Alla Kipelova

The crystallography of M23C6carbides in a martensitic 9% Cr steel after tempering, aging and creep

The orientation relationships of M23C6 carbides in a martensitic creep resistant steel were studied. Almost all M23C6 carbides were located at (sub)grain boundaries after tempering and aging. The carbides were slightly elongated along the boundary planes and obeyed the Kurdjumov-Sachs, Nishiyama-Wassermann, and Pitsch orientation relationships as well as two new orientation relationships, that is and , with α-Fe matrix. On the other hand, the M23C6 particles in the neck portion of crept specimen lost their orientation relationships with α-Fe.

Development of Ultrafine Grained Austenitic Stainless Steels by Large Strain Deformation and Annealing

The development of ultrafine grained structures in 316L and 304-type austenitic stainless steels subjected to large strain cold working and subsequent annealing and their effect on mechanical properties were studied. The cold rolling was accompanied by a mechanical twinning and a partial martensitic transformation and resulted in the development of elongated austenite/ferrite grains with the transverse size of about 50 nm at a strain of 4. The grain refinement by large strain cold working resulted in an increase of tensile strength above 2000 MPa in the both steels. Annealing at temperatures above 500°C resulted in ferrite-austenite reversion. However, the transverse grain/subgrain size remained on the level of about 100-150 nm after annealing at temperatures up to 700°C.

Microstructure evolution in a 316L stainless steel subjected to multidirectional forging and unidirectional bar rolling

The formation of ultrafine-grained structures was studied in a 316L stainless steel during severe plastic deformation. The steel samples were processed up to a total amount of strain of 4 at ambient temperature using two different methods, i.e., multidirectional forging and unidirectional bar rolling. The large strain developed upon cold working resulted mechanical twinning and partial martensitic transformation. The latter was readily developed during multidirectional forging. After straining to the total amount of strain of 4, the austenite fractions comprised approximately- 0.45 as well as 0.15 in the rolled and forged samples, respectively. Both the multidirectional forging and bar rolling led to extensive grain refinement. The uniform microstructures consisting of austenite and ferrite crystallites with the transverse size of 60 nm and 30 nm were evolved at a total amount of strain of 4 in the rolled and forged samples, respectively. The grain refinement by severe plastic deformation was accompanied by an increase for the microhardness to 5380 and 4970 MPa for the forged and rolled samples, respectively.

Nanocrystalline structures and tensile properties of stainless steels processed by severe plastic deformation

The development of nanocrystalline structures in austenitic stainless steels during large strain cold rolling and their tensile behavior were studied. The cold rolling to total equivalent strains above 2 was accompanied by the evolution of nanocrystalline structures with the transverse grain size of about 100 nm. The development of deformation twinning and martensitic transformation during cold working promoted the fast kinetics of structural changes. The development of nanocrystalline structures resulted in significant strengthening. More than fourfold increase in the yield strength was achieved. The strengthening of nanocrystalline steels after severe plastic deformation was considered as a concurrent operation of two strengthening mechanisms, which were attributed to grain size and internal stress. The contribution of internal stresses to the yield strength is comparable with that from grain size strengthening.

Effect of Tempering on Microstructure and Creep Properties of P911 Steel

The microstructure and creep properties of a P911-type steel normalized at 1060°C and then subjected to one-step tempering at 760°C for 3 h or two-step tempering at 300°C for 3 h + 760°C for 3 h were examined. The transmission electron microscope (TEM) observations showed that the tempered martensite lath structure (TMLS) with a lath thickness of 340 nm evolved after both tempering regimes. High dislocation densities of 3×1014 or 5×1014 m-2 retained after one-and two-step tempering respectively. M23C6 carbides with a mean size of 120 nm and V-rich MX carbonitrides having a “wing” shape with an average length of about 40 nm precipitated on high-and low-angle boundaries and within ferritic matrix, respectively. A number of Nb-rich M(C,N) carbonitrides with a mean size of 20 nm precipitated on dislocations during low temperature tempering. The creep tests were carried out under constant load condition at 650°С at applied stresses of 100 and 118 MPa. Analysis of creep rate versus time curves showed that the use of two-step tempering decreases the minimum creep rate providing an increase in the creep strength in long-term conditions.

Microstructure Evolution in a 304-Type Austenitic Stainless Steel during Multidirectional Forging at Ambient Temperature

The formation of nanocrystalline structure in a 304-type austenitic stainless steel during multidirectional forging (MDF) at room temperature was investigated. Initial coarse austenite grains with an average size of 50 μm were refined to about 80 nm by martensitic transformation during MDF to a total true strain of 2 and remained unchanged upon further deformation up to a strain of 4. The volume fraction of martensite achieved ~0.9 after forging to a strain of 1.6. The MDF at room temperature was accompanied by a significant hardening of the 304-type steel. The microhardness and the flow stress increased during forging and approached their saturations on the levels of about 5 GPa and 1.7 GPa, respectively, after total true strain of 2. The structural mechanisms responsible for microstructure evolution during severe deformation are discussed.

Portevin-Le Chatelier effect in an E911 creep resistant steel with 3%Co additives

The effect of tempering temperature on mechanical properties of an E911+3%Co creep resistant steel was investigated. The mechanical tensile tests were carried out at temperatures from 298 to 1073 K and at strain rates varying from 2.1 ? 10?5 s?1 to 2.1 ? 10?1s?1. The Portevin-Le Chatelier (PLC) effect was found in the temperature range of 473 to 623 K. Various attributes of dynamic strain aging (DSA) like serrated flow with an acoustic emission were observed. With increasing temperature the ultimate tensile strength (UTS) and the yield strength (YS) increased while the ductility decreased. The dependences of the critical plastic strain on strain rate and temperature exhibited inverse behavior that was associated with concentrated solid solution in the DSA regime.

Formation of Ultrafine-Grained Structures in 304L and 316L Stainless Steels by Recrystallization and Reverse Phase Transformation

The microstructure evolution and mechanical properties of 316L and 304L austenitic stainless steels subjected to large strain cold bar rolling and subsequent annealing were studied. The cold working was accompanied by mechanical twinning and strain-induced martensitic transformation. The latter was readily developed in 304L stainless steel. The uniform microstructures consisting of elongated austenite and martensite nanocrystallites evolved at large total strains, resulting in tensile strength above 2000 MPa in the both steels. The subsequent annealing at temperatures above 700°C was accompanied by the martensite-austenite reversion followed by recrystallization, leading to ultrafine grained austenite.

Zener Pinning Pressure in Tempered Martensite Lath Structure

The Zener drag force exerted by M23C6 carbides, Fe2(W,Mo) Laves phase and M(C,N) particles for migration of different grain boundaries in P92-type and P911+3%Co heat-resistant steels was calculated. In particular, the prior austenite grain boundaries (PAGB), boundaries of packets and blocks, which are mainly high-angle boundaries (HAGB), were addressed. Zener pinning pressures were determined for each type of dispersoids separately taking into account that the M23C6 carbides, Fe2(W,Mo) Laves phase are inhomogeneously distributed such that they are mainly located at the boundaries, and the M(C,N) dispersoids are uniformly distributed throughout the metallic matrix. In the both steels, the pinning pressure from the second phase particles located at grain boundaries is about an order of magnitude higher than that caused by homogeneously distributed MX precipitates. In spite of numerous second phase particles precipitated during tempering, grain growth (although rather moderate) occurred during the creep tests of the studied materials. The driving pressure for grain boundary motion might be mostly associated with high dislocation density retained in the tempered martensite structure. The resulting pressure for grain growth in the P92-type steel under creep conditions at 600 and 650°C is somewhat higher than that for the P911 steel.

Mechanical Properties of an Al-5.4%Mg-0.5%Mn-0.1%Zr Alloy Subjected to ECAP and Rolling

Superplasticity and microstructural evolution of a commercial Al-5.4%Mg-0.5%Mn-0.1%Zr alloy subjected to severe plastic deformation through equal-channel angular pressing (ECAP) and subsequent rolling was studied in tension at strain rates ranging from 1.4×10-4 to 5.6×10-2 s-1 in the temperature interval 400-550°C. The alloy had an unrecrystallized microstructure with an average crystallite size less than 5 m. The alloy exhibited the yield strength of ~370 MPa, ultimate strength of ~450 MPa and elongation-to-failure of ~15% at ambient temperature. In spite of small crystallite size the alloy shows moderate superplastic properties. The highest elongation-to-failures of ~450% appeared at a temperature of ~500°C and an initial strain rate of ~1.4×10-3 s-1, where the strain rate sensitivity coefficient, m, is of about 0.57. The relationship between superplastic ductilities and microstructure is discussed.