Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks. In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.

Influence of grain size on the mechanical behaviour of some high strength materials

The boron and carbon levels of a P/M nickel base superalloy were systematically varied in order to determine the mechanisms by which these elements strengthen the alloy, and their optimum concentration. Carbon levels were reduced to 20 ppm while the boron level was varied from 0.02 to 0.10 wt pct. Carbon levels of 0.002 and 0.05 wt pct were also studied, while maintaining a boron concentration of 0.02 wt pct. Zirconium levels were maintained at 0.06 wt pct. The resulting alloys were subjected to identical heat treatments and examinedvia SEM, TEM, and STEM microscopy. The alloys were also subjected to tensile, creep, stress-rupture, and fatigue crack growth tests. Results show that both carbon and boron have a strong influence on the formation of grain boundary precipitates, as expected. Carbon was present as the MC and M23C6 type carbides, while boron combined to form an intergranular M3B2 boride. Boron and zirconium were observed to be critical to the alloys' mechanical properties, although boron levels above the solubility limit resulted in no further improvement or debit in strength. Carbon additions resulted in no improvement in properties, indicating the feasibility of a carbon-free P/M superalloy. The role of the minor element additions is discussed in terms of both microstructural features and related strengthening mechanisms.

Effects of B, C, and Zr on the structure and properties of a P/M nickel base superalloy

The interrelationships between composition, γ′ morphology, hardness and γ–γ′ mismatch were identified for a set of advanced nickel-base polycrystalline superalloys. When cooling from a temperature above the γ′ solvus, i.e. a supersaturated γ matrix, it was found that the cooling rate determines the γ′ morphology and particle distribution that develops. This in turn dictates the hardness of the alloy system and the mismatch in lattice parameters between the γ and γ′ phases. This work shows that the development of these parameters is relatively insensitive to alloy composition. Moreover, the thermal history, especially cooling rate through the γ′ solvus temperature is found to be the critical factor in determining alloy response.

The influence of cooling rate from temperatures above the γ′ solvus on morphology, mismatch and hardness in advanced polycrystalline nickel-base superalloys

Microstructural stability and microstructure-property relationship during long-term thermal exposure in K452 alloy (a new Ni-base cast superalloy with similar to 21 pct Cr, 11 pct Co, 3.5 pct W, 2.5 pct Al, 3.5 pct Ti, and others) are investigated. It is found that exposure temperature and time have significant effects on the microstructure and properties of the alloy. During exposure, the microstructure is degraded by gamma' coarsening, MC carbide (M mainly represents Ti. W, and Nb) degeneration, precipitation and evolution of grain interior (GI) M23C6 carbide, evolution of grain boundary (GB) microstructure, and precipitation of eta phase. Among them, the gamma' coarsening is the leading reason for the decrease of strength of the alloy. The GI M23C6 and the eta phase have negligible influence on the properties due to their relatively small populations. Blocky, closely spaced GB M23C6 particles engulfed in gamma' increase the stress-rupture life, whereas the formation of a continuous GB M23C6 chain has an opposite effect. A life peak occurs when the M23C6/gamma' structure at the GBs is in an optimal form. The degenerated MC is the preferred initiation site of microcracks. Its presence at the GBs promotes the onset of intergranular fracture; and leads to the decrease in mechanical properties.

Effects of Long-Term Thermal Exposure on the Microstructure and Properties of a Cast Ni-Base Superalloy

The temperature and strain rate dependence of yield and work hardening behavior in elevated temperature stress-strain testing was investigated in the nickel-base superalloy UDIMET 115. This alloy was heat treated to produce a bimodal distribution of hyperfine and coarse γ1 precipitates. Yield behavior is shown to be controlled primarily by dislocation pair cutting of the hyperfine precipitates. Stage II work hardening appears to be governed mainly by the dislocation networks generated on the coarse precipitates as these are bypassed under the Orowan mechanism. Analysis of Stage III work hardening using ϕvs σ plots shows that a steady state exists for the stress-strain deformation of this system. Results are also reported on extremely strong serrated flow at moderately elevated temperatures and a high temperature ductility minimum.

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J.K. Tien

Papers

The effects of grain boundary carbides on the creep and back stress of a nickel-base superalloy