Grain Boundary Precipitation Research Articles

Grain-boundary precipitation in Allvac 718Plus

Allvac 718Plus is a newly developed nickel-based superalloy designed to replace Inconel 718 in static and rotating applications in gas turbine engines. Fine γ′ precipitates act as its principal strengthening phase and a second plate-like phase is generally present at grain boundaries. This has been reported to be the δ phase (Ni3Nb, D0a, orthorhombic), and is used to pin grain boundaries and improve resistance to intergranular fracture. In Allvac 718Plus non-uniform precipitation of a δ-like phase was observed along the grain boundaries; however, no relation was found between grain-boundary misorientation and the occurrence of precipitation. The crystal structure and chemistry of this phase was found to be different from the orthorhombic Ni3Nb δ phase reported previously in Inconel 718 and Allvac 718Plus. The true structure was found to be consistent with the hexagonal η-Ni3Ti D024 structure, but its chemistry was close to Ni6AlNb with partial ordering of Al and Nb over the prototype Ti sites. It was found that the serrated boundaries observed in the alloy were a result of discontinuous precipitation of η-Ni6AlNb, which was a predominant precipitation mechanism throughout the microstructure.

Structure and mechanical properties of corrosion-resistant high-nitrogen 04Kh22AG15N8M2F and 05Kh19AG10N7MFB steels after hot deformation

The structure and mechanical properties of corrosion-resistant high-nitrogen austenitic 04Kh22AG15N8M2F and 05Kh19AG10N7MFB steels are studied after hot rolling at 950 and 1100°C. The following specific features of the structure of hot-rolled 04Kh22AG15N8M2F steel are revealed: the presence of coarse grain-boundary precipitates of the molybdenum-rich σ phase and its nonuniform distribution over the volume of austenite grains. The 05Kh19AG10N7MFB steel hot rolled at 950°C contains ultrafine carbonitrides particles and has the best combination of a high strength and a sufficient elasticity and impact toughness. The structures of the hot-rolled steels have no ferrite, martensite, and traces of recrystallized austenite grains.

Surface and Interface Energies of Complex Crystal Structure Aluminum Magnesium Alloys

Alloying Al with Mg can improve its structural properties but also can lead to the formation of grain-boundary precipitates of β-Mg2Al3 that lead to failure by intergranular fracture and corrosion. Simulating the properties of the β phase is difficult because it has a complex structure with more than 1000 atoms per unit cell. We approximate the experimental β structure by the β′ structure, which has about 300 atoms per unit cell, and we compute the fracture behavior of the material from density functional theory calculations of relevant surface and interface energies. We report also on experimental measurements of the orientation and fracture properties of the α-Al(Mg)–β-Mg2Al3 interface and compare them with the atomistic simulations. We have computed the surface energy of face-centered cubic α-Al with up to 10 at. pct Mg, as well as the decohesion energy of β′-Mg2Al3 and the interfacial decohesion energy between β′-Mg2Al3 and pure α-Al with geometry similar to that observed experimentally. We find that the β′-Mg2Al3 decohesion energy is nearly isotropic and is lower than the pure Al surface energy and the α-Al–β′-Mg2Al3 interface decohesion energy. This result is consistent with the experimental observations of fracture within the β phase rather than at the α-Al(Mg)–β-Mg2Al3 interface or within the α-Al(Mg) phase.

Complex reaction of the DIGM and discontinuous precipitation and dissolution of phases

The complex reaction of diffusion-induced grain-boundary migration (DIGM), discontinuous precipitation, and dissolution of phases in a precipitation-hardened lead-antimony alloy has been studied. The concentration profile of Sb in front of the migrating boundary has been measured. At the reaction front, there is observed an intense bulk diffusion, which leads to the formation of precipitate-free zones in front of the migrating boundary. Driving forces of the boundary migration have been analyzed. The activation energy of the reaction has been determined. A comparison of the theoretical and experimental values of the rate of the complex reaction of the discontinuous precipitation of phases and DIGM has been carried out.

The effect of pre-ageing temperature and retrogression heating rate on the strength and corrosion behaviour of AA7150

The effects of processing condition variations in the retrogression and re-ageing (RRA) treatment of AA7150 have been investigated. The results reveal that the corrosion resistance and strength of RRA-treated samples are sensitive to the pre-ageing temperature and retrogression heating rate. With an intermediate pre-ageing temperature of 60 °C and a slow retrogression heating rate of 5 °C/min, RRA-treated samples can be tailored towards a good combination of both corrosion resistance and strength, owing to a smaller potential difference between grain-boundary precipitates and the matrix, and a larger volume fraction of strengthening precipitates in the matrix.

Effect of rotational speeds on microstructure and mechanical properties of friction stir-welded 7075-T6 aluminium alloy

In this article, effects of rotational speed on the microstructure and mechanical properties of friction stir-welded 7075-T6 Al alloys were investigated. Good correlation existed between the rotational speeds and weld properties. It was found that with increase in the rotational speed, higher pick temperatures and wider thickness of the heat-affected zone were obtained, and then grain coarsening, dissolution, and accumulation of hardening precipitates in grain boundaries. The average grain size of nugget increased from 6.8 to 8.9 µm, with increasing rotary speed from 600 to 1550 r/min. In addition, the best mechanical properties were obtained at moderate rotational speed of 825 r/min while ultimate tensile strength and elongation were 405 MPa and 6.1 per cent, respectively. Also, it was observed that at low welding speed of 60 mm/min, the weld properties are found to be dominated by the thermal cycle rather than the plastic deformation induced by tool, so as to make a reduction in hardness and strength of the weld when using higher rotational speeds.

STEM-HAADF tomography investigation of grain boundary precipitates in Al–Cu–Mg alloy

The high angle annular dark field scanning transmission electron microscopy (STEM-HAADF) tomography technique was applied to understand the three dimensional (3D) morphology and distribution characteristic of grain boundary precipitate (GBP) in peak-aged Al–Cu–Mg alloy. The results indicate that GBPs show both spherical and lenticular shapes and triangularly distribute along grain boundary (GB). Based on 3D observations from various directions, the values of GBP relevant parameters such as GBP size, center to center distance, number of GBPs per unit GB area and area fraction of GB covered by GBPs are further determined. The 3D method for GBP relevant parameter determination seems an effective way to avoid misunderstandings in the conventional two dimensional (2D) methods induced by GBP overlapping and projection effect as well as curved GB surface.

On the Crack Defect of CrZrCu Alloy DC Casting Ingot

The CrZrCu alloy cylindrical ingots have been tried to produce by the direct chill (DC) casting process. However crack defect is contained in the central area of the ingots. Metallographic analysis proving a kind of hot crack is cause by the precipitate in grain boundaries. A 3D coupled thermal-stress mathematical model has been formulated to describe thermal and stress–strain fields during the direct chill (DC) casting process for CrZrCu alloy cylindrical ingots. The model has been applied to analyzing the crack defect elimination, so that the need for further refinement of the DC casting process parameters has been satisfied.

Influence of repetitious-RRA treatment on the strength and SCC resistance of Al–Zn–Mg–Cu alloy

Influence of repetitious-retrogression and reaging (repetitious-RRA) treatment on the strength and SCC resistance of Al–Zn–Mg–Cu alloy was investigated by the slow strain rate technique (SSRT) and electrical conductivity test, using TEM, EDS and SEM. Additionally, the strength of alloy was characterized by Vickers hardness. The results show that within 3 times RRA treatments, SCC resistance increases with the increase of time, while maintaining strength as compared to RRA treatment, TEM observation along with EDS analysis indicates that within 3 times, matrix precipitates (MPs) in repetitious-RRA treatment are similar to those by RRA treatment and grain boundary precipitates (GBPs) become more discrete and coarser with the time. However, RRA treatment reaches 4 times, the SCC resistance gets worse because MPs are also coarse. Besides, the Cu content of GBPs increases with the time, inversely for Zn content, which is another reason for improving the SCC resistance.

Precipitation in solution-treated aluminium–4 wt% copper under cyclic strain

Solution-treated Al–4 wt% Cu was strain-cycled at ambient temperature and above, and the precipitation and deformation behaviours investigated by TEM. Anomalously rapid growth of precipitates appears to have been facilitated by a vacancy super-saturation generated by cyclic strain and the presence of continually refreshed dislocation density to provide heterogeneous nucleation sites. Crystallographic texture appears to be responsible for latent hardening in specimens tested at room temperature. Increasing temperatures lead to a gradual hardening throughout life due to precipitation. Specimens machined at 45° from the rolling direction, which exhibit rapid precipitation hardening, show greater texture hardening due to increased axial stress required to cut precipitates in specimens. In the temperature range 100–200°C, precipitation of Θ″ is suppressed by cyclic strain, and precipitation of Θ′ promoted. The rapid growth of precipitates generated by cyclic strain operates with diminishing effect at higher temperatures due to faster recovery of non-equilibrium vacancy concentrations. Θ′ precipitates generated under cyclic strain are smaller and more finely dispersed than those produced via quench-ageing due to heterogeneous nucleation on dislocations and possess a low aspect ratio and rounded edges of the broad faces caused by the introduction of ledges into the growing precipitates by dislocation cutting. Frequency effects indicate that dislocation action is responsible for the observed reduction in aspect ratio. Accelerated formation of grain-boundary precipitates appears partially responsible for rapid inter-granular fatigue failure at elevated temperatures, resulting in coexistent fatigue striations and ductile dimples on the fracture surface.

Correlations between intergranular stress corrosion cracking, grain-boundary microchemistry, and grain-boundary electrochemistry for Al–Zn–Mg–Cu alloys

There is little or no correlation between grain-boundary, or matrix, microstructures and stress corrosion cracking (SCC) susceptibility. Grain-boundary microchemistry, especially the copper content of grain-boundary precipitates (GBP) is the most important factor. Further evidence for this is a correlation between the initial open-circuit potential of ‘fresh’ brittle intergranular fracture surfaces, the copper content of GBP, and the plateau SCC velocities of overaged 7079 and 7075 alloys. Preliminary comparisons of overaged (T7) and peak-aged (T651) material have also been made, and support the view that beneficial effects of overaging on SCC resistance is associated with increasing copper content of GBPs.

Investigation of Bar-Like Carbides in HR3C Boiler Steel

After the heat treatment at 1200 oC 30 minutes followed by water cooling and 700 oC 4 hours for HR3C (S31042) boiler steel, the bar-like carbides precipitate in grain boundaries and within grains. It was found that the bar-like carbides nucleate in the grain boundaries and grew into grains in a parallel manner. With the help of TEM, the carbides were characterized to be M23C6, with a length of about 6μm and width of 0.6-0.8μm. Through observing crept specimens after testing at 700 oC, it was found that fine and dispersed MX (about 20nm) as well as dislocation walls exist inside grains, except bar-like and granular M23C6 carbide.

Grain Boundaries and Mechanical Properties of Ultrafine-Grained Metals

The concept of grain boundary (GB) engineering of ultrafine-grained (UFG) metals and alloys is developed for enhancement of their properties by tailoring different GBs (low-angle and high-angle ones, special and random, or equilibrium and nonequilibrium) and formation of GB segregations and precipitations by severe plastic deformation (SPD) processing. In this article, using this approach and varying regimes and routes of SPD processing, we show for several light alloys (Al and Ti) the ability to produce UFG materials with different GBs, and this can have a dramatic effect on the mechanical behavior of the processed materials. This article demonstrates also several new examples of attaining superior strength and ductility as well as enhanced superplasticity at low temperatures and high strain rates in various UFG metals and alloys.

Effect of the microstructure of Al 7050-T7451 on anodic oxide formation in sulfuric acid

The effect of the microstructure of an Al 7050-T7451 substrate on the anodic oxide formation in sulfuric acid was studied in this article. The microstructure of the substrate was assessed by optical microscope (OM) and transmission electron microscope (TEM). The surface and cross-section morphologies of the oxide films were examined by scanning electron microscope (SEM). The chemical composition of intermetallic particles in the alloys and films was investigated using energy dispersive spectroscope (EDS). The roles of intermetallic phases and grain or subgrain boundaries on the oxide film formation were researched using the potentio-dynamic and potentiostatic polarization technique in sulfuric acid solution. The results show that the transition of coarse intermetallic particles or grain (subgrain) boundaries at the surface of Al alloys can be characterized by potentiodynamic polarization curves. The surface and cross-section micrographs of the anodic layer seem to preserve the microstructure of the substrate. Large cavities in the anodic films are caused by the preferential dissolution of coarse Al 2CuMg particles and the entrance of Cu-rich remnants into the electrolyte during anodizing. The Al 7Cu 2Fe particles tend to be occluded in the oxide layer or lose from the oxide surface because of peripheral trenching. Small pores in the films are induced by the dissolution of precipitates in grain or subgrain boundaries. The film surface of recrystallized grain bodies is smooth and homogeneous.

Influence of γ′ and grain boundary carbide on tensile fracture behaviors of Nimonic 263

Tensile fracture behaviors of Nimonic 263 after three different heat treatments have been investigated. Standard heat treatment produces 10% γ′ coupled with precipitate free zones (PFZs), and M 23C 6 carbide densely distributed at grain boundaries (GBs). Alternative heat treatment causes dense distribution of GB carbides and precipitation of 1–3% γ′ with the absence of PFZs. Solution-treated sample has neither γ′ nor M 23C 6 carbide. Room temperature tensile tests indicate both standard and alternative heat-treated samples show intergranular fracture, but the elongation of the latter is two times larger than that of the former. The combined effect of GB carbides and intensive slip bands accounts for the intergranular fracture. The γ′ volume fraction affects elongation. Solution-treated sample displays transgranular fracture mode. Lack of restriction to dislocation mobility in grains interior and the weak interaction between GBs and dislocations are responsible for the transgranular fracture. The effect of PFZs on fracture mode may be negligible.

Nb-Base FS-85 Alloy as a Candidate Structural Material for Space Reactor Applications: Effects of Thermal Aging

The proposed uses of fission reactors for manned or deep space missions have typically relied on the potential use of refractory metal alloys as structural materials. Throughout the history of these programs, a leading candidate has been Nb-1Zr, due to its good fabrication and welding characteristics. However, the less-than-optimal creep resistance of this alloy has encouraged interest in the more complex FS-85 (Nb-28Ta-10W-1Zr) alloy. Despite this interest, only a relatively small database exists for the properties of FS-85. Database gaps include the potential microstructural instabilities that can lead to mechanical property degradation. In this work, changes in the microstructure and mechanical properties of FS-85 were investigated following 1100 hours of thermal aging at 1098, 1248, and 1398 K. The changes in electrical resistivity, hardness, and tensile properties between the as-annealed and aged materials are compared. Evaluation of the microstructural changes was performed through optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The development of intragranular and grain-boundary precipitation of Zr-rich compounds as a function of aging temperature was followed. Brittle tensile behavior was measured in the material aged at 1248 K, while ductile behavior occurred in samples aged above and below this temperature. The effect of temperature on the under- and overaging of the grain-boundary particles is believed to have contributed to the mechanical property behavior of the aged materials.

Characterization of Grain-boundary Precipitates after Hot-ductility Tests of Microalloyed Steels

The hot ductility of microalloyed steels was investigated by interrupted tensile tests at the temperatures of 850 and 950°C. Analyses of microstructural damage during plastic straining of the steels were performed using an experimental setup that allowed rapidly quenching the tensile specimens after straining to a predefined level. Microstructural investigations on the materials were carried out on longitudinally sectioned samples. Further analyses on crack surfaces were performed by fracturing the strained specimens in liquid nitrogen and by analyzing the surfaces formed by high-temperature decohesion through conventional and field emission SEM. It was demonstrated that AlN and Nb(C,N) precipitates, in isolated or combined form, affected the prior-austenite grain boundaries. Differences in hot cracking sensitivity among the steels was accounted for by modifications of the precipitate size and volume fraction.

Blade Failure in Second Stage Turbine

The present paper reports the investigation of a sudden blade fracture leading to a fatal engine failure. The blade sample was subjected to a series of examinations, including visual examination and photographic documentation, optical microscopy, scanning electron microscopy (SEM), using both energy dispersive spectroscopy (EDS) and X-ray mapping. Analysis of all elements show that loss of aluminum from coating due to oxidation and coating phases changing; decreasing of alloy ductility and toughness due to carbides precipitation in grain boundaries; degradation of the alloy gamma prime γ' phase (aging and coarsening) and micro cavities. These were found on fracture surfaces which served as an origin of creeping failure mechanism and are the most important factor for failure of this blade.

Microstructure of annealed Ti48.5Ni(51.5– x )Cu x (x = 6.2–33.5) thin films

(Ni, Cu)-rich Ti–Ni–Cu amorphous films with a Cu content of 6.2–33.5 at. % formed by sputtering were annealed at 773, 873 and 973 K for 1 h and their microstructures investigated. Two types of precipitate were observed in the annealed films: a Ti(NiCu)2 phase for the Ti48.5Ni40Cu11.5, Ti48.6Ni35.9Cu15.5, Ti48.3Ni28.4Cu23.3 and Ti48.3Ni23.9Cu27.8 films, plus a TiCu phase for the Ti48.5Ni18Cu33.5 films. These precipitates were found to have coherency with the B2 matrix in the films annealed at 773 K and were densely distributed within the grains. However, in the films annealed at 873 K, their size increased 10-fold and their density decreased. Annealing at 973 K promoted grain-boundary precipitation and, accordingly, the density of the precipitates in the grain interiors decreased. On the other hand, the annealed Ti48.9Ni44.9Cu6.2 films showed no precipitates in their grain interiors, but the number of grain-boundary precipitates increased with increasing annealing temperature. It was also found that grain size decreased with increasing Cu content and was significantly decreased for the Ti48.5Ni18Cu33.5 films.

Precipitation and its effect on the mechanical properties of a cast Mg–Gd–Nd–Zr alloy

Microstructural characterization of a Mg–11Gd–2Nd–0.5Zr (wt.%) alloy processed by isothermal aging at 250 °C has been performed using transmission electron microscopy, and the mechanical properties of the alloy after aging for different time have been evaluated using Vickers hardness, room- and high-temperature tensile tests. A four-stage precipitation sequence of the alloy can be described as follows: supersaturated α-Mg solid solution → β″(D0 19) → β′(bco) → β 1(fcc) → β(fcc), where the later precipitate can coexist with the former. The precipitation inside the grains is accompanied by the formation and growth of grain boundary precipitates (GBPs) and precipitation free zones (PFZs). Discussion on the structure–properties relationships reveals that microstructure with dense β′ precipitates in triangular arrangement is the optimum strengthening structure for the alloy. The existence of GBPs and PFZs in the alloy deteriorates the mechanical properties.