Favorable Slip System Research Articles

Instrumented micro-indentation of NiTi shape-memory alloys

We study the instrumented Vickers micro-indentation of single-crystal Ti–50.9 at% Ni shape-memory alloys with systematically varied surface normal orientations ([100], [210], [111] and [221]) and Ti 3Ni 4 precipitate sizes (0 nm, 10 nm, 50 nm, 100 nm, 300 nm and 500 nm). Based on transmission electron microscopy observations, indentation of solutionized NiTi induces inelastic deformation via dislocation activity and a stress-induced martensitic transformation. The room-temperature hardness, Hv, and recoverable energy, E r, of NiTi are shown to be a maximum for very small precipitate sizes, decrease for intermediate precipitate sizes, and increase for large precipitate sizes. The maximization of Hv and E r at small precipitate sizes (10 nm) is attributed to the relatively high resistance to both dislocation motion and a recoverable stress-induced martensitic transformation. The decreases in Hv and E r at intermediate precipitate sizes (50–300 nm) are attributed to a decrease in the resistance to dislocation motion and a measured increase in the transformation temperatures with respect to the indentation temperature. The increases in Hv and E r at large precipitate sizes (500 nm) are attributed solely to measured decreases in the transformation temperatures with respect to the indentation temperature, since the resistance to dislocation motion remains constant as the precipitates grow from 300 nm to 500 nm. For nearly all heat treatments, the [100] and [221] surfaces demonstrate the highest and lowest values of Hv and E r, respectively, an effect attributed primarily to orientation of favorable slip systems.

Slip systems and dislocation emission from crack tips in single crystal TiC at low temperatures

Low temperature (20–900°C) plastic deformation in TiC 0.91 single crystals has been studied using microindentations on (111), (001) and (110) surfaces, the dislocation structures around microindents being characterized by transmission electron microscopy (TEM). Deformation occurs primarily by {111}〈11̄0〉 and {110}〈11̄0〉 slip, with the favored slip system determined by crystal orientation. Indentations below 300°C produced distinct dislocation half loops-hexagonal loops arising from {111} slip and elongated loops from {110} slip. At 500°C, much more extensive plastic deformation occurred, accomplished mainly by the motion of edge dislocations from these same systems. The dislocation configurations suggest a relatively high mobility of edge segments and a large Peierls stress for screw dislocations. Thermal activation apparently increases the mobility of screw segments, and results in dislocation structures containing mixed dislocations with no preferred orientation; this signals the onset of the brittle-ductile transition between 700 and 900°C. Cleavage cracks around indents on {111} and {110} surfaces introduced below 500°C, but not those on {001}, arrested with dislocation emission at the crack tips. The emitted dislocations were coplanar dislocation half loops, arising from {001}〈110〉 slip, and resulted from mode II or mode III loading of the cleavage crack. The local mode II and mode III stress intensity factors must have been sufficiently high to activate {001} slip, even through this slip system has not, to date, been reported in macroscopic tests.

Fatigue Crack Initiation Under Cyclic Torsion

Lin’s ratchet mechanism developed by Lin and his associates for fatigue crack initiation under tension and compression loading is here applied to fatigue under torsional loadings. Under the former loading, the most favorable slip system has a slip plane and a slip direction, both making 45 deg with the free surface; while under the latter the most favorable slip system has a slip direction parallel to the free surface. High cycle fatigue loading is considered. A slip direction parallel to the free surface does not produce extrusion or intrusion. A surface crystal with a slip plane normal to the shaft axis and with a slip direction making an angle β with the free surface was considered. Displacement component normal to the free surface is considered to be a measure of the amount of intrusion, which, in turn, is taken as a measure of crack initiation. It was found that the angle β, giving a maximum displacement component normal to the free surface, varies with the torsional stress. Under the same maximum range of shear stress (zero mean stress), the growth of the displacement component normal to the free surface is much larger in the tension and compression specimen than in torsional ones.

Textural Studies of Thermomechanically Treated Eutectoid Steel

The important class of metallurgical treatments known as thermomechanical treatment (TMT) is especially appropriate for textural studies because the crystallographic disposition of the matrix largely determines the final properties of the material. The current work utilized regular and inverse pole figure measurements of the ferrite matrix in TMT eutectoid steel to monitor the effects of various processing steps such as degree of cold rolling and the temperature and time of rapid annealing. Textural results provided the key to understanding fatigue life differences between longitudinally and transversely oriented sheet samples. The most favored ferrite slip system was not well oriented to act in the transverse samples. This in turn retarded the slip which nucleates fatigue cracking and hence extended the total fatigue life of transverse samples.