Effects of temper level on the dependence of fatigue crack growth threshold and crack closure on the prior austenitic grain size

An attempt has been made to systematically investigate the effects of microstructural parameters, such as the prior austenite grain size (PAGS), in influencing the resistance to fatigue crack growth (FCG) in the near-threshold region under three different temper levels in a quenched and tempered high-strength steel. By austenitizing at various temperatures, the PAGS was varied from about 0.7 to 96 μm. The microstructures with these grain sizes were tempered at 200 °C, 400 °C, and 530 °C and tested for fatigue thresholds and crack closure. It has been found that, in general, three different trends in the dependence of both the total threshold stress intensity range, ΔK th , and the intrinsic threshold stress intensity range, ΔK eff, th , on the PAGS are observable. By considering in detail the factors such as cyclic stress-strain behavior, environmental effects on FCG, and embrittlement during tempering, the present observations could be rationalized. The strong dependence of ΔK th and ΔK eff, th on PAGS in microstructures tempered at 530 °C has been primarily attributed to cyclic softening and thereby the strong interaction of the crack tip deformation field with the grain boundary. On the other hand, a less strong dependence of ΔK th and ΔK eff, th on PAGS is suggested to be caused by the cyclic hardening behavior of lightly tempered microstructures occurring in 200 °C temper. In both microstructures, crack closure influenced near-threshold FCG (NTFCG) to a significant extent, and its magnitude was large at large grain sizes. Microstructures tempered at the intermediate temperatures failed to show a systematic variation of ΔKth and ΔKeff, th with PAGS. The mechanisms of intergranular fracture vary between grain sizes in this temper. A transition from “microstructure-sensitive” to “microstructure-insensitive” crack growth has been found to occur when the zone of cyclic deformation at the crack tip becomes more or less equal to PAGS. Detailed observations on fracture morphology and crack paths corroborate the grain size effects on fatigue thresholds and crack closure.

Effects of tempered structure on the near threshold fatigue crack growth behavior of a coarse grained high strength steel

Near threshold fatigue crack growth behavior of a high strength steel under different temper levels was investigated. It is found that the observed variations in ΔKth could predominantly be attributed to roughness induced crack closure. The closure-free component of the threshold stress intensity range, ΔKeff,th showed a systematic variation with monotonic yield strength.

Micromechanics Of Fatigue Crack-Growth At Low Stress Intensities In A High-Strength Steel

An attempt to systematically investigate the effects of microstructural parameters in influencing the resistance to fatigue crack growth (FCG) in the near-threshold region under three different temper levels has been made for a high strength low alloy steel to observe in general, widely different trends in the dependence of both the total threshold stress intensity range, DELTA-K(th) and the intrinsic or effective threshold stress intensity range, DELTA-K(eff-th) on the prior austenitic grain size (PAGS). While a low strain hardening microstructure obtained by tempering at high temperatures exhibited strong dependence of DELTA-K(th) on the PAGS by virtue of strong interactions of crack tip slip with the grain boundary, a high strength, high strain hardening microstructure as a result of tempering at low temperature exhibited a weak dependence. The lack of a systematic variation of the near-threshold parameters with respect to grain size in temper embrittled structures appears to be related to the wide variations in the amount of intergranular fracture near threshold. Crack closure, to some extent provides a basis on which the increases in DELTA-K(th) at larger grain sizes can be rationalised. This study, in addition, provides a wide perspective on the relative roles of slip behaviour embrittlement and environment that result in the different trends observed in the grain size dependence of near-threshold fatigue parameters, based on which the inconsistency in the results reported in the literature can be clearly understood. Assessment of fracture modes through extensive fractography revealed that prior austenitic grain boundaries are effective barriers to cyclic crack growth compared to martensitic packet boundaries, especially at low stress intensities. Fracture morphologies comprising of low energy flat transgranular fracture can occur close to threshold depending on the combinations of strain hardening behaviour, yield strength and embrittlement effects. A detailed consideration is given to the discussion of cyclic stress strain behaviour, embrittlement and environmental effects and the implications of these phenomena on the crack growth behaviour near threshold.

Thermodynamics and phase equilibria in the system Cu2O CuO ()Ga2O3

The standard Gibbs free energies of formation of CuAlO2 and CuAl2O4 were determined in the range 700° to 1100°C, using emf measurements on the galvanic cells (1) Pt,CuO +] Cu2O/CaO-ZrO2/O2,Pt; (2) Pt,Cu +] CuAlO2+] Al2O3/CaO-ZrO2/ Cu +] Cu2O,Pt; and (3) Pt,CuAl2O4+] CuAlO2+]Al2O3/CaO-ZrO2/O2,Pt. The results are compared with published information on the stability of these compounds. The entropy of transformation of CuO from tenorite to the rock-salt structure is evaluated from the present results and from earlier studies on the entropy of formation of spinels from oxides of the rock-salt and corundum structures. The temperatures corresponding to 3-phase equilibria in the system Cu2O-CuO-Al2O3 at specified O2 pressures calculated from the present results are discussed in reference to available phase diagrams.

Effects of different modes of hot cross-rolling in 7010 aluminum alloy: part II. mechanical properties anisotropy

The influence of microstructure and texture developed by different modes of hot cross-rolling on in-plane anisotropy (A (IP)) of yield strength, work hardening behavior, and anisotropy of Knoop hardness (KHN) yield locus has been investigated. The A (IP) and work hardening behavior are evaluated by tensile testing at 0 deg, 45 deg, and 90 deg to the rolling direction, while yield loci have been generated by directional KHN measurements. It has been observed that specimens especially in the peak-aged temper, in spite of having a strong, rotated Brass texture, show low A (IP). The results are discussed on the basis of Schmid factor analyses in conjunction with microstructural features, namely grain morphology and precipitation effects. For the specimen having a single-component texture, the yield strength variation as a function of orientation can be rationalized by the Schmid factor analysis of a perfectly textured material behaving as a quasi-single crystal. The work hardening behavior is significantly affected by the presence of solute in the matrix and the state of precipitation rather than texture, while yield loci derived from KHN measurements reiterate the low anisotropy of the materials. Theoretic yield loci calculated from the texture data using the visco-plastic self-consistent model and Hill's anisotropic equation are compared with that obtained experimentally.

Finite Element Analysis of Stress Evolution in Al-Si Alloy

A 2D multi-particle model is carried out to understand the effect of microstructural variations and loading conditions on the stress evolution in Al-Si alloy under compression. A total of six parameters are varied to create 26 idealized microstructures: particle size, shape, orientation, matrix temper, strain rate, and temperature. The effect of these parameters is investigated to understand the fracture of Si particles and the yielding of Al matrix. The Si particles are modeled as a linear elastic solid and the Al matrix is modeled as an elasto-plastic solid. The results of the study demonstrate that the increase in particle size decreases the yield strength of the alloy. The particles with high aspect ratio and oriented at 0A degrees and 90A degrees to the loading axis show higher stress values. This implies that the particle shape and orientation are dominant factors in controlling particle fracture. The heat treatment of the alloy is found to increase the stress levels of both particles and matrix. Stress calculations also show that higher particle fracture and matrix yielding is expected at higher strain rate deformation. Particle fracture decreases with increase in temperature and the Al matrix plays an important role in controlling the properties of the alloy at higher temperatures. Further, this strain rate and temperature dependence is more pronounced in the heat-treated microstructure. These predictions are consistent with the experimentally observed Si particle fracture in real microstructure.