fa Liu

Effects of precipitation phases on the hydrogen embrittlement sensitivity of Inconel 718

Abstract We investigated the effects of precipitation phases on the hydrogen embrittlement (HE) sensitivity of Inconel 718 by means of tensile tests. Hydrogen was charged into the test specimens via a cathodic charging process prior to the tensile tests. Various heat treatments were applied to conventionally aged specimens to fabricate specimens with different precipitation conditions for the γ″ phase and the δ phase. For each precipitation condition, we fabricated two specimens, one of which was charged with hydrogen before the tensile test. All specimens were tensioned under identical tensile conditions. The percent loss of the reduction of area (RA) caused by pre-charged hydrogen was used to assess HE sensitivity. Both the δ phase and the γ″ phase were found to play significant roles in altering HE sensitivity of Inconel 718. When these phases were totally dissolved, the HE sensitivity of the alloy was very low. The percent loss of RA decreased along with a decrease in the fractional volume of γ″. The δ-free aged alloy had greatly enhanced HE resistance, the same level as that of conventionally annealed alloy, and its strength was equal to that of the conventionally aged alloy. Fracture origins noted on the specimens were located on the surface layers and displayed brittle cleavage when pre-charged hydrogen was utilized. Local transgranular cleavages initiated from the δ/matrix were also observed in conventionally aged specimens, where there was a presence of pre-charged hydrogen. Therefore, the δ phase was considered to promote HE by initializing micro-cracks from δ/matrix interfaces. Since the d-free aged alloy has both good strength and good ductility, we propose that it is advantageous for fabricating some hydrogen-containing parts.

A numerical approach for predicting laser surface annealing process of Inconel 718

Abstract Laser surface annealing (LSA) was performed on a 3-mm-thick plate to reduce the hydrogen embrittlement (HE) sensitivity of aged Inconel 718, and the applicable parameter ranges were obtained. A finite element method (FEM) program was developed to simulate the LSA processes. The laser absorption coefficients of the sample surface under different LSA conditions were deduced using the trial and error method. All LSA processes were simulated with the deduced laser absorption coefficients. Thermal cycling experiments were then used to prove that the dissolution kinetics of the γ″ phase in Inconel 718 follows the Avrami equation, the coefficients for which were further deduced. The relationship between the volume fraction of the γ″ phase and the alloy hardness was established based on the coherency strengthening mechanism of the γ″ phase. The hardness distributions and depths of the laser-annealed surface layers were calculated by applying all of the above results. The good agreement between the calculated and measured results demonstrated the feasibility of applying this approach to predictions of LSA processes.

Investigation of the hot compression behavior of the Mg?9Al?1Zn alloy using EBSP analysis and a cellular automata simulation

In this paper, the flow behavior of the magnesium alloy AZ91 during hot compression with dynamic recrystallization (DRX) is studied by the electron back-scattered diffraction pattern (EBSP) method and a two-dimensional cellular automata (CA) simulation. The flow behavior characteristics of the alloy during compression are studied and analyzed in detail. The influence of second phase particles on the accumulation of dislocation and the migration of grain boundaries during the compression process are taken into account in the CA model. The current CA simulation results agree well with the experimental data in terms of both flow stress and microstructure evolution, suggesting that the proposed CA model is a reliable numerical approach for studying the DRX process of the AZ91 alloy. The simulated results show that the average size of the recrystallized grains increases to a peak value and then slowly reduces to a steady value due to simultaneous multi-cycle recrystallization during hot compression of the AZ91 alloy.

Experimental and numerical analyses of the mechanism for decreasing hydrogen-embrittlement sensitivity of aged Inconel 718 by laser surface annealing

Laser surface annealing (LSA) treatments were conducted to decrease the hydrogen embrittlement (HE) sensitivity of Inconel 718. Hydrogen was charged into the alloy by a cathodic-charging process. The LSA treatment significantly reduced the strength loss of the hydrogen-precharged notched specimen. Tensile tests and elasto–plastic simulations were performed to study how LSA affects the HE sensitivity of the as-aged Inconel 718. Various heat treatments were applied to the notched tensile specimens to obtain different precipitation states of γ″. All tensile tests were conducted in air at room temperature under identical conditions. One specimen of each kind was free of hydrogen; another was charged with hydrogen prior to the tensile test. A correlation between the HE sensitivity and the precipitation state of the alloy was confirmed by assessing the loss in the notch tensile strength due to hydrogen pre-charging. The γ″ particles, especially those in the surface layers, were proved to have deleterious effect...