Compressive Strength and Interface Microstructure of PCBN Grains Brazed with High-Frequency Induction Heating Method

In order to prepare monolayer brazed superabrasive wheels, the polycrystalline cubic boron nitride（PCBN）grains were brazed to AISI 1045 steel matrix with Ag–Cu–Ti filler alloy using the high-frequency induction heating technique. The compressive strengths of brazed grains were measured. Morphology, chemical composition and phase component of the brazing resultant around PCBN grain were also characterized. The results show that the maximum compressive strength of brazed grains is obtained in the case of brazing temperature of 965 °C, which does not decrease the original grain strength. Strong joining between Ag–Cu–Ti alloy and PCBN grains is dependent on the brazing resultants,such as TiB_2, TiN and AlTi_3, the formation mechanism of which is also discussed. Under the given experimental conditions, the optimum heating parameters were determined to be current magnitude of 24 A and scanning speed of0.5 mm/s. Finally, the brazing-induced residual tensile stress, which has a great influence on the grain fracture behavior in grinding, was determined through finite element analysis.

Effects of grain size on phase transition behavior of nanocrystalline shape memory alloys

We report recent advances in the experimental and theoretical study of grain size(GS)effects on the thermal and mechanical properties of nanostructured NiTi polycrystalline shape memory alloy(SMA).It is shown that when GS<60 nm,the superelastic stress-strain hysteresis loop area(H)of the polycrystal decreases rapidly with GS and tends to vanish as GS approaches 10 nanometers.At the same time,the temperature dependence of the transition stress also decreases with GS and eventually approaches zero,leading to a wide superelastic temperature window and breakdown of the Clausius-Claperyon relationship.Rate dependence of the stress-strain responses is significantly reduced and the cyclic stability of the material is improved by the nanocrystallization.It is proposed that the emergence of such significant changes in the behavior of the material with GS reduction originate from the large increase in the area-to-volume ratios of the nanometer-thick interfaces(grain boundary and Austenite-Martensite(A-M)interface)in the polycrystal.In particular,with GS reduction,interfacial energy terms will gradually become dominant over the bulk energy of the crystallite,eventually bring fundamental changes in the phase transition responses of the material.Modelling strategy leading to the establishment of quantitative relationships among GS,grain boundary,A-M interfaces and the macroscopic responses of the material are outlined.

Unique Duplex Microstructure and Porosity Effect on Mechanical Properties of AlCoCrFeNi_(2.1) Eutectic High-Entropy Alloys Processed by Selective Laser Melting

Selective laser melting(SLM)was performed on AlCoCrFeNi_(2.1) eutectic high-entropy alloys(EHEAs),and the unique duplex microstructure and porosity were investigated as well as the negative effect on mechanical properties.SLM printed AlCoCrFeNi_(2.1) EHEAs is composed of face-centered cubic and BCC/B2 phases,and the surface texture of samples perpendicular to and parallel to the building direction is different.From the bottom of the molten pool to the inside,the structure changes from dendrites to columnar crystals and then to cellular crystals,the distribution of elements in and around the molten pool is uniform,and there is no element segregation.The surface hardness of as-printed AlCoCrFeNi_(2.1) EHEAs is higher than that of as-cast ones due to the grain refinement strengthening,and there is anisotropy in different surface hardness,the highest hardness is 580 HV(X–Y plane)and 560 HV(X–Z plane).The reduction of bearing area and tip stress concentration caused by pores have adverse effects on the tensile strength and elongation of the alloy,the ultimate tensile strength increased from 1060 to 1289 MPa.