Key Engineering Materials Vols. 378-379

Abstract: The high cycle fatigue characteristics of magnesium alloys under low humidity, high humidity (80% RH) and sprayed 5%NaCl solution environments have been introduced. Fatigue limit of bulk magnesium alloy was significantly reduced even under high humidity condition, while other structural materials such as steel and aluminum alloy showed no influence of humidity on fatigue limit. The reduction of fatigue limit under 5% NaCl environments was much larger than that under high humidity environment. The remarkable reduction of fatigue limit under corrosive environments was attributed to the formation of corrosion pit, which was induced by simultaneous action of mechanical loading and corrosive environment. To improve the reduced fatigue strength under corrosive environment, coating used to apply on the surface. Non-chromium chemical conversion coating showed superior effect on the improvement of fatigue strength under corrosive environment compared to anodized coating. Fatigue strengths of the coated and painted AZ61 alloy under high humidity and 5%NaCl environments showed almost the same fatigue strength as bulk material under low humidity.

Abstract: This paper reviews the understanding of fretting fatigue with an emphasis on the behavior of Ti-6Al-4V. Advances in life prediction and assessment approaches are highlighted. The role of microstructure on fretting fatigue and its use to detect fretting fatigue damage can now be considered in assessment strategies. Various palliatives are used to enhance the fretting fatigue resistance. These include treatments to introduce compressive residual stress and surface coatings that reduce friction and/or protect the underlying structural material.

Abstract: Conventional depth-sending indentation and novel cyclic indentation were performed to study the mechanical properties and strengthening mechanism of pure gold and Ca-doped gold of up to 90 ppm. The system was chosen because it is popular materials for microelectronics interconnection, yet the strengthening mechanism is not well-understood. Conventional depthsensing indentation showed an increase in hardness and modulus of gold with increasing Ca content of up to 90 ppm, which remained significantly higher after annealing. Cyclic depth sensing indentation, where the specimen is loaded to a specific value, unloaded and immediately reloaded, produced a cyclic loop between unloading and reloading curves. The path of the unloading and reloading curves tell the story of the different response of the dislocations generated under the indentation, in different materials, pure gold and Ca-doped gold at different level. Thus their dislocation movement and deformation behavior could be studied by cyclic depth-sensing indentation method. This paper reports cyclic indentation study performed in calcium doped gold widely used in wire bonding and shows that a few ppm level of calcium could result in dislocationssolute interactions, leading to significant strengthening of the materials. TEM micrographs of the material shows that it consists of elongated grains parallel to the drawing direction about 200 nanometers in diameter and a few micrometers in length. It is concluded that cyclic nanoindentation could be used as an extended technique to extract sensitive material information that are not reflected in the conventional test.

Abstract: In this research paper, the cyclic stress amplitude controlled fatigue response and fracture behavior of an Al-Cu-Mg alloy (Aluminum Association designation 2024) is presented and discussed. The alloy was friction stir welded in the T8 temper to provide two plates one having high tensile ductility and denoted as Plate A and the other having low tensile ductility and denoted as Plate B. Test specimens of the alloy, prepared from the two plates, were cyclically deformed under stress amplitude control at two different load ratios with the primary objective of documenting the conjoint influence of magnitude of cyclic stress, load ratio and intrinsic microstructural effects on cyclic fatigue life and final fracture characteristics. The high cycle fatigue resistance of the alloy is described in terms of maximum stress, R-ratio, and microstructural influences on strength. The final fracture behavior of the friction stir welded alloy is discussed in light of the concurrent and mutually interactive influences of intrinsic microstructural effects, deformation characteristics of the alloy microstructure, magnitude of cyclic stress, and resultant fatigue life.

Abstract: In this research paper, the cyclic stress amplitude controlled fatigue response and fracture behavior of an Al-Cu (Aluminum Association designation 2219) is presented and discussed. The alloy was provided as a thin sheet in the T62 temper in the fully anodized condition. A small quantity of the as-provided sheet was taken and the surface carefully prepared to remove the thin layer of anodized coating. Test specimens of the alloy, prepared from the two sheets (anodized and non-anodized), were cyclically deformed under stress amplitude control at two different load ratios with the primary objective of establishing the conjoint influence of magnitude of cyclic stress, load ratio and intrinsic microstructural effects on cyclic fatigue life and final fracture characteristics. The high cycle fatigue resistance of the alloy is described in terms of maximum stress, load ratio, and microstructural influences on strength. The final fracture behavior of the alloy sheet is discussed in light of the concurrent and mutually interactive influences of intrinsic microstructural effects, deformation characteristics of the alloy microstructure, magnitude of cyclic stress, and resultant fatigue life.

Abstract: Low cycle fatigue of porosity free Al-Si alloys containing between 11 and 18 wt.% Si and produced through directed solidification has been investigated. All alloys contain acicular Al-Si eutectic as the principal element of microstructure, completed by different amounts of primary (Al) and (Si) phases. Crack initiation and propagation modes have been determined for all alloys: crack initiation is always associated with brittle Si particles while propagation takes place across layers of the ductile aluminium which act as effective microstructural barriers. A simple energetic model allows a semi-quantitative interpretation of experimental results concerning damage evolution at the surface: single or multiple cracking. LCF data are analysed both on the basis of Coffin-Manson relation and taking into account the effect of the maximal stress on the fatigue life. The second approach gives a coherent and complete interpretation of experimental results in all investigated materials. The fatigue life of two phase Al-Si model alloys is determined by a combination of the macroscopic response of alloys to cyclic straining which depends on the overall microstructure, including phases which do not participate directly in fatigue, and of local parameters which act at the level of short crack propagation. Since the damage mechanisms at the microstructure size scale are the same in all investigated alloys, the parameter which really determines the fatigue life is the maximal stress. Concerning the effect of microstructure, it is emphasized that it is necessary to take into account both extreme and average values of parameters associated with microstructure elements which effectively play a role in fatigue. Finally, it is shown that the conclusions of the present work can be easily generalised to the fatigue of various single and two phase materials, the unifying element being the physical nature and the resistance of microstructural barriers to the propagation of short cracks.

Abstract: The low-cycle fatigue (LCF) properties of a nickel-base precipitation-strengthened superalloy (GH4145/SQ), obtained at a temperature of 538 o C, were reported and discussed in this paper. The properties investigated include cyclic stress response, fatigue life, deformation microstructure and final fracture features as a function of applied strain amplitude. It was shown that the alloy exhibited a pronounced initial hardening followed by continuous softening to failure at high plastic strain amplitudes ( > 0.2% ap ε ), while at low plastic strain amplitudes ( < 0.2% ap ε ) the initial hardening was followed by a well-defined saturation stage. Bilinear behavior with a change of slope at a plastic strain amplitude of about 0.2% was observed in the cyclic stress-strain (CSS) and Coffin-Manson (C-M) plots. TEM observations revealed that slip band density increased with increasing total strain amplitude and precipitate degradation resulting from dislocation-precipitate interactions took place with continuous cyclic straining. The change in the microstructure during cycling is thus responsible for the fatigue hardening / softening behavior of the alloy. SEM examinations indicated that at low plastic strain amplitudes ( < 0.2% ap ε ) crack propagation was basically transgranular, while at high plastic strain amplitudes ( > 0.2% ap ε ) crack propagation exhibited intergranular features, as a whole. The variation in both the number of operating slip systems and the fracture modes with the strain amplitude employed was used to explain the observed two-stage LCF behavior of the present investigated superalloy.

Abstract: In this technical manuscript the cyclic stress amplitude controlled fatigue properties and fracture behavior of an emerging titanium alloy (referred to by its designation as ATI 425TM by the manufacturer) is presented and discussed. The alloy was provided as rod stock in the fully annealed condition. Test specimens of the as-received alloy were cyclically deformed under total stress amplitude control at two different stress ratios (R = 0.1 and R = 0.3) with the purpose of establishing the conjoint and mutually interactive influences of magnitude of cyclic stress, load ratio and intrinsic microstructural effects on cyclic fatigue life, final fracture behavior and viable mechanisms governing failure at the microscopic level. The high cycle fatigue resistance of this titanium alloy is described in terms of maximum stress, load ratio, and maximum elastic strain. The final fracture behavior of the alloy under cyclic loading conditions is discussed in light of the mutually interactive influences of intrinsic microstructural features, magnitude of cyclic stress, load ratio and resultant fatigue life.

Abstract: This paper deals with the fatigue behaviour of NiTiCu shape memory alloy (SMA) wire actuators on thermo-mechanical cycling (TMC). Cyclic loading in SMA actuators is invariably associated with both functional and structural fatigue. The characteristic of the actuators such as austenite (hot shape) remnant deformation and recovery strain undergo changes upon TMC. These in turn result in continuous change in strain response (functional fatigue) during application. It has been shown that the functional fatigue can be minimized by adopting TMC at higher stress than that of the working stress prior to the application. On the other hand, failure of the actuators by fracture (structural fatigue) due to cyclic stress/strain is inevitable. Study shows that the fatigue life of the actuators is strongly dependent on the type of loading and the temperature range of operation. This has been explained in terms of damage accumulation, crack initiation and fracture behaviour.

Abstract: In the present study, the fatigue crack propagation tests of Zr-based metallic glass were conducted in laboratory air, and the fracture surface was observed to clarify the effects of loading frequency and the stress ratio. In spite of being brittle material, the metallic glass showed stable fatigue crack propagation behaviour, and the relationship between the crack propagation rate, da/dN, and the stress intensity range,
K, can be divided into three regions as well as conventional crystalline metals. The crack propagation rate can be expressed as a function of the stress intensity range by Paris law in the middle region. The power in Paris law was 1.4, and it is considerably smaller than the value for conventional crystalline metals. The threshold stress intensity range,
Kth, was 1.8 MPam1/2. The effects of the stress ratio and the loading frequency were not observed on the relationships, da/dN-
K and da/dN-
Keff. Then, the fatigue crack propagation of the metallic glass is cycle dependent in laboratory air.