An efficient method to engage oxide ceramics in low-temperature interfacial reactions: Microstructure evolution and kinetics behaviors based on supercooling in a transient liquid phase bonding joint

Ductile transient liquid phase(TLP)bonding joints reinforced by multiple precipitates were produced using novel pre-sintered coatings and Au-Si fillers;therefore,the highest strength of NiTi/sapphire joints brazed at 460℃ for 30 min reached 72 MPa.The pre-sintering process improved the surface-active of sapphire by forming metastable Ti_(3)O and non-stoichiometric Al_(2)O_(3).The typical brazing seam consisted of O-rich compounds,TiSi_(2),and Ti-Ni-Si,wherein the O-rich phase featured different crystallinity depending on the oxygen content.The sapphire/seam interface was either a nanoscale diffusion region or a Si-rich amorphous layer.The breakdown of the Stokes-Einstein relation(SER)occurred,and the deviation from SER increased with a higher cooling rate.The influence of coating thickness was reflected in(i)the supercooling related to the viscosity and fractional exponent of liquids and(ii)the microstructural change of the joint related to the driving force for crystal growth.This work presented a new strategy for joining ceramics to metals at lower temperatures but using the joint at higher temperatures;furthermore,gave an insight into the microstructure evolution and kinetics behaviors based on supercooling in a transient liquid phase bonding joint.

Cu-Cu low-temperature diffusion bonding by spark plasma sintering:Void closure mechanism and mechanical properties

In this study,combining the single point diamond turning(SPDT)and spark plasma sintering(SPS),we achieved high-strength diffusion bonding of copper at an ultra-low temperature of 202℃(0.35 T_(m),T_(m):absolute temperature of the melting point).Additionally,the closure mechanism of interfacial micro-and nano-voids during the Cu-Cu SPS diffusion bonding is systematically revealed for the first time.For micro-voids,the pulsed current is found to induce additional diffusion flux and plastic deformation,thereby facilitating the void closure.Molecular dynamics(MD)simulation revealed that at the atomic scale,high-energy Cu atoms induced by the pulsed current can significantly promote the diffusion of low-energy atoms in their vicinity and accelerate the void closure.This study also proposes a novel“evaporation-deposition”nano-void closure mechanism for the previously unstudied nano-void closure process.The results show that the synergistic effect of the pulsed current and nanoscale surface rough-ness can significantly improve joint strength.At a low temperature of 405℃(0.5 T_(m)),on combining the computerized numerical control(CNC)turning and SPS diffusion bonding,the joint strength can reach 212 MPa,while that for the joint obtained by traditional hot pressing diffusion bonding at the same tem-perature is only 47 MPa.We obtained an ultra-high joint strength of 271 MPa using the combined process of SPDT and SPS diffusion bonding at an ultra-low temperature of 202℃(0.35 T_(m)),which is approximately 600℃ lower than the traditional diffusion bonding process temperature of 800℃(0.79 T_(m)).To sum up,this study provides a novel method and theoretical support for realizing low-temperature high-strength diffusion bonding.

Effect of Brazing Conditions on Microstructure and Mechanical Properties of Al_2O_3/Ti—6Al—4V Alloy Joints Reinforced by TiB Whiskers

Al2O3 and Ti-6Al-4V alloy were brazed with Ag-Cu-Ti ＋B fillers in different brazing conditions. Effects of brazing temperature, holding time and additive Ti content on joints microstructure and shear strength were investigated by scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, transmission electron microscopy and shear testing. Results indicate that TiCu and Ti（Cu,Al） decrease, but Ti2Cu and -Ti2（Cu,Al） increase in brazing seam with increasing brazing temperature, holding time and additive Ti content. Area consisting of Ti3（Cu,Al）30 and TiO near Al2O3 becomes gradually discontinuous from continuity when brazing temperature rises or holding time extends. As Ti additive content increases, TiO is absent near Al2O3, area consisting of only Ti3（Cu,Al）30 thickens. TiB whiskers are in situ synthesized by Ti and B atoms during brazing process. The brazing temperature, holding time and additive Ti content on joints microstructure influence the joints shear strength directly. The shear strength of joints, obtained at 850 ℃ holding for 10 min, reaches the maximum of 78 MPa. According to the experimental results, phase diagram and thermodynamics calculation, the interface evolution mechanism of the Al2O3/Ti-6Al-4V alloy joint was analyzed.