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The number density of Z phase measured at 550 o C was lower that that at 600o C, indicating that the preferential recovery of martensitic lath structure around prior austenite grain boundary is not remarkable at 550 o C in contrast with 600 o C.
The MX carbonitrides are located at PAGB, lath, block and packet boundaries and inside lath grains before creep [16].
Additionally, grain boundary diffusion may accelerate the Z phase formation.
Figure 7 demonstrates change of number density of Z phase particle during creep exposure.
The stress can influence grain boundary diffusion relates to the Z phase formation around PAGB.

In this study, HPT is adopted for grain refinement and subsequent aging is conducted at reduced temperatures.
Results and Discussion The Vickers microhardness increases with an increasing distance from the disc centre and with the increasing number of turns as shown in Fig. 1a.
At the last stage where the hardness is saturated, the grain has been further refined.
The TEM observation in Fig. 2 thus shows that grain refinement is developed with increasing amount of deformation.
TEM observation reveals that the grain size was refined to ~200 nm at the saturation. 4.

Plate-like Mg2Sn-particles were found coupled with MgZn2-needles inside the grain, as well as at sub-grain and grain boundaries.
Large precipitates are located at grain boundaries; near grain boundary (NGB) zones are depleted of precipitates.
parallel to the grain boundary.
The number of fully dissolved layers may serve as a dimensionless width of a precipitate depleted zone Discussion In agreement with previous reports [10,11] and according to the present work, during the aging of solution treated and quenched Mn-SnZn alloys, the hcp-MgZn2 precipitates nucleate homogeneously (possibly on vacancies or vacancy-Sn clusters) in the body of grains as well as heterogeneously on grain boundaries.
The dimensionless calculated width of PDZ's (the number of fully dissolved precipitate layers) and experimental values of the PDZ width after different aging periods correspond with the dependence (w/d)~1/3 that is characteristic of normal coarsening.

A number of issues, such as high strength, high stiffness and deformation mechanisms [1-3], have been intensively studied in the last twenty years.
On the other hand, due to the increasing volume percent of grain boundaries (GB) and triple junctions, the GB induced mechanisms, such as mass diffusion through grain boundary, grain boundary sliding, grain rotation, etc., would dominate the deformation of nano-grained metals.
Based on the Ashby-Verrall model [1] of grain insertion, Yang and Wang [3] proposed a 9 grain cluster model that also accommodate a grain rotation stage in the deformation scheme.
The second term on the left hand side represents grain boundary sliding, with Bη being the grain boundary viscosity, and slideD the amount of grain boundary sliding along the grain boundary.
These equations provide the formulation to solve for the grain center coordinates X , the grain rotation Θ , and the mass flow along the grain I.

The tensile strength of the alloys increased with increasing the number of ARB cycles.
In the Al-Si and Al-Ag alloys, the post-uniform elongation increased with increasing the number of the ARB cycles.
The mean grain size (thickness) was 750nm.
The main results can be summarized as follows: (1) The 4N-Al ARB processed by 7cycles showed an equiaxed grain structure having mean grain size of 750nm.
On the other hand, the strength of the solid solution alloys monotonously increased with increasing the number of the ARB cycles

Functional Steel Hardness and Wear Improving on a Basis of Phenomena of Grain Boundaries Phase Transition Y.A.MINAEV Leninsky prt.4, Moscow 119049, Russia e-mail: ymin36@mail.ru Keywords: Steel Alloys; Grain Boundaries Phase Transition; Nitrided Coatings.
Based on the phenomena the grain boundaries first-order phase transition in range 0.55 – 0.86 of metal melting point with formation of two-dimensional liquid was elaborated the technology of coatings by synthesis of nitrides using of gaseous nitrogen.
For a development of a technology we focus on a grain boundaries and description of its fundamental property i.e. first-order grain boundaries phase transition (GBPhT) with formation of two-dimensional liquid[1-3].
In referred works it has been shown that an interval of temperature near melting point TS0 the surface layers or grain boundaries of solid metal can be considered as quasi-liquid, i.e. as a separate phase.
The contents of small-sized nitrides (less 300 nm) makes till 65-75% from a total number.

After homogeneous grains developed, further grain growth became restrained. 1.
Introduction Though nanocrystalline materials possess a large number of grain boundaries that may act as driving force for the grain growth, experimental results indicate that most nanocrystalline materials exhibit a remarkable resistance to grain growth [1-3].
After homogeneous grains developed, further grain growth became very slow.
These grain boundaries motion resulted in selective grain growth.
After homogeneous grains developed, further grain growth became Fig.3.

It is concluded that the fine ferrite grains or austensite grains are stabilized by the grain boundary carbides, and grain-boundary sliding controlled by grain boundary diffusion is the principal superplastic deformation mechanism at temperatures in the range of 700-850 °C.
The average grain size is 4.5µm, which is approximately equivalent to the initial grain size; and the grains retain nearly equiaxed as the initial grains (Fig. 1(b)).
I.e. dynamic grain growth had not occurred during testing in this case, whereas grain growth occurs for the cases of very-fine-grained UHCS superplastic deformation in most other studies [4-5].
The grain-boundary carbides are coarsened (comparing Fig. 1(b) with Fig. 1(c)), with resulting average size of 1.5µm; the intragrain carbides partly dissolve, and the number of carbides decrease.
It can be concluded that the grains are stabilized by the largeish grain boundary carbides.

The number of measurements was at least 200 for each state.
Taking into account the size of the area analyzed by X-ray diffraction, the number of grains including is not less than 4×106.
This alteration in the distribution histogram likely results from the formation of new grains smaller than 0.1 μm during annealing alongside a modest increase in the size of the existing grains.
Additionally, the nucleation of new grains at grain boundaries may contribute to a reduction in the size of the larger grains appeared after compression.
Pshenichnyuk, Superplasticity and grain boundaries in ultrafine-grained materials, Woodhead Publishing Ltd., 2011

Grain Boundary Engineering of High-Nitrogen Austenitic Stainless Steel H.
During welding, an HNSS is ready to precipitate rapidly immense amounts of chromium nitride in the HAZ, as intergranular or cellular morphologies at or from grain boundaries into grain interiors [9].
Recent grain boundary studies of austenitic stainless steels have revealed that the sensitization depends strongly on grain boundary structure, and that low energy grain boundaries such as CSL boundaries have strong resistance to intergranular precipitation and corrosion [14,15].
In contrast, the GBEed HNSS having a very high CSL frequency (87 %) shows a small number of isolated random boundaries surrounded by many CSL boundaries as shown in Fig. 2(b).
During the thermomechanical-processing, a migrating grain boundary inevitably interacts with lattice dislocations and other grain boundaries.