Basic alloying features

8.4.1 Basic alloying features

These alloys have been developed for high-temperature service and include iron-, cobalt- and nickel- based materials, although nowadays they are principally nickel based. The production of these alloys over several decades (see Figure 8.9) illustrates the transition in the development of engineering materials from basic alloy composition achievements to a more process-dominated control.

In these alloys γ ′ (Ni 3 Al) and γ ∗ (Ni 3 Nb) are the principal strengtheners by chemical and coherency strain hardening. The ordered γ ′ -Ni 3 Al phase is an equilibrium second phase in both the binary Ni– Al and Ni–Cr–Al systems and a metastable phase in the Ni–Ti and Ni–Cr–Ti systems, with close matching of the γ ′ and the fcc matrix. The two phases have very similar lattice parameters ( $ < 0.25%,

depending on composition) and the coherency (interfacial energy γ I ≈ 10–20 mJ m 2 ) confers a very low coarsening rate on the precipitate so that the alloy overages extremely slowly even at 0.7T m . In alloys containing Nb, a metastable Ni 3 Nb phase occurs but, although ordered and coherent, it is less stable than γ ′ at high temperatures. Another source of strengthening is due to solid-solution hardening; Cr is a major element, Co may

be added up to 20% and Mo, W and Ta up to a total of 15%. These elements also dissolve in γ ′ so

Advanced alloys 459

20mm (a)

(d) Figure 8.8 Microstructure and fracture mode of silicon spheroidal graphite (SG) iron: (a, b)

as-cast and (c, d) austempered at 350 ◦

C for 1 h (L. Sidjanin and R. E. Smallman, 1992; courtesy of Institute of Materials, Minerals and Mining).

that the hardening effect may be twofold. Additions of Cr, like Co, also increase the γ ′ solvus and lower the stacking-fault energy.

In high-temperature service, the properties of the grain boundaries are as important as the strength- ening by γ ′ within the grains. Grain boundary strengthening is produced mainly by precipitation of chromium and refractory metal carbides; small additions of Zr and B improve the morphology and stability of these carbides. Optimum properties are developed by multistage heat treatment; the inter- mediate stages produce the desired grain boundary microstructure of carbide particles enveloped in a film of γ ′ and the other stages produce two size ranges of γ ′ for the best combination of strength at both intermediate and high temperatures. Table 8.3 indicates the effect of the different alloying elements.

Some of the nickel-based alloys have a tendency to form an embrittling σ-phase (based on the composition FeCr) after long-term in-service applications, when composition changes occur removing σ -resisting elements such as Ni and enhancing σ-promoting elements such as Cr, Mo or W. This tendency is predicted in alloy design by a technique known as Phacomp (phase computation), based on Pauling’s model of hybridization of 3d electrons in transition metals. While a fraction of the 3d orbitals hybridize with p and s orbitals to create the metallic bond, the remainder forms non-bonding

460 Physical Metallurgy and Advanced Materials

DS MM002

SC alloys

MM002 IN100

DS cast alloys

cast alloys

Temperature ( 850

N105 Wrought alloys

800 Nimonic 80A

Approximate year available

Figure 8.9 Increases in temperature capability for turbine blade alloys, based on creep rupture in 1000 h at 150 MN m −2 ( from Driver, 1985, by permission of Institute of Materials, Minerals and Mining).

Table 8.3 Influence of various alloying additions in superalloys. Influence

B Zr C Nb Hf Ta Matrix strengthening

Cr Al Ti Co Mo W

γ ′ formers

√ √ Carbide formers

√ √ √ Grain boundary strengthening

√ √ √ √ Oxide scale formers

orbitals which partly fill the electron holes in the d-shell, increasing through the transition series to give electron hole numbers N v for Cr (4.66), Mn (3.66), Fe (3.66), Co (1.71) and Ni (0.66). Computation shows that the γ/σ phase relation depends on the average hole number N v, given by

m i (N v ) i ,

i =1

where m i is the atomic fraction of the ith element of electron hole number N v and n is the number of elements in the alloy. The limit of γ-phase stability is reached at N v ≈ 2.5.