Shi Kang Wu

The in-situ anomalous small-angle X-ray scattering (ASAXS) technique was used to investigate the strain glass transition (SGT) in as-quenched Ti_48.7Ni_51.3 shape memory alloy during a thermal cycle of 30 °C to the SGT temperature T_g (−50 °C) and then to 30 °C again. The Ni atoms play a critical role as point defects in the SGT mechanism and are very difficult to characterize using conventional tools. ASAXS identified the distribution of Ni atoms in nanodomains, which have a disk-like core–shell configuration with a Ni-rich shell and a highly Ni-rich core. Moreover, the morphological evolution, growth and shrinkage of the highly Ni-rich core domains during the thermal cycle through T_g are demonstrated. The enhancement and reversible behavior of the local lamellar ordering arrangement of nanodomains during the SGT process at T_g are revealed. The structural evolution and local ordering arrangement of nanodomains can play a role in hindering martensitic transformation. The ASAXS results provide new knowledge about the SGT beyond that from current simulation works. However, this corresponding structure of the nanodomains was destroyed when the specimen was heated to 250 °C.

Dissimilar infrared brazing of CoCrFeMnNi/CoCrFeNi equiatomic high entropy alloys and 316 stainless steel using MBF601 and BNi-2 foils was evaluated. The wetting angles of the two fillers at 50 °C above their liquidus temperatures on the three substrates were below 40 degrees. The CoCrFeMnNi/316 SS joint had the highest shear strength of 361 MPa with BNi-2 filler brazing at 1020 °C for 180 s, and fractured at the CrB compound in the joint. The CoCrFeMnNi/MBF601/316 SS joint contained a CoCrFeMnNi-based matrix, phosphides and B-containing compounds. The CoCrFeNi/316 SS joint had the highest shear strength of 374 MPa when brazed with BNi-2 filler at 1020 °C for 600 s, and fractured at the CrB in the joint. The CoCrFeNi/MBF601/316 SS joint consisted of a (Fe,Ni)-rich matrix, phosphides and B/Cr/Fe/P compounds, and the highest shear strength of 324 MPa was achieved when it was brazed at 1080 °C for 600 s.

In TiNi-based shape memory alloys (SMAs), the effects of thermal cycling on the transformation peak temperatures of B2 ↔ B19′, B2 ↔ R, B2 ↔ B19, B2 ↔ R ↔ B19′, and B2 ↔ B19 ↔ B19′ one-stage and two-stage transformations have been investigated and compared. Experimental results of the differential scanning calorimeter and hardness tests indicate that the alloy’s intrinsic hardness and the shear strain, s, associated with martensitic transformation, are two important factors, due to their relation to the ease of introducing dislocations during cycling. The temperature decrease by cycling for one-stage transformation was in the order of B2 ↔ B19′ > B2 ↔ B19 > B2 ↔ R according to the orders of magnitude of their s values. This phenomenon also affected the suppression of B19 ↔ B19′ and R ↔ B19′ transformation peak temperatures in two-stage transformation. Both Ti₅₀Ni₄₈Fe₂ and Ti_48.7Ni_51.3 SMAs aged at 450 °C for 4 h exhibited B2 ↔ R ↔ B19′ transformation, but the hardness of the latter was much higher than that of the former due to the precipitation hardening of the Ti₃Ni₄ precipitates. This causesd the decrease of the R ↔ B19′ transformation peak temperature in the Ti₅₀Ni₄₈Fe₂ SMA to be much higher than that in Ti_48.7Ni_51.3 SMAs aged at 450 °C for 4 h, which directly affected the sequential B2 ↔ R transformation of Ti₅₀Ni₄₈Fe₂ SMA in the next thermal cycle and decreased this transformation peak temperature. The Ti₄₈Ni₅₂ SMA aged at 600 °C for 150 h underwent B2 ↔ B19′ transformation and then B2 → R → B19′/B19′ → B2 transformation as the cycle number increased, in which the B2 ↔ R transformation peak temperature raised slightly by cycling. This characteristic is uncommon and may have resulted from the strain field around the thermal-cycled dislocations favoring the formation of the R-phase.

The wettability, microstructures, and bonding strength of infrared brazing Ti-15-3 and Ti₅₀Ni₅₀ shape memory alloy using 72Ag-28Cu (wt.%) and 68.8Ag-26.7Cu-4.5Ti (wt.%) filler metals have been investigated. Only Ticusil^® active braze readily wets both Ti₅₀Ni₅₀ and Ti-15-3 substrates. Wetting of eutectic 72Ag-28Cu melt on Ti₅₀Ni₅₀ base metal is greatly ameliorated by adding 4.5 wt.% Ti into the alloy. The brazed Ti-15-3/BAg-8/Ti₅₀Ni₅₀ joint consists of Cu-Ti intermetallics in the Ag-rich matrix. The formation of interfacial Cu(Ti,V) and (Cu_xNi_1−x)₂Ti intermetallics next to Ti-15-3 and Ti₅₀Ni₅₀ substrates, respectively, is attributed to the wetting of both substrates. The brazed Ti-15-3/Ticusil^®/Ti₅₀Ni₅₀ joint shows a vigorous reaction, which results in the formation of a large amount of Ti₂Ni intermetallics in the joint. The maximum joint strengths using BAg-8 and Ticusil^® filler metals are 197 MPa and 230 MPa, respectively.

The recrystallization behavior, grain growth kinetics, and corresponding hardness variation of homogenized and 80% cold-rolled FeCoNiCrPd, FeCoNiCrMn, and their quaternary/ternary FCC-structured high/medium entropy alloys (H/MEAs) annealed under different conditions were investigated. Experimental results indicate that the grain size and hardness of these H/MEAs follow the Hall–Petch equation, with the Hall–Petch coefficient K_H value being mainly dominated by the alloy’s stacking fault energy and shear modulus. The FeCoNiCrPd alloy exhibits the highest hardness of the H/MEAs at the same grain size due to the largest Young’s modulus difference between Cr and Pd. The grain growth exponent n, kinetic constant k, and activation energy for grain growth Q_G of all H/MEAs are calculated. The k can be expressed by the Arrhenius equation with Q_G, which is attributed to the diffusion rate. The results demonstrate that the Q_G values of these H/MEAs are much higher than those of conventional alloys; most notable is FeCoNiCrPd HEA, which has an unusually lattice distortion effect that hinders grain growth.

Infrared vacuum brazing of CoCrFeMnNi high entropy alloy (HEA) using BNi-2 and MBF601 fillers has been investigated. Both brazes show poor wettability at temperatures only 20 °C above their liquidus temperatures. However, the wettability of BNi-2 and MBF601 fillers on CoCrFeMnNi HEA is greatly improved with increasing the test temperatures, 50 °C above their liquidus temperatures. The BNi-2 brazed joints are dominated by Ni-rich matrix with huge CrB and a few tiny boride precipitates. Average shear strengths of joints increase with increasing brazing temperature and/or time, and fracture location changes from blocky CrB in the brazed zone to grain boundary boride in the substrate. The MBF601 brazed joints are composed of CoCrFeMnNi-based matrix, particles of B/Co/Cr/Fe/Mn/Ni/P compounds, and some phosphides form along the grain boundaries of the substrate. The specimen brazed with MBF601 filler foil at 1050 °C for 600 s has the highest average shear strength of 321 MPa, while that brazed at 1080 °C for 600 s has a lower average shear strength of 271 MPa due to the presence of solidification shrinkage voids.