New Creep Deformation Concept Based on Creep under Lower Stresses

Through the analysis of many creep rate-time or creep rate-strain curves of -single phase Ni-20mass% Cr alloy single crystals with various stress axes, it was clarified that the creep deformation manners at lower stresses are drastically different to those at higher stresses. These creep features at lower stresses are summarized into three ones as follows. (i) The fully extended transient stage occupies the considerable ratio of rupture life. (ii) The steady state stage disappears, because the transient stage directly connected with the accelerating stage. (iii) The origin of the onset of accelerating stage is regarded as the formation of the dynamic recrystallized grain. These difference in creep deformation manner were caused by the predominant operation of the primary slip system and then the homogeneous evolution of dislocation substructures.


Introduction
Creep deformation well understood. The creep deformation manner defined as dislocation creep was composed mainly by J.Weertman [1] and a few researchers [2,3,4], and the most of their works were based on the creep curves of pure aluminum as shown in Fig. 1 [4]. Using this Figure, it was indicated that the creep strain in transient stage  t , and the strain during steady state creep stage  s , are insensitive to temperature, but are slightly sensitive to applied stress. That is, the strains of  s become larger with increasing the stress. Here, it is notable that most of these creep tests had been carried out in a shorter time for less than 100h, in other words at higher stress. Under such conditions, the steady state stage has been regarded as the most important stage in creep, because it has been believed that the ratio of the transient stage to rupture life is quite smaller than the other two stages, and the accelerating stage is regarded as the dangerous stage.
In practical alloys, it has been also believed that the large part of the steady state stage turns to the accelerating stage [5], because of the coarsening of despersoids, as the result the ratio of accelerating stage to rupture life becomes larger. Based on this conception, the  projection methods [6] and the method [7] were proposed as the methods to predict the creep curve and to estimate the creep life. The creep curves estimated by these two methods were composed only by the accelerating stage and the transient stage was completely ignored.  Intent to conduct the creep of  single phase single crystal. So far the origin of onset of accelerating creep has been regarded as the formation of voids and cracks on grain boundary. Therefore, the creep curve in single crystal removing the grain boundary has been characterized by the curve having the extended steady state and accelerating stages as shown in Fig. 2 [10]. To confirm these features, the creep tests at lower stresses using single crystals are conducted as follows.

Experimental Procedure
The  single phase Ni-20Cr single crystal with 13mm in diameter and 280mm in length was prepared  But it was not well understood that the dynamic recrystalization leads onset of accelerating creep, because the creep rate was not depend on grain size in dislocation creep. To interpret the grain size dependence of creep rate in dislocation creep, the Core Mantle Model was proposed by Terada et al. [11]. According to this model, the portion along grain boundary where dislocation substructures is free increases with decreasing the grain size. And this portion along grain boundary was designated as Mantle. By using this model, the increase in creep rate during the accelerating stage can be interpreted.

Conclusions
By investigating the creep rate-time curves or creep rate-strain curves of  single phase Ni-20Cr single crystals conducted the creep tests at lower stress at 1173K, it was clarified that the creep deformation manners at lower stresses are drastically different to those at higher stresses.
These creep features at lower stresses are summarized into three ones as follows. These difference in creep deformation manner were caused by the predominant operation of the primary slip system and then the homogeneous evolution of dislocation substructures.