Microstructure Refinement Strategies in Carburized Steel

Microstructure refinement strategies in carburized microstructures were evaluated because of their potential for improving the fatigue performance of case carburized components. Commercial 52100 steel was used to simulate the high carbon content in the case. Specimens were subjected to various thermal treatments in a quenching dilatometer. Reheating cycles to austenitizing temperatures were evaluated with respect to both prior austenite grain size (PAGS) and associated martensite and retained austenite refinement. Quantitative stereological measurements were performed to evaluate the micro-geometry of plate martensite and the size distribution of retained-austenite regions. Decreasing the reheating temperature resulted in finer PAGS and multiple reheating cycles resulted in a narrow PAGS distribution. Refinement in PAGS led to a reduction in martensite plate size and finer distribution of RA.

In Situ Observation on the Effects of Prior Martensite Formation on Nanostructured Low–Temperature Bainite Transformation

Nanobainite transformation behavior was comparably studied using in situ observations for two heat treatments: With and without partial quenching before isothermal holding at 300 °C. It was found that the prior martensite formation significantly accelerated the rate of the subsequent nanobainite transformation. Bainitic laths formed adjacent to a prior martensite plate and grew up to austenites. Bainite phase also formed both at the grain boundaries of the parent phase and inside the grains. Regarding the growth mode, bainite grows along the longitudinal direction and hardly grows along the lateral direction.

Non-Modulated Martensite Microstructure With Internal Nanotwins In Ni-Mn-Ga Alloys

The self-accommodated non-modulated martensite of Ni-Mn-Ga single crystal was studied by transmission and scanning electron microscopy in the latter case using the electron backscatter diffraction technique. Three kinds of interfaces existing at different length scales were reported. The first, is the wavy and incoherent interface separating martensite variants observed on the micro-level with no-common crystallographic plane between them. The second is within a single martensite plate where the lattice rotates around one of the {110} pole to accommodate the interfacial curvature between martensite plates. Finally, at the nanoscale the third interface exists, a twin boundary separating internal nanotwins with the {112} type habit plane.

Nano- and Microcrystal Investigations of Precipitates, Interfaces and Strain Fields in Ni-Ti-Nb by Various TEM Techniques

In the present contribution several advanced electron microscopy techniques are employed in order to describe chemical and structural features of the nano- and microstructure of a Ni45.5Ti45.5Nb9 alloy. A line-up of Nb-rich nano-precipitates is found in the Ni-Ti-rich austenite of as-cast material. Concentration changes of the matrix after annealing are correlated with changes in the transformation temperatures. The formation of rows and plates of larger Nb-rich precipitates and particles is described. The interaction of a twinned martensite plate with a Nb-rich nano-precipitate is discussed and the substitution of Nb atoms on the Ti-sublattice in the matrix is confirmed.

Crystallographic Features of Martensite Transformed from Ultrafine Grained Austenite Fabricated by Severe Plastic Deformation

We studied the crystallographic features, especially the orientation relationship with respect to austenite, of martensite in a steel transformed from coarse-grained equiaxed austenite (35 μm), ultrafine-grained equiaxed (2.5 μm) or lamellar (300nm) austenite fabricated by sever plastic deformation. With decreasing the grain size of equiaxed austenite, the orientation relationship changed from Kurdjumov - Sachs relationship to Greninger - Troiano relationship. We inferred that this change of orientation relationship could be attributed to the small size of martensite plate transformed from the ultrafine-grained equiaxed austenite. The martensite transformed from the ultrafine-grained lamellar austenite did not have a definite orientation relationship with austenite. We considered that a high density of dislocations or a high density of low angle boundaries within the ultrafine lamellar austenite grain resulted in the large deviation of orientation relationship.

Transformation Behavior of Ni-Mn-Ga Ferromagnetic Shape Memory Alloy

Ni-Mn-Ga Ferromagnetic Shape Memory Alloy (FSMA) has been prepared by melt casting technique. The alloy was annealed at different temperatures. The samples are characterized in prepared condition and after three different annealing treatments. Microstructure of the alloy has been investigated using SEM at room temperature. Microstructure study reveals that the magnetic domains run diagonally across the surface. Differential Scanning Calorimetry (DSC) result shows the ferromagnetic transition temperature of the alloy is 105°C. In-situ study of structure during heating has revealed that the martensite to austenite transformation takes place in the temperature range of 28°C to 36°C. The present study focuses the effect of annealing on phase transformation and magnetic transformation temperature of Ni-Mn-Ga alloy. It has been observed that the thickness of the martensite plate increases as the alloy is annealed at 950°C for 30 hrs.

Microstructure Control of High Nitrogen Alpha + Beta Type Titanium Alloy

Solution nitriding and aging treatment were applied to Ti-4mass%Cr alloy in order to fabricate a ductile high-nitrogen titanium alloy with fine (α + β) structure. The solution-nitrided specimen withα’ martensitic structure was significantly hardened by solid solution strengthening by the absorbed nitrogen. During the aging treatment, fine β grains with a size of 1 microns in thickness precipitated along the martensite-plate boundaries. Although the specimen was softened to some extent after the aging treatment, the hardness is kept much higher than that of the aged Ti-4mass%Cr alloy without solution nitriding. This indicates that the nitrogen is still in solid solution of α phase even after the aging treatment, and contributes to strengthening of the fine-structured Ti-4mass%Cr-N alloy.

Internal Structure of B19 Martensite in AuTi Shape Memory Alloy

Internal twin of B19 martensite in equiatomic AuTi binary alloy was examined by conventional transmission electron microscopy observation and the phenomenological theory of martensite crystallography (PTMC). The crystal structure of martensite was B19 (orthorhombic) with the lattice parameters of 0.2944nm, 0.4900nm and 0.4633nm. Most of martensite plates were internally twinned by {111}typeI twin. <211>typeII twin was occasionally observed and {011}compound-twin relationship was observed at boundaries between adjacent martensite plates. However, no martensite plate entirely twinned by the <211>typeII twinning or the {011}compound twinning was observed. PTMC analysis showed that the invariant plane is formed only by the introduction of the internal twin of {111}typeI or <211>typeII twin in the present geometry of the transformation. Geometry of a typical martensite plate with internal twin of {111}typeI twin was in good agreement with that required for the formation of habit plane with the invariant plane character. The observed {111}typeI twin is, therefore, considered to be the lattice invariant shear to minimize the elastic strain energy due to the transformation.