Powder Metallurgy Ni-base Research Articles

Investigation of room temperature strengthening mechanism on PM Ni-base superalloys with tantalum addition

In this paper, the mechanical properties of the high-performance powder metallurgy (PM) superalloys with Ta addition fabricated by the Central Iron and Steel Research Institute (CISRI) were evaluated at room temperature (RT). We focus on the reason why the solute element Ta affects RT properties of PM superalloys. To this end, the necessary advanced characterization methods were used to study the plastic deformation and fracture mechanisms. The results revealed that the leading cause for the enhancement in RT yield strength with Ta increasing is the antiphase boundary (APB) strengthening of the bimodal γ' precipitates, the extended stacking fault (ESF) shear and the interaction between ESF and dislocation. Importantly, the microtwin structure was found in the deformed substructure, which is an extraordinary phenomenon in RT plastic deformation for PM superalloys. • The microtwin configuration was found inL1 2 -γ' strengthening Ni-base PM superalloys after RT tensile defromation. • The microtwins are formed by the a/6 〈112〉Shockley partials shearing precipitates on adjacent {111} planes. • The deformation induced ESFs (or microtwins) is the major reason for the increase in yield strength.

Effect of tantalum on the microstructure stability of PM Ni-base superalloys

The effect of Ta on the microstructure stability of powder metallurgy (PM) Ni-base superalloy was examined at standard heat treatment and after long-term thermal exposure. The results reveal that the previous particle boundary structure is eliminated with the addition of Ta. As Ta content increases, the γ' morphology changes from spheres to cuboids and begins to split, which is caused by the increase in the γ/γ' mismatch. Reversal coarsening of γ' particles in Ta-containing alloys are observed during exposure at 750 °C. The main reasons are related to γ/γ' lattice mismatch, elements partitioning and soft impingement. Besides, the increase of Ta content promotes the precipitation of σ phase at grain boundaries and in grains. • The PM Ni-based superalloys were fabricated by plasma rotation electrode process and hot isostatic pressing. • Systematical study on microstructure evolution of the PM superalloy with increase of Ta content. • The addition of Ta effectively inhibits the previous particle boundary structure in PM superalloy. • The addition of Ta changes the partitioning of alloying elements in γ and γ'. • Reversal coarsening of γ' particles occurred in the Ta-containing alloys during the aging at 750 °C.

Morphology evolution of γ′ precipitates in a powder metallurgy Ni-base superalloy

Effect of the cooling rate on the morphological evolution of the γ′ precipitate in a powder metallurgy Ni-base superalloy FGH96 was studied. Results showed that as the cooling rate decreased from 600 °C/min to 2 °C/min, the precipitate morphology transformed from sphere to cuboid, octet and dendrite. As the cooling rate increased, the mean size of the γ′ precipitates decreased while the nucleation density increased. When the cooling rate was lower than 6 °C/min, the precipitate would evolve from a sphere to cuboids with concave faces, octets, dendrite to final split-off branches. The morphological transition was driven by energy balance between coherent strain induced by the γ′/γ lattice misfit and interface energy. The dendritic shape γ′ exhibited an eight-primary-branch pattern with preferred growth direction along 〈111〉, and secondary arms along 〈100〉.

Microstructure and mechanical properties of the PM precipitation-hardening alloy N07626: effect of HIP temperature and solution annealing

The microstructure and mechanical properties of the precipitation-hardening powder metallurgy Ni-base alloys UNS N07626 are investigated. The material was compacted by hot isostatic pressing and solution treated adopting distinct conditions to reveal the effect of processing temperature on material properties. The microstructure is analysed by scanning electron microscopy, optical microscopy and electron backscatter diffraction and energy-dispersive X-ray spectroscopy. The prior particle boundaries (PPBs) are related to the presence of thermally stable nanometric Nb-carbides and aluminium oxides. An increase of processing temperature conditions is associated to a slight decrease of mechanical strength and to an appreciable improvement of toughness, as suggested by Instrumented V-Notch Charpy experiments. This behaviour is attributed to the reduction of the density of PPBs precipitates acting as void-nucleating sites in a micro-ductility mechanism. The precipitates’ density is related to a limited grain growth and the resulting separation of PPBs from grain boundaries.

Effect of oxygen content and heat treatment on carbide precipitation behavior in PM Ni-base superalloys

The influence of oxygen content and heat treatment on the evolution of carbides in a powder metallurgy (PM) Ni-base superalloy was characterized. The results reveal that oxygen content has little influence on the precipitation of carbides inside the particles. However, under the consolidated state, stable Ti oxides on the particle surface act as nuclei for the precipitation of prior particle boundaries (PPB). Also, oxygen can diffuse internally along grain boundaries under compressive stress, which favors the precipitation of carbides inside the particles. Therefore, a higher amount of carbides will appear with more oxygen content in the case of consolidated alloys. It is also observed that PPB can be disrupted into discontinuous particles at 1200°C, but this carbide network is hard to be eliminated completely. The combined MC-M23C6 morphology approves the nucleation and growth mechanism of carbide evolution.

Effects of temperature and shot peening on S-N behavior of a PM Ni-base superalloy UDIMET 720

Fatigue S-N (stress vs life) curve behavior for a powder metallurgy (PM) Ni-base superalloy UDIMET 720 in both polished and shot-peened surface conditions was investigated at room temperature and 600 °C in air. Tests were carried out under four-point bending at a load ratio of 0.1 and a frequency of 10 Hz. At 600 °C, an offset S-N curve was found for both polished and shot-peened surface conditions. This offset S-N curve is deduced to be controlled by the location of crack nucleation sites. Observations and deductions of crack nucleation sites, crack nucleation life, crack closure, environmental attack, and cyclic softening and hardening have been used to explain the experimental results.

Fatigue failure probability in a powder metallurgy Ni-base superalloy

A probabilistic model of fatigue failure of PM superalloy is presented which takes into account the size, shape and location of the defects which initiate fatigue cracks. The fatigue failure probability is linked with the growth rate of cracks initiated from inclusions, which is described using fracture mechanics. The risk of fatigue failure from surface, sub-surface and internal particles is evaluated and discussed. The model predictions are found in fairly good agreement with experimental results.

Effect of Hot Working on Creep Rupture Properties of PM Ni-base Alloy Consolidated by Hot Extrusion.

Effects of hot working on elevated temperature tensile and creep rupture properties at 1000°C were investigated in powder metallurgy(PM) Ni-base alloy, Alloy X, with 50 ppm oxygen content consolidated by hot extrusion process in comparison with cast and wrought materials. It was found that the ductility of PM material at temperatures above 850°C was improved by the application of hot forging while the tensile strength was relatively unchanged. Furthermore, it was found that creep rupture properties of PM materials were not significantly affected by extrusion ratio ranging from 6 to 25, but improved by hot forging. Although aluminum oxides originating from powder surface oxidation during atomization aligned along grain boundaries in the extruded material, these oxides were redistributed within grains by nonuniform deformation of hot forging. It was considered that the improvement of creep rupture properties was realized by a decrease in cavity formation initiated at oxides at grain boundaries.