Wenming Tang

Solid state interfacial reactions in electrodeposited Cu/Sn couples

Abstract Cu/Sn couples, prepared by sequentially electroplating Cu and Sn layers on metallized Si wafers, were employed to study the microstructures, phases and the growth kinetics of Cu-Sn intermediate phases, when electroplated Cu/Sn couples were aged at room temperature or annealed at temperatures from 373 K to 498 K for various time. Only Cu6Sn5 formed in aged couples or couples annealed at temperature below 398 K. The Cu6Sn5 layer was continuous, but not uniform, with protrusions extending into the Sn matrix. When Cu/Sn couples were annealed at temperatures from 423 K to 498 K, two continuous and uniform Cn6Sn5/Cu3Sn layers formed within the reaction region between Sn and Cu. There were many voids near the Cu3Sn/Cu interface and within the Cu3Sn layer. Cu6Sn5 and Cu3Sn formations both follow parabolic growth kinetics with activation energies of 41.4 kJ/mol for Cu6Sn5 and 90.4 kJ/mol for Cu3Sn, respectively.

Synthesis of TiB2 nanocrystalline powder by mechanical alloying

TiB2 nanocrystalline powder was synthesized by mechanical alloying of Ti-67B elemental powder. X-ray diffraction(XRD) and transmission electron microscopy(TEM) were used to study the structural evolution of the powder during ball milling. The effects of heat treatment on the structural evolution and thermal stability of the mechanically alloyed(MAed) Ti-67B powder were also discussed. During ball milling the Ti-67B powder, a solid solution of B in Ti, Ti(B) is firstly formed. When the powder is milled for 10 h, the amorphous transition of Ti(B) from the crystalline to the amorphous phase occurs. When the powder is milled for 20 h, nanocrystalline TiB2 is formed from the amorphous Ti(B). When the powder is milled for 60 h, only TiB2 is detected with grain size of 10 nm. The formation of TiB2 nanocrystalline is controlled by the gradual diffusion reaction mechanism. During heat-treatment of the MAed Ti-67B powder, the structural changes of TiB2, including grain growth and lattice ordering degree increasing may occur.

Synthesis and grain growth kinetics of in-situ FeAl matrix nanocomposites (II): Structural evolution and grain growth kinetics of mechanically alloyed Fe-Al-Ti-B composite powder during heat treatment

Abstract Morphological changes, structural evolutions and grain growth kinetics of mechanically alloyed(MAed) Fe 50 Al 50 , Fe 42.5 Al 42.5 Ti 5 B 10 and Fe 35 Al 35 Ti 10 B 20 (mole fraction, %) powders were investigated by XRD and SEM, when being isothermally annealed at 1 073–1 373 K. The effect of different Ti and B addition on the grain growth of FeAl phase was also discussed. The results show that the nanocrystalline FeAl and in-situ TiB 2 /FeAl nanocomposite powders can be synthesized by subsequent heat treatment. Besides the relaxation of crystal defects and lattice stress, the transformation from Fe-based solid solution into B2-FeAl and TiB 2 occurs upon heating of the MA-processed alloys. Although the grain growth takes place, the grain sizes of both FeAl and TiB 2 are still in nanometer scale. The activation energies for the nanocrystalline FeAl growth in the three alloys are calculated to be 534.9, 525.6 and 1 069.6 kJ/mol respectively, according to kinetics theory of nanocrystalline growth. Alloys with different TiB 2 contents exhibit unequal thermal stability. The presence of higher content TiB 2 plays significant role in the impediment of grain growth.

Effect of high temperature aging on microstructure and mechanical properties of HR3C heat resistant steel

Abstract Microstructure and mechanical properties of the HR3C austenite heat resistant steel were investigated after artificial aging at 650°C for time up to 3000 h. The results show that as the aging time increased, the room temperature tensile and impact fracture mechanisms of the HR3C steel change from trans- to intergranular fracture. M23C6 type carbides and MX type carbonitrides continuously precipitate during aging, leading to the change of the mechanical properties and fracture mode of the steel. Moreover, the dissolution of the coherent twins and the transformation from the incoherent twins to the thermodynamically stable austenite subgrains have great effects on the mechanical properties of the aged steel, too. When increasing the aging time to ≧2000 h, the microstructure and mechanical properties of the steel are nearly constant, indicating a good thermal stability of the HR3C steel at elevated temperature.

Preparation of Cu/Invar composites by powder metallurgy

Abstract An orthogonal experiment scheme was employed to study the influences of forming pressure, sintering temperature and holding time and Cu content on microstructure, hardness and electrical resistivity of the Cu/Invar composites prepared by the powder metallurgy (PM) technique. The interdiffusion of the Fe, Ni and Cu atoms of the composites during sintering was also investigated. The results show that the Invar alloy is distributed continuously in the composites, when the Cu content is 30 wt-% and below; when the Cu content is 40 wt-% and above, a continuous net structure of Cu forms. Properties, especially the electrical and thermal conductivities, depend on the relative density and atom interdiffusion of the Cu/Invar composites. Taking the electrical resistivity of the composites as index, the optimum processing parameters are: forming pressure of 600 MPa, sintering temperature of 1000°C, holding time of 60 min and Cu content of 50 wt-%.

Structural evolution of Si-50%C powder during mechanical alloying and heat treatment

Abstract The nanocrystalline β-SiC powder was successfully synthesized by ball milling the Si-50%C elemental powder. During ball milling, a solid solution of C in Si, Si(C), firstly forms, followed by SiC. The formation of SiC is controlled by the mixing mechanism of the gradual diffusion reaction(GDR) and the mechanically induced self-propagating reaction(MSR). The amount of β-SiC increases with milling time increasing. After 40 h milling, there exists only β-SiC in the milled powder. The grain size of β-SiC is about 6.4 nm after the powder is milled for 60 h. After the 60 h-milled Si-50%C elemental powder is heat treated at 1 100 °C for 1 h, the grain size of β-SiC does not change, but the lattice ordering degree of β-SiC increases.

Electroless plating and growth kinetics of Ni–P alloy film on SiCp/Al composite with high SiC volume fraction

After Sn/Pd activating, the SiCp/Al composite with 65% SiC (volume fraction) was coated by electroless Ni–P alloy plating. Surface morphology of the composite and its effect on the Ni–P alloy depositing process and bonding action of Ni and P atoms in the Ni–P alloy were studied. The results show that inhomogeneous distribution of the Sn/Pd activating points results in preferential deposition of the Ni–P alloy particles on the Al alloy and rough SiC particle surfaces and in the etched caves. The Ni–P alloy film has an amorphous structure where chemical bonding between Ni and P atoms exists. After a continuous Ni–P alloy film formed, electroless Ni–P alloy plating is not affected by surface morphology and characteristics of the SiCp/Al composite any longer, but by the electroless plating process itself. The Ni–P alloy film follows linear growth kinetics with an activation energy of 68.44 kJ/mol.

Effects of rolling and annealing on microstructures and properties of Cu/Invar electronic packaging composites prepared by powder metallurgy

The Cu/Invar composites of 40% Cu were prepared by powder metallurgy, and the composites were rolled with 70% reduction and subsequently annealed at 750 °C. Phases, microstructures and properties of the composites were then studied. After that, the amount of α-Fe(Ni,Co) in the composites is reduced, because α-Fe(Ni,Co) partly transfers into γ-Fe(Ni,Co) through the diffusion of the Ni atoms into α-Fe(Ni,Co) from Cu. When the rolling reduction is less than 40%, the deformation of Cu takes place, resulting in the movement of the Invar particles and the seaming of the pores. When the rolling reduction is in the range from 40% to 60%, the deformations of Invar and Cu occur simultaneously to form a streamline structure. After rolling till 70% and subsequent annealing, the Cu/Invar composites have fine comprehensive properties with a relative density of 98.6%, a tensile strength of 360 MPa, an elongation rate of 50%, a thermal conductivity of 25.42 W/(m·K) (as-tested) and a CTE of 10.79×10−6 /K (20–100 °C).

Microstructures and properties of Cu/Ag(Invar) composites fabricated by powder metallurgy

Abstract The Ag (Invar) composite powder prepared by ball milling was used to fabricate the Cu/Ag (Invar) composites. Microstructures and properties of the composites were studied after sintering and thermo-mechanical treatment. The results indicate that during ball milling, micro-forging weld and work-hardening fracture result in that the average particle size of the Ag (Invar) powder increases rapidly at first, and then decreases sharply, finally tends to be constant. Compared with the Cu/Invar ones, the sinterability of the composites is greatly improved, resulting in that the pores in them are smaller in amount and size. After the thermo-mechanical treatment, the Cu/Ag (Invar) composites are nearly fully dense with the optimum phase composition and element distribution. More importantly, Cu and the Invar alloy in the composites distribute continuously in a three-dimensional (3D) network structure. Cu/Invar interface diffusion is effectively inhibited by the Ag barrier layer, leading to a great improvement of the mechanical and thermal properties of the Cu/Ag (Invar) composites.

Formation and growth of Cu–Al IMCs and their effect on electrical property of electroplated Cu/Al laminar composites

Abstract Cu/Al laminar composite was prepared by dipping Zn layer and then electroplating Cu thick layer on pure Al sheet. During annealing the Cu/Al composites at temperature from 473 to 673 K, the Cu/Al interfacial diffusion and reaction and its kinetics and also the electrical resistivity of the composites were studied. The results show that no Cu–Al IMC layer is observable as the composites are annealed at 473 K for time till 360 h, indicating that the Zn intermediate layer can effectively suppress the Cu/Al interfacial diffusion. However, as the composites are annealed at 573 K and above, Zn atoms in the Zn layer dissolve into the Cu layer. Tri-layered reaction product of CuAl 2 /CuAl/Cu 9 Al 4 then forms from the Al side to the Cu side. The IMC layer follows the diffusion-controlled growth kinetics. Electrical resistivity of the Cu/Al composites increases with the increase of the annealing temperature and time.