Nickel-base oxide-dispersion-strengthened (ODS) alloys produced by the mechanical alloying process are being used for high temperature applications because of their excellent high temperature strength which can be useful up to relatively high fraction of their melting points. Mechanical alloying is a dry, high-energy ball milling operation that produces composite metal powders with controlled, extremely fine, micro-structures. Mechanically alloyed powders are consolidated by placing them in sealed cans for extrusion or hot pressing, followed by conventional hot and cold working processes. A final anneal at very high temperatures is required to develop the stable, coarse grain structure suitable for the most demanding stress-rupture applications. The high temperature strength of ODS alloys is attributed to the presence of inert, finely dispersed oxide particles which give dispersion strengthening effect by acting as barriers to dislocation motion. In order to develop high temperature strength, fine grains should be transformed into elongated coarse grains which contribute to the retardation of creep process and prolongation of rupture life. This is possible with a high temperature anneal during which secondary recrystallization occurs. INCONEL alloy MA 754 which has been used as a turbine vane material in advanced jet engines obtains a desired grain structure by isothermal annealing.
The general objectives of thermomechanical processing (TMP) of ODS alloys is to achieve coarse grain morphologies by means of secondary recrystallization, and to increase the availability of the necessary product forms. However, subsequent secondary recrystallization depends strongly on the temperature, strain, strain rate of the prior TMP operation and such compositional factors as yttria content and the amount of excess oxygen. The problem is more severe in complex shaped parts.
The purposes of this work are to investigate the effect of hot deformation conditions, such as hot rolling and hot compressing, on the microstructure and secondary recrystallization response of hot rolled INCONEL alloy MA 754 plate. Also, this work addressed the relationship between as-hot deformed microstructure and recrystallized grain structure after secondary recrystallization.
From this work, we obtained the results as follows. The effect of the finishing temperature of hot rolling on the microstructure and the secondary recrystallization response of as-primary rolled INCONEL alloy MA 754 plate was investigated. This material exhibits an elongated coarse grain (Grain Aspect Ratio (GAR): 12.1) after secondary recrystallization heat treatments when the finishing temperature of hot rolling was above about 983℃ with average grain size of about 0.71㎛ under this rolling condition. When the finishing temperature of hot rolling was below about 886℃, the GAR value after annealing of rolled material was below 5.9. The effects of reduction ratio of hot rolling were also investigated. The grain aspect ratio after annealing increased as the grain size before annealing increased in this work. The average grain size of as-hot rolled material depended inversely on reduction ratio of hot rolling. The grain structure of hot rolled material was also affected by the secondary recrystallization, which was related to the grain size of annealed material. The average grain size of as-hot rolled material linearly depended on the finishing temperature and reduction ratio of hot rolling. The critical secondary recrystallization temperature (T$_{sRx}$) of hot rolled materials also showed the dependence on the reduction ratio of rolling. $T_{sRx}$ of 34%-reduced sample was 1250℃, and was 1160℃ in the case of 72%-reduced sample. Creep and stress-rupture life were decreased with decreasing finishing temperature and reduction ratio of hot rolling related to grain aspect ratio. It was found that the stress dependence was large, with power law exponents which depends on the grain morphology. There was a transition in the fracture mode from the partly transgranular at the high GAR to intergranular at low GAR. The creep activation energy was larger than the activation energy for self diffusion. A texture study showed that all of the as-rolled samples had the as-rolled texture of {110}〈100〉 preferred orientation at any reduction ratio on this hot rolling condition and transformed into the secondary recrystallized texture of {047}〈113〉 preferred orientation after isothermal annealing. This means that the secondary recrystallization response does not depends on the as-deformed texture.
The effects of temperature and strain rate of hot compression on the microstructure and the secondary recrystallization response were also investigated. Under the given hot compressing conditions, the grain structure depended on the temperature and strain rate of hot compression. The specimens deformed below 1000℃ with high strain rate (10-1s-1, 1s-1) showed elongated coarse grain structures after annealing. The strain rate and the deformation temperature determine whether a specimen can be secondary recrystallized after hot-compressing. In order to explain the relationship between the average grain size before annealing and the hot compression condition, we used the diffusion compensated strain rate parameter for the assessment of the strain rate-temperature influence. The secondary recrystallization response and average grain diameter of as-compressed material depended on the diffusion compensated strain rate of hot compression. This material exhibited an elongated coarse grain after annealing when the average grain diameter at as-compressed state was below the critical grain diameter (about 0.9㎛). The increase of grain size during hot compression was shown to occur by normal grain growth. X-ray diffraction (XRD) patterns of specimens showed strong (111) peaks before and after compressing. The samples compressed below 1000℃ had strong (220) recrystallized peak after annealing. The other samples in which secondary recrystallization did not occur had no changes in XRD patterns after annealing. Texture analysis was done on specimens made from the as-compressed and annealed samples. Most as-compressed specimens exhibited random orientation before and after annealing. But the samples in which secondary recrystallization occurred showed increased values of orientation distribution function, f(g) compared to other samples.
