Processing developments

8.7.4 Processing developments

8.7.4.1 Superplastic aluminum alloys

Superplastic forming is a cost-effective manufacturing process for producing both simple and complex shapes from aluminum alloy sheet, because of its low-cost tooling and short lead times for production.

A range of alloys is available, including 2004 (Al–6Cu–0.4Zr) or Supral, 5083 SPF, 7475 SPF and Lital 8090 SPF (Al–Li–Mg–Cu). Supral and Lital dynamically recrystallize to a fine grain size during the early stages of deformation ( ∼500 ◦ C), which is stabilized with ZrAl 3 particles. The grain size in 7475 is stabilized by TMP and submicron Cr particles and, in 5083, by an Mn-dispersoid. Components are formed by clamping the alloy sheet in a pressure chamber and then applying gas or air pressure to force the sheet slowly into contact with a tool surface; both the tool and sheet are maintained at the forming temperature throughout the process. During normal superplastic forming, the alloys tend to develop voiding. This void formation is minimized by forming in hydrostatic conditions by introducing a gas pressure on both the front and back surfaces of the sheet being formed. The sheet is forced against the tool surface with a small pressure differential.

8.7.4.2 Rapid solidification processing of aluminum alloys

Rapid solidification processing (RSP) has been applied to aluminum alloys to produce a fine grain size and extend solid solubility, particularly for the transition metals iron, molybdenum, chromium and zirconium, which usually have low solid solubility and low diffusion rates in aluminum. Interest- ingly, RSP alloys containing Fe and Cr, on annealing, precipitate metastable spherical quasi-crystals of icosahedral phase with fivefold symmetry. These are extremely stable and hardly coarsen after extensive heat treatment, which indicates a potential for alloy development.

A series of commercially available high-temperature Al–Fe–V–Si alloys has been developed and consist of very fine, sphericalAl 13 (FeV) 3 Si silicides uniformly dispersed throughout the matrix, which display much slower coarsening rates than other dispersoids. A typical alloy with 27 vol.% silicides is 8009 (Al–8.5Fe–1.3V–1.7Si) and, without any needle or platelet precipitates in the microstructure, has a K lc ∼ 29 MN m −3/2 . The tensile properties as a function of temperature are shown in Figure 8.18 in comparison with a conventional 7075-T6 alloy. At all temperatures up to 480 ◦

C, the 8009 alloy has

a higher specific stiffness than a Ti–6A1–4V alloy. The fatigue and creep rupture properties are better than conventional aluminum alloys with excellent corrosion resistance. These RSP silicide alloys can

be readily fabricated into sheet, extruded or forged, and the combination of attractive properties makes them serious candidates for aerospace applications. Other alloys developed include Al–Cr–Zn–Mn, Al–8Fe–2Mo and Al–Li.

8.7.4.3 Mechanical alloys of aluminum

Mechanical alloys of aluminum contain dispersions of carbides or oxides, which not only produce dispersion strengthening but also stabilize a fine-grained structure. An advantage of these alloys arises because the strength is derived from the dispersoids and thus the composition of the alloy matrix can

be designed principally for corrosion resistance and toughness rather than strength. Thus, the alloying elements which are usually added to conventional aluminum alloys for precipitation strengthening and grain-size control may be unnecessary.

Mechanical alloying is carried out with elemental powders and an organic process control agent, such as stearic acid, to balance the cold-welding and powder-fracture processes. No dispersoid is added because the oxide on the surface of the powders and process control agent are consolidated

478 Physical Metallurgy and Advanced Materials

2219-T851 7075-T651

Tensile elongation (%)

0 100 200 300 400 Temperature (°C)

7075-T651 2219-T851

Figure 8.18 Tensile properties of RSP Al alloy 8009 as a function of temperature compared with conventional aluminum alloys ( from Gilman, 1990; courtesy of Institute of Materials, Minerals and Mining).

during mechanical alloying as hydrated oxides and carbonates. The process produces a fine disper- sion of ∼20 nm particles in a dynamically recrystallized structure with grains as fine as 0.05 µm.

Subsequent vacuum degassing at elevated temperature removes the H 2 and N 2 liberated, improves the homogenization of the matrix and reduces carbonates to Al 4 C 3 , which forms most of the dis- persoid. The final grain size is around 0.1 µm. The powder is then compacted by HIPing or vacuum hot-pressing and conventionally extruded to produce a material with a stable grain size of 0.3 µm, with grain boundaries pinned by the dispersoid.

Mechanical alloys have been developed corresponding to the 2000, 5000 and 7000 aluminum series alloys. IN 9021 is heat treatable by solution treatment and natural or elevated temperature ageing to give 500–560 MN m −2 proof stress, 570–600 MN m −2 TS, 12% elongation and 40 MN m −3/2 K 1c . IN 9052 is the equivalent of a 5000 series alloy, requiring no heat treatment and offering good strength in thick sections, 390 MN m −2 proof stress, 470 MN m −2 TS, 13% elongation and 46 MN m −3/2 K 1c . Mechanically alloyed Al–Mg–Li offers inherent high strength in thick section, 430 MN m −2 proof

stress, 500 MN m −2 TS, 10% elongation and 30 MN m −3/2 K 1c .

Problems

8.1 What properties are required of steels for cold-forming applications?

8.2 Use of dual-phase steels is now more widespread in automobile applications. Describe the heat- treatment used for this type of steel and the microstructure developed. How are the mechanical properties optimized? What are the advantages gained by their use? Write down a typical dual- phase steel composition.

8.3 The composition of Cr steel for aircraft landing gear components is: Fe − 0.40C − 0.70Mn − 0.8Cr − 1.8Ni − 0.25Mo − 1.6Si − 0.05V wt.%.

Advanced alloys 479 (a) What role does (i) vanadium, (ii) molybdenum and (iii) silicon play in the development of

the required mechanical properties? (b) Given that the yield stress is approximately 1650 MPa and the K 1c is approximately

60 Mpa m 1/2 , estimate the critical crack size for catastrophic failure. Comment on the practical significance of this value.

8.4 The surfaces of steel specimens can be hardened by enrichment in their nitrogen content. One route is to maintain a nitrogen-rich atmosphere around a heated steel specimen. If this atmosphere gives a constant N content of 1.53 wt.% at 1000 ◦

C and the minimum hardness requires a nitrogen content of 0.25 wt.%, calculate the time required to achieve a hardened depth of (i) 1 µm and (ii) 1.75 µm under these conditions.

Diffusion of N in γ-Fe: D o = 9.1 × 10 −5 m 2 s −1 ,Q = 170 kJ mol −1 , R = 8.314 J mol −1 K −1 . The error function is:

0.85 0.90 0.95 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Erf(z) 0.770 0.797 0.8209 0.8427 0.8802 0.910 0.934 0.9523 0.9661 0.9763 0.9838

8.5 Recently, steel producers have produced automobile steels which are designed to bake-harden during the paint-drying process, which involves time at about 200 ◦

C. What is the magnitude of the increase in strength and outline the strengthening mechanism involved.

8.6 Steels for structural applications rarely have yield strengths in excess of 500 MPa, whereas steels having yield strengths up to 1500 MPa are used in aeroplane landing gear. Explain the reasons for this difference.

8.7 Outline the factors in the development of gas-turbine blades over the last 50 years that have required changes in the methods of processing nickel-based superalloys, including forging, investment casting, directional solidification and single-crystal production.

8.8 What are the principal advantages of directionally solidified and single-crystal nickel-based superalloys over conventionally cast material in relation to the mechanical performance of gas- turbine blades?

8.9 The intermetallic compounds NiAl and Ni 3 Al are both fully ordered up to their melting point. NiAl deforms by the motion of unit dislocations, whereas Ni 3 Al deforms by the operation of superdislocations. (a) Describe the processes involved and explain the reasons for the different deformation behavior. (b) Sketch how the yield stress of these two compounds varies with increase in temperature and explain the characteristics.

8.10 Over the last few decades the composition of line-pipe steels has changed significantly. Outline the changes and give the reasons for them.

8.11 Cast iron contains approximately 2.5% Si. What is its role and what is the desired microstructure in austempered ductile iron (ADI)? What are the two important heat treatments to achieve this structure?

8.12 A commonly used versatile titanium alloy contains 6% Al and 4% V. What is the role of these additions and what is the microstructure produced?

Further reading

Baker, C. (ed.) (1986). Proc. 3rd International Aluminium–Lithium Conference. Institute of Metals, London.

480 Physical Metallurgy and Advanced Materials Bhadeshia, H. K. D. H. (1992). Bainite in Steels. Institute of Materials, London.

Honeycombe, R. W. K. (1981). Steels, Micro-structure and Properties. Edward Arnold, London. Janowak, J. R. et al. (1984). A review of austempered ductile iron metallurgy. First International Conference

on ADI. Materials in defence (1988). Metals and Materials, 4(7). Institute of Materials, London. Meetham, G. W. (1981). The Development of Gas Turbine Materials. Applied Science, London. Peters, H. (ed.) (1991). Proc. 6th International Aluminium–Lithium Conference. Deutsche Gesellschaft für

Material-kunde. Polmear, I. J. (1989). Light Alloys. Edward Arnold, London. Sims, C. T. and Hagel, W. C. (eds) (1972). Superalloys. John Wiley, Chichester. Sims, C. T., Hagel, W. C. and Stoloff, N. S. (1987). Superalloys II. John Wiley, Chichester. Stoloff, N. S. (ed.) (1984). Ordered alloys. International Metal Reviews, 29(3).

Stoloff, N. S. (1989). Physical and mechanical metallurgy of Ni 3 Al and its alloys. International Metal Reviews, 34(4). Yoo, M. H. et al. (1993). Deformation and fracture of intermetallics. Overview No. 15. Acta Metall. Mater., No. 4. Kim, Y-W. (1989). Physical metallurgy of titanium aluminides. TMS/ASM Symposium on High Temperature Aluminides and Intermetallics.

Chapter 9

Oxidation, corrosion and surface treatment