Relationship between Colored Microstructure under Polarized Light and Shape Recovery Characteristics on the Thermomechanical Cycled CuAlNi Single Crystals

The paper studied the relationship between microstructure and shape recovery characteristics by using colored microstructure analysis under polarized light on the thermomechanical cycled CuAlNi single crystals. The two-way shape memory effect in quenched thin bar resulted from the preferential formation/extinction of martensite variant due to the internal quench stress, and the variant was formed at an angle of about 45 deg. with the tension direction ([001] of the βphase). Initial thermomechanical cycling under relatively low stress single variant stress-induced martensite was formed at an angle of 45 deg. with the tension and its morphology was a lath of parallel twins. More than one group of variants were formed after several training cycles and such variants also caused tilting of some thermally formed accommodated martensite. By overheating the trained sample containing stabilized multi-variants of stress-induced martensite, very coarse martensite structure with a strong asymmetry was produced, which caused the reverse two-way shape memory effect.

Hybrid post-processing effects of magnetic abrasive finishing and heat treatment on surface integrity and mechanical properties of additively manufactured Inconel 718 superalloys

Desired microstructure and surface integrity are critical to achieving the high performance of additively manufactured components.In the present work,the hybrid post-processes of magnetic abrasive finishing(MAF)and post-heat treatment(HT)were applied to the additively manufactured Inconel718 superalloys.Their hybrid effects and influencing mechanism on the surface quality and mechanical properties of the additively manufactured samples have been studied comparatively.The results show that the MAF process effectively reduces the surface roughness by more than an order of magnitude due to the flexibility and geometric consistency of the magnetic particles and abrasives with the finished surfaces.The proper sequence of MAF and HT obtains enhanced mechanical properties for the homogenized-MAF-aged sample with the yield strength of 1147 MPa,the ultimate tensile strength of 1334 MPa,and the elongation of 22.9%,which exceeds the standard wrought material.The surface integrity,compressive residual stress field,and grain refinement induced by the MAF and subsequent aging heat treatment increase the cracking resistance and delay the fracture failure,which significantly benefits the mechanical properties.The MAF process combined with proper post-heat treatment provides an effective pathway to improve the mechanical properties of additively manufactured materials.

Effect of mechanical alloying on sinterability and phase evolution in pressure-less sintered TiB_(2)-TiC ceramics

Phase relation and microstructure evolution in the pressure-less sintered TiB_(2)‒TiC ceramics preceded with mechanical alloying were systematically studied by a combination of SEM analysis.WC debris from milling balls promotes sintering by dissolving into the TiC phase to achieve dense microstructures at 1600℃.Variation of W solution in TiC grains exposes two types of core-rim structures,with no or more W in dark and white cores respectively but with common medium W in both rims.Diminishing whitecores reveal an exchange reaction between WC and TiC via mechanical alloying to form the Ti_(1-z)W_(z)C phase prior to sintering.The dark-cores inherit from the as-milled TiC power to further enable the reprecipitation of rims from a mixed liquid-phase,which facilitated also the anisotropic growth of TiB_(2) grains.The dark-cores grow persistently in the second-step at 2000℃ enabled by this liquid-phase,which coarsens the TiB2 grains too.With more alloyed phase,sintering was insufficient at 1500℃ with only the surface fluidity from the primary powders,and the second-step sintering increased the fluidity in the liquid-phase to fully densify the binary microstructure.Re-distribution of the alloyed W by two-step sintering rationalizes the evolution process of the binary microstructures and leads to better understanding of the mechanical behaviors.

Vertical sections of Tb_(0.15)Ho_(0.85)Fe_y-Tb_(0.3)Dy_(0.7)Fe_y(y=1.85, 1.9, 2.0) in Tb-Dy-Ho-Fe system

The x（Tb0.15Ho0.85Fey）＋（1–x）（Tb0.3Dy0.7Fey）（x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9; y=1.85, 1.9, 2.0） samples were prepared by a vacuum arc furnace, and annealed at 1000 oC for 1 d and at 950 oC for a week. Three vertical sections of Tb0.15Ho0.85Fey-Tb0.3Dy0.7Fey（y=1.85, 1.9, 2.0） in the Tb-Dy-Ho-Fe system were determined using optical microscopy, scanning electron microscopy, energy dispersion X-ray spectroscopy, X-ray diffraction, and differential thermal analysis. These vertical sections consisted of two single-phase regions： L and（Tb,Dy,Ho）Fe2; four two-phase regions： L＋（Tb,Dy,Ho）Fe3, L＋（Tb,Dy,Ho）Fe2,（Tb,Dy,Ho）Fe2＋（Tb,Dy,Ho）Fe3, and（Tb,Dy,Ho）Fe2＋（Tb,Dy,Ho）. The high Ho content of（Tb,Dy,Ho）Fey alloys led to the elevation of the peritectic temperature of L＋（Tb,Dy,Ho）Fe3→（Tb,Dy,Ho）Fe2. The region of（Tb,Dy,Ho）Fe2 phase was shifted towards the side of rich-Ho with the Fe content increasing. It meant that the substitution of Ho for Dy or Tb had a marked effect on the solidification process of（Tb,Dy,Ho）Fe2 compounds.