into the factor of±2 times scatter band

Low Cycle Fatigue Behaviors of Single Crystal Nickel-based Superalloy at Temperatures of 600~760 <b>℃</b>

Low cycle fatigue (LCF) tests were conducted on a single crystal nickel-based superalloy DD6 with crystallographic orientations [001] at different elevated temperatures. The typical temperatures selected are 600 ℃, 650 ℃, 700 ℃ and 760 ℃, which represent the temperature field of turbine blade root or dovetail. The experimental results indicate that the characteristic of cyclic hardening exists in DD6 superalloy at middle temperature range. The LCF strain range-life curves are temperature-dependent. A modified cyclic damage accumulation (CDA) life prediction model is proposed. In this modified model, an approximate logarithmic linear function is introduced as the additional terms to consider the temperature effect. The prediction results are mainly distributed in scatter band of ±3. The temperature-modified CDA models agree well with the experimental data

Microstructure heterogeneity and creep damage of DZ125 nickel-based superalloy

The creep behavior of the DZ125 superalloy at high temperatures has been investigated based on the creep properties measurement and microstructure observations. The experimental results show that, after full heat treatment, the fine and coarser cuboidal γ′ precipitates distributed in the dendrite arm and inter-dendrite regions, respectively, the boundaries with various configurations located in the inter-dendrite regions. In the primary creep stage, the cuboidal γ′ phase in the alloy transformed into the rafted structure along the direction vertical to the stress axis. The dislocations slipping and climbing over the rafted γ′ phase are attributed the deformation mechanism of the alloy during steady-state creep. The (1/2)〈1 1 0〉 dislocations slipping in the γ matrix and 〈1 1 0〉 super-dislocations shearing into the γ′ phase are the deformation mechanisms of the alloy in the latter stage of creep. And then the alternate slipping of dislocations results in the initiation and propagation of the micro-cracks along the boundaries until the occurrence of the creep fracture. Since the grain boundaries with various angles relative to the stress axis distribute in the different regions, the initiation and propagation of micro-cracks along the boundaries display the various features

High Temperature Mechanical Constitutive Modeling of a High-Nb TiAl Alloy

Uniaxial tensile, low cycle fatigue, fatigue-creep interaction and creep experiments of a novel high-Nb TiAl alloy (i.e. Ti-45Al-8Nb-0.2W-0.2B-0.02Y (atom fraction/%)) were conducted at 750℃ to obtain its tested data and curves. Based on Chaboche visco-plasticity unified constitutive model, Ohno-Wang modified non-linear kinematic hardening was introduced in Chaboche constitutive model to describe the cyclic hardening/softening, and Kachanov damage was coupled in Chaboche constitutive model to characterize the accelerated creep stage. The differential equations of the constitutive model discretized by explicit Euler method were compiled in to ABAQUS/UMAT to simulate the mechanical behavior of high-Nb TiAl alloy at different test conditions. The results show that Chaboche visco-plasticity unified constitutive model considering both Ohno-Wang modified non-linear kinematic hardening and Kachanov damage is able to simulate the uniaxial tensile, low cycle fatigue, fatigue-creep interaction and creep behavior of high-Nb TiAl alloy and has high accuracy