Atomic structure of interphase boundary enclosing bcc precipitate formed in fcc matrix in a Ni-Cr alloy

The atomic structure of the interphase boundaries enclosing body-centered cubic (bcc) lath-shape precipitates formed in the face-centered cubic (fcc) matrix of a Ni-45 mass pct Cr alloy was examined by means of conventional and high-resolution transmission electron microscopy (HRTEM). Growth ledges were observed on the broad faces of the laths. The growth ledge terrace (with the macroscopic habit plane ) contains a regular array of structural ledges whose terrace is formed by the (111)_fcc//(110)_bcc planes. A structural ledge has an effective Burgers vector corresponding to an transformation dislocation in the fcc → bcc transformation. The side facet (and presumably the growth ledge riser) of the bcc lath contains two distinct types of lattice dislocation accommodating transformation strains. One type is glissile dislocations, which exist on every six layers of parallel close-packed planes. These perfectly accommodate the shear strain caused by the stacking sequence change from fcc to bcc. The second set is sessile misfit dislocations (∼10 nm apart) whose Burgers vector isa/3[111]_fcc =a/2[110]_bcc. These perfectly accommodate the dilatational strain along the direction normal to the parallel close-packed planes. These results demonstrate that the interphase boundaries enclosing the laths are all semicoherent. Nucleation and migration of growth ledges, which are controlled by diffusion of substitutional solute atoms, result in the virtual displacement of transformation dislocations accompanying the climb of sessile misfit dislocations and the glide of glissile dislocations simultaneously. Such a growth mode assures the retention of atomic site correspondence across the growing interface. formerly Graduate Student, Kyoto University, Kyoto 606-01, Japan This article is based upon a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18–22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

铝元素抑制超高碳钢中网状碳化物析出机理

借助相图计算、能谱分析及组织观察研究了铝元素对超高碳钢中网状碳化物析出过程的影响,含碳1．5wt％的超高碳钢添加不同含量的铝后显著影响网状碳化物的析出：随铝含量增加,奥氏体晶粒尺寸愈小、晶界愈多,导致网状碳化物减薄;未溶碳化物愈多、奥氏体固溶碳含量愈少,导致网状碳化物析出量减少;晶界聚集铝量增加,聚集的铝抑制碳化物的形核及长大。当铝含量接近2．0wt％时,网状碳化物出现断续,甚至有的晶界没有碳化物析出,在球化过程中,该网状碳化物很容易球化。     

EBSD and DTA Characterization of A356 Alloy Deformed by ECAP During Reheating and Partial Re-melting

Recrystallization and partial re-melting processes have been developed for producing semi-solid feedstock in a solid state in which a globular microstructure is obtained by plastic deformation followed by reheating. In this research, to induce strain, a cast- and solution-treated Aluminum A356 (7 wt pct Si) alloy was subjected to a repetitive equal channel angular pressing process using a 90 deg die, up to a total accumulated strain of approximately 8 in route A (increasing strain through a sequence of passes with no rotation of the sample after each pass) at ambient temperature. The microstructural evolutions of deformed and reheated materials were studied by optical microscopy, scanning electron microscopy, and electron back-scattered diffraction analysis. In addition, the influences of pre-deformation on the recrystallization mechanism and liquid formation of A356 alloy were presented and discussed. The results are also supported by differential thermal analysis experiments. Evaluation of the observations indicated that the average cell boundary misorientation increased with increasing strain, so this increased misorientation accelerated the mobility of boundaries and recrystallization kinetics. Therefore, the recrystallization mechanism and kinetics affected by deformation, reheating condition, and intrinsic material properties determined the particle size in the semi-solid state.     

Role of severe plastic deformation on the formation of nanograins and nano-sized precipitates in Fe–Ni–Mn steel

In this research, the effect of severe plastic deformation (SPD) on the formation of nano-scaled grains and precipitation of nano-sized particles which consequently control mechanical properties of Fe–Ni–Mn alloy was investigated. Fe–Ni–Mn martensitic steels show excellent age hardenability but suffer from embrittlement after aging. Discontinuous coarsening of grain boundary precipitates, resulting in the formation of precipitate free zone (PFZ) along prior austenite grain boundaries, has been found as the main source of embrittlement in the previous studies. In this paper, severe plastic deformation has been carried out on the Fe–10Ni–7Mn steel to improve its mechanical properties. It is found that substantial improvement of tensile properties in cold-rolled steels occurs at thickness reductions larger than 60% where formation of ultrafine grains is realized. According to transmission electron microscopy (TEM) observations, formation of nano-scaled grains less than 100 nm along with the copious precipitation of nanometer-sized precipitates take place in the severely-deformed steels.     

Improvement of Strength–Ductility Balance by the Simultaneous Increase in Ferrite and Martensite Strength in Dual-Phase Steels

Metallurgical and Materials Transactions A - This work investigates the effect of increasing both martensite phase and ferrite phase strength on tensile properties and fracture behavior of...     

Role of defects on microstructure development of beta titanium alloys

Microstructures of α precipitation from a deformed β matrix by thermomechanical processing were studied in a representative β titanium alloy, i.e. Ti-15V-3Cr-3Sn-3Al alloy. In the aged specimens, after solution treatment, the grain boundary is the most preferential nucleation site in the a precipitation. Localized slip occurs in the β matrix by cold rolling after solution treatment whereas (332)<113> deformation twins are formed by subzero rolling at 77 K. After subsequent aging of the rolled specimens, the α phase preferentially forms on those defects in the β matrix with a low energy orientation relationship (Burgers relationship). There is strong variant restriction in heterogeneous nucleation on such defects. Controlled short time annealing above the β transus (β recovery treatment) after cold rolling produces the fine β subgrain. By subsequent aging, α precipitates form preferentially on β subgrain boundaries and also within β subgrains. A great number of α variants are observed locally in the recovered and aged specimens than those in the cold rolled and aged specimens. β recovery treatment prior to aging improves the strength-ductility balance mostly because of the increase of local elongation.     

Crystallography and interphase boundary of (MnS+VC) complex precipitate in austenite

Crystallography and interphase boundary of (MnS+VC) complex precipitates formed in austenite (γ) matrix are studied by transmission electron microscopy (TEM) in an austenitic Fe-36 mass pct Ni alloy containing small amounts of manganese, sulfur, vanadium, and carbon. When VC is formed directly within the γ matrix grain, it displays a cube-cube orientation relationship (OR) with respect to γ. When VC is formed on MnS, which precipitated in γ with a cube-on-edge OR, three distinctive VC/γ ORs are found: (1) (111)_γ‖(001)_VC, , (2) the cube-cube OR, and (3) the cube-on-edge OR. The MnS/γ OR becomes irrational after γ recrystallization. When VC forms on such incoherent MnS particles, which hold irrational ORs with respect to γ matrix, a wide variety of VC/γ ORs are observed. Geometrical analysis by near coincidence site lattice (NCS) model achieves reasonable success in explanation of the observed planar facets for VC precipitates having rational ORs with respect to γ. However, as for the VC formed on the incoherent MnS with irrational ORs, there is rather poor agreement between observation and prediction. T. KIMORI, formerly Graduate Student, Department of Materials Science and Engineering, Kyoto University This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committeee.