Precipitation-Strengthened Materials

Precipitation-Strengthened Materials 

metals or alloys that are strengthened by disperse particles of refractory compounds, mainly oxides, which are neither soluble nor capable of coagulation in the matrix (base) of the alloy at high operating temperatures. Precipitation-strengthened materials differ from the industrially widely used age-hardenable alloys by their structure, composition, and methods of preparation, as well as by a higher structural and thermal stability, which is manifested in the retention of long-term strength by precipitation-strengthened materials at high temperatures. The increased high-temperature stability of the widely used age-hardenable nickel alloys is due mainly to the presence of intermetallic hardening agents (Ni₃Al and Ni3Ti). However, at temperatures above 1000°-1100°C, the hardening agents dissolve and coagulate in the base alloy, which results in a loss of strength. The increased high-temperature stability of precipitation-strengthened materials based on nickel is achieved by the introduction of 2-5 percent refractory oxygen compounds into the nickel (ThO₂, HfO₂, and Y₂O₃). The optimum dislocation structure of the matrix is formed when the particle dispersity (100-600) angstroms) and the distance between the particles (0.5-0.8 microns) are strictly observed, as well as when thermomechanical treatment procedures—cold deformation and high-temperature annealing—are used. Graphs of the dependence of long-term strength on the time and the temperature for precipitation-strengthened and age-hardenable nickel alloys are given in Figure 1.

Articles made of precipitation-strengthened materials are usually manufactured in three stages: preparation of the initial powders, mainly by the simultaneous chemical precipitation of the base and the hardener from aqueous solutions of their salts; formation of the ingots; and their subsequent pressure treatment (extrusion, drawing, and rolling). The use of precipitation strengthening makes it possible to increase high-temperature stability and to broaden the temperature ranges of application for virtually all metals and alloys based on copper, nickel, cobalt, iron, zirconium, titanium, and molybdenum.

Figure 1. Dependence of long-term strength of precipitation-strengthened and age-hardenable nickel alloys on temperature (a) and time (b)

REFERENCES

Sovremennye kompozitsionnye materialy. Edited by L. Brautman and R. Croca. Moscow, 1970. (Translated from English.)
Portnoi, K. I., and A. T. Tumanov. “Kompozitsionnye i dispersnouprochnennye zharoprochnye nikelevye splavy.” In Sbornik nauchnykh dokladov na soveshchanii po probleme: “Struktura’ i svoistva zharoprochnykh metallicheskikh materialov.” Moscow, 1970.
Tumanov, A. T., and K. I. Portnoi. “Novye puti povysheniia zharoprochnosti nikelevykh splavov.”Dokl. AN SSSR, 1971, vol. 197, no. 1.

K. I. PORTNOI and A. T. TUMANOV