A two-stage approach of manufacturing FeAl40 iron aluminides by self-propagating synthesis and pressureless sintering

This is an Accepted Manuscript of an article published by Taylor & Francis Group in Powder Metallurgy on 11 June 2018, available online at https://doi.org/10.1080/00325899.2018.1478778. Under embargo until 11 June 2019.A two-stage sintering process was successfully used to sinter FeAl to densification levels of just above 95% at a temperature of 1300 ºC. In the first stage, mixed iron and aluminium powders were synthesised at 750°C via Self-Propagating High-temperature Synthesis (SHS) to form brittle and porous Fe2Al5. Then the pellets were crushed and milled to various sizes and mixed with iron powders in the nominal composition of FeAl40 and pressurelessly sintered at a higher temperature to obtain a higher densification by taking advantage of the less violent exothermic reaction of Fe2Al5 and Fe. The intermediate and end products in SHS and sintering were characterised by SEM/EDX and XRD. The porosity level of the final FeAl40 product was controlled by the heating rate and powder size, which was also strongly influenced by the temperature, holding time and the ratio of the two powders.Peer reviewedFinal Accepted Versio

Ceramic Conversion Treatment of Commercial Pure Titanium with a Pre-Deposited Vanadium Layer

Titanium is characterized by poor wear resistance which restricts its application. Ceramic conversion treatment (CCT) is used to modify the surface; however, it is a time-consuming process. In this work, a thin vanadium layer was pre-deposited on the commercial pure titanium (CPTi) samples’ surface, and it increased the oxygen absorption significantly and assisted in obtaining a much thicker oxide layer than those samples without a V layer at the treatment temperatures of 620 °C and 660 °C. The oxidation of the samples pre-deposited with the V layer had a much higher oxidation rate, and V was evenly distributed in the oxide layer. After CCT, all samples had a low wear volume and stable coefficient of friction in comparison to the untreated CPTi sample. A slightly higher wear area in the wear track was observed on the V pre-deposited samples than those samples without vanadium, especially those with a thicker oxide layer (>4 µm). This might be associated with defects in a thicker oxide layer and insufficient support from a shallower oxygen diffusion zone or hard debris created at the initial stage. Vanadium in the oxide layer reduced the contact angles of the surface and increased the wettability significantly

Cyclic oxidation behaviour of N-type (Zr,Ti)Ni(Sn,Sb) and P-type (Zr,Ti)Co(Sn,Sb) thermoelectric materials

In this study, the fabricated Hf-free N-type (Zr,Ti)Ni(Sn,Sb) and P-type (Zr,Ti)Co(Sn,Sb) thermoelectric materials were subjected to cyclic oxidation testing at 500 °C for 10, 30, and 50 cycles. The oxidation behaviour of the materials was systematically investigated by evaluating mass gain to study the oxidation kinetics and by analysing surface morphology, phase constitution and elemental distribution to investigate the oxidation mechanism. The results indicated that both of the materials were severely oxidised during the cyclic oxidation testing, and the mass gain followed the parabolic kinetics and the parabolic rate constant (kp) being 0.006165 mg2cm−4s−1 and 0.000109 mg2cm−4s−1 for the N-type and the P-type TE materials, respectively. Alternated multilayers of Ni3Sn4+SnO2+(Zr,Ti)O2 and CoSb + SnO2+Sb2O4+(Zr,Ti)O2 were identified on the surface of the N-type and P-type materials, respectively, after the cyclic testing, which would deteriorate the thermoelectric performance of the materials. The outcome of this study strongly suggests that it is essential to improve the oxidation resistance and the thermal stability of the N-type (Zr,Ti)Ni(Sn,Sb) and P-type (Zr,Ti)Co(Sn,Sb) thermoelectric materials for high-temperature applications

Tribological Properties of the Fast Ceramic Conversion Treated Ti-6Al-2Sn-4Zr-2Mo Alloy with a Pre-Deposited Gold Layer

Ceramic conversion treatment (CCT) is an effective way to modify the surface of titanium alloys. However, this process normally needs more than a 100-h treatment at 600–700 °C to form a hard and wear-resistant titanium oxide layer. In this paper, we pre-deposited a thin gold layer on the surface of Ti-6Al-2Sn-4Zr-2Mo (Ti6242) samples before CCT to investigate if Au can speed up the treatment. Treatments at 640/670/700 °C were carried out for 10 or 120 h. After CCT, the surface roughness, surface morphology, microstructure, elemental composition, and phase constituents were characterized. Surface hardness and the nano-hardness depth distribution were measured. Finally, reciprocating sliding tribological tests were carried out to study the friction and wear of the surface layers. Thin gold layers accelerated the CCT significantly with a much thicker oxide layer. The friction of the untreated Ti6242 alloy against the WC ball was unsteady and high, but it was much lower and stable for the CCTed samples pre-deposited with Au because of the formation of titanium oxides and lubrication effect of the gold particles. The wear resistance of the CCTed Ti6242 alloy samples with gold was reinforced significantly. By pre-depositing a thin gold layer on the surface of Ti6242, the treatment time can be cut significantly, and CCT becomes more efficient