Recovery and Recrystallization during High Temperature Deformation

The production of dislocation substructures by cold working and recovery, fatigue, creep and hot working are reviewed. The relationships of subgrain size and dislocation density to the causal parameters of strain, strain rate, strain amplitude, temperature, stress and time (as applicable) are presented for each process. The importance of dislocation mechanisms such as climb, cross-glide, annihilation and subboundary formation are explained. The relative capabilities and limitations of each mode of creation with respect to both external processing and internal mechanisms are explored. The effects of the metal's stacking fault energy, of solid solution and of particle dispersion on structure and behavior are presented. The properties of the different kinds of substructures for room temperature and creep service are examined. The need for modification of the Petch relationship between yield strength and subgrain size is explored. The thermal stability is shown to be an important factor for creep service. It is concluded that the most suitable modes of substructure preparation are either cold working and recovery or hot working both from the view point of fitting into current industrial practice and from that of dependable, useful service properties.

The production and utility of recovered dislocation substructures

The fundamental mechanisms responsible for the low rate of strain hardening during high temperature deformation and for a steady state of flow at high strains have been confirmed to be:

I.

Dynamic recovery, which limits the accumulation of dislocations through annihilation and which operates at all strains in all metals; and




II.

Dynamic recrystallization, which eliminates dislocations through the migration of grain boundaries and which only operates beyond a critical strain when the dislocation density becomes high enough to give rise to the nucleation and growth of new grains.

These softening processes are retarded by the presence of solute atoms and second phase particles which reduce the mobility of both dislocations and high angle boundaries. These effects have some similarities to those observed under cold working and annealing but there is a strong dynamic element introduced by straining at the elevated temperature. As a result of the high strains imposed there is much more microstructural change than during creep loading.

Industrial hot forming processes generally involve several stages of deformation separated by intervals during which static recovery or recrystallization take place. The interaction between dynamic and static softening processes under industrial conditions will be the subject of a sequel paper. This work will also consider the trends in hot ductility; the latter depends on the retardation of grain boundary cracking by dynamic recovery and recrystallization. Finally, since the effects of alloying in hot working have been treated only in a general way, the behavior of specific materials and their thermomechanical processing will be reviewed in the third and fourth papers of this-series.

Recent advances in hot working: Fundamental dynamic softening mechanisms

This chapter focuses on two kinds of environmental fracture, stress corrosion cracking (SCC) and hydrogen embrittlement, and discusses the role in such failures of a number of metallurgical variables. These include chemical composition; microstructural components such as precipitate type and structure, and grain size and shape; crystallographic texture; heat treatment and its effect on the foregoing variables; and processing, particularly the thermomechanical treatments (TMT), which are attracting increased attention for property optimization. These variables are expected to be of great importance in the development of new engineering materials to meet demanding service conditions.

The Role of Metallurgical Variables in Hydrogen-Assisted Environmental Fracture

To repair or join sections of a metal article, matching metal powder is sintered in the solid state to form a porous structure in the joint. A layer of matching powder, modified to incorporate melting point depressants such as boron and silicon is subsequently added to the surface of the sintered region. The joint is processed at a temperature where the modified layer melts while the sintered layer and base metal remain solid. The modified material flows into the sintered layer by capillary action and promotes liquid phase sintering, resulting in a dense joint with properties approaching those of the base metal.

Powder metallurgy repair technique

Diffusion bonding is effected utilizing a thin alloy interlayer which melts at the desired diffusion bonding temperature forming a transient liquid phase and which subsequently resolidifies at temperature as a result of constituent interdiffusion, continued heat treatment providing a homogeneous solid-state diffusion bond.

Diffusion bonding utilizing transient liquid phase

1,263,228. Welding by pressure. UNITED AIRCRAFT CORP. 6 April, 1970 [24 April, 1969], No. 16117/70. Heading B3R. [Also in Division C7] A process for diffusion welding components of nickel-base superalloys of the γ-γ 1 type having a combined Al/Ti content in excess of 6% by weight comprises coating each of the surfaces to be joined with a nickel-base coating containing up to 50% by weight cobalt to a thickness of 0A63-3A8 A, and diffusion bonding the components under pressure and at a temperature exceeding the γ 1 solvus temperature. The cobalt content is preferably between 25 and 35% by weight and the interlayer may be electroplated or applied by vacuum deposition on the surfaces to be joined. Specific alloy compositions referred to include the following:-

Diffusion welding of the nickel-base superalloys

The advantages of using select thermomechanical processing for improving mechanical properties of γ′ strengthened nickel-base superalloys in the range room temperature to 1400°F are demonstrated in the present study. Based upon analysis of the deformation and precipitation behavior of this class of alloys, several processing conditions have been evaluated for the alloy Udimet 700. The working operation and resulting dislocation substructure are shown to be of critical importance. In the preferred treatment the alloy is warm worked in the two-phase γ-γ′ condition at a temperature below that of rapid recrystallization such that a polygonal substructure is formed on the scale of the γ′ interparticle spa cing. A thermomechanical treatment consisting of 1) solution heat treatment, 2) precipitation aging at 1950°F to form γ′, 3) warm working to 1.50∈ (92 pct RA) at 1950°F, plus 4) final aging at 1550° and 1400°F was found to be beneficial for Udimet 700. The resulting property improvements included large increases in ultimate and yield strength, creep resistance, and low and high cycle fatigue resistance. These were achieved with minimal loss in ductility. The mechanical properties characterized were intended to be broad in scope and no exhaustive attempt was made to optimize processing in order to achieve maximum improvement. However, the experimental results confirm the assessment that nickel-base superalloys can be effectively strengthened through controlled thermomechanical processing.

Thermomechanical strengthening of a γ′ precipitation-hardened nickel-base alloy

In filling voids in or between metallic articles, as in the repair of cracks or other surface defects in castings, the void is filled with a powder mix which upon heat treatment fills the void. The filler powder mix is a blend of at least two distinct powder components and in terms of overall composition preferably corresponds to that of article being repaired. The components comprising the mix are selected to provide a transient liquid phase and isothermal resolidification of a portion of the mix at the temperature below the article melting point. A subsequent heat treatment is preferably utilized for homogenization.

Metallic filler material

A class of age hardenable nickel base alloys for use as filler wires in fusion welding superalloys is described. The alloys contain manganese in levels of from about 0.5 to 3 percent to greatly reduce the incidence of heat-affected zone cracking in the metal being welded. The weld filler alloys also contain significant amounts of aluminum, titanium, tantalum, and columbium, and therefore the resultant welds can be age hardened to relatively high strength levels.

William Anthony Owczarski

Papers

Diffusion bonding utilizing transient liquid phase

Diffusion welding of the nickel-base superalloys

Thermomechanical strengthening of a γ′ precipitation-hardened nickel-base alloy

Metallic filler material

Age hardenable nickel superalloy welding wires containing manganese