Enhancement of Cu nano-precipitation by Al addition in a Cu-Ni bearing low alloy steel

Abstract Effect of copper addition on microstructure and mechanical properties of an ultra high strength low alloy NiSiCrCoMo steel is investigated in the present study. Fe–0.38C–3Ni–2Si–1Cr–

For conventional structural and tool materials, in particular WC–Co cemented carbides, hardness and wear-resistance can usually be increased only at the expense of toughness and strength. For the first time we have achieved a dramatically increased combination of hardness, wear-resistance, fracture toughness and strength as a result of precipitation of extremely fine nanoparticles in the cobalt binder of cemented carbides. These nanoparticles are ∼3 nm in size, coherent with the Co matrix and consist of a metastable phase.

Abstract High-strength low-carbon ferritic steels attaining a maximum yield strength of 1600 MPa by combined Cu and NiAl precipitation-strengthening were developed. The yield strength of the alloys increases monotonically with the total concentration of the principal alloying elements i.e. Mn, Cu, Ni and Al. At 12.40 at.%, a 1600 MPa yield strength is achieved after solution treatment at 950 °C followed by aging at 550 °C for 2 h. For all three alloys investigated, the hardness reached a maximum after 1–2 h aging at 500–550 °C. At peak hardness, the combined precipitation of the body-centered cubic (bcc) Cu-alloy and B2-ordered NiAl-type intermetallic precipitates is observed by atom probe tomography (APT). The morphology, composition and structure of the Cu-alloy and NiAl-type precipitates were characterized using APT and transmission electron microscopy, as a function of aging time at 550 °C. In peak hardness conditions, the equiaxed bcc Cu-alloyed precipitates contain substantial amounts of Fe and are enriched in Ni, Al and Mn. Ni and Mn segregate at the Cu-alloy precipitate/ferrite matrix interface. In addition to the segregation, B2 NiAl-type precipitates nucleate at the Cu-alloy precipitates. After aging for 2 h, most Cu-alloy precipitates have a NiAl-type precipitate attached to their side. On subsequent further aging, the Cu-alloyed precipitates enrich progressively with Cu and elongate, indicating a transformation to the 9R or face centered cubic structure. The Cu-alloyed precipitates coarsen slower than the NiAl-type precipitates due to interfacial energy differences between the two types of precipitates, slower diffusion kinetics of Cu through the NiAl precipitates, different matrix equilibrium solubility and solute transfer from Cu-alloyed precipitates to NiAl-type precipitates. The relatively slow growth and coarsening of Cu-alloyed precipitates are consistent with the observation of an only modest decrease of hardness with extended aging.

A series of six-component (FeCoNiCrMn) 100− x Al x ( x = 0–20 at.%) high-entropy alloys (HEAs) was synthesized to investigate the alloying effect of Al on the structure and tensile properties. The microstructures of these alloys were examined using transmission electron microscopy, and crystalline phase evolution was characterized and compared with existing models. It was found that the crystalline structure changed from the initial single face-centered cubic (fcc) structure to a duplex fcc plus body-centered cubic (bcc) structure and then a single bcc structure as the Al concentration was increased. Resulting from the structural changes there were also corresponding variations in tensile properties. In the single fcc region, alloys behaved like a solid solution with relatively low strength but extended ductility. In the mixed structure region, alloys behaved like a composite with a sharp increase in strength but reduced ductility. In the single bcc region, alloys became extremely brittle. In this study, close correlation between the microstructure and mechanical properties was also discussed and presented.

Abstract We elucidate here the determining role of aluminum on the precipitation behavior and mechanical properties in Cu-Ni-bearing low alloy steel. The microstructure and precipitation behavior were studied by electron microscopy in conjunction with the thermodynamic analysis. Aluminum had significant influence on the precipitation behavior of niobium such that the precipitation temperature of Nb(C, N) was decreased, and prior austenite grain size was governed by Al and Nb precipitation. The nano-size B2 structure of particles was confirmed to be Ni(Al, Mn), and Mn atoms preferred to substitute for Al in the sublattice site in the B2 structure. Furthermore, aluminum effectively increased precipitate density by increasing the nucleation rate of ordered-B2 Ni(Al, Mn) and copper precipitation, thereby increasing the hardness of Cu-Ni-bearing low alloy steel during tempering. Excellent tensile properties were obtained in low carbon steel after quenching and tempering, with tensile strength of 1150 MPa and yield strength of 1020 MPa. The high strength is attributed to the martensite matrix and precipitation of copper.

Abstract Intercritical tempering at 680 °C for different times was carried out in a low carbon copper-bearing steel to study the evolution of the crystal structure of Cu precipitates by high resolution transmission electron microscopy. With increased tempering time, four different types of crystal structure of copper precipitates with different sizes were observed, namely, (a) nano-ordered clusters comprised of B2 FeCu nano-ordered clusters (2–3 nm) and weak ordered BCC Cu nanoclusters (2–3 nm), (b) 9R Cu (5–12 nm) - multiple twinned structure consisted of two, six or seven 9R twins with an orientation relationship of (1 1–4) 9R  ∥(0 1 1) α , [− 1 1 0] 9R  ∥[1 − 1 1] α , (c) detwinned 9R Cu (24–26 nm) consisted of two 9R parts and a removable interface (1 1–4) 9R , and (d) FCC Cu (~ 37 nm) precipitates consisted of two parts of FCC Cu and a micro-twinned region. The evolution sequence of crystal structure of Cu precipitates was: nano-ordered clusters → 9R Cu → detwinned 9R Cu → FCC Cu. The maximum contribution to precipitation hardening is attributed to nano-ordered clusters.

Abstract A unique batch tempering treatment for industrial scale direct-quenched steel coils has been studied using laboratory simulations. The tempering treatment was non-isothermal with slow heating to 570 °C and slow cooling to simulate the tempering of large steel coils. The paper presents the effect of finishing rolling temperature (FRT) relative to the non-recrystallization temperature (TNR) and the effect of long time tempering on the microstructure, dislocation density and mechanical properties of direct-quenched coiled strips. Conditioning austenite below the recrystallization stop temperature resulted in a finer effective grain size distribution, which correlated strongly with the impact toughness of the final product. Furthermore low finish rolling temperature resulted in partially ferritic microstructures while higher finishing rolling temperatures led to mixtures of bainite and martensite. Dislocation densities determined with TEM and XRD showed somewhat different trends regarding the effect of tempering: intra-lath dislocation density, as measured with TEM, showed a statistically significant drop in only one case, while XRD analysis indicated a drop in all cases. Furthermore, no significant correlation between finishing rolling temperature and dislocation density existed in XRD studies. The XRD results indicate that the decrease in dislocation density corresponds to about 100 MPa lower dislocation strengthening. However, precipitation hardening and potential internal micro stress relief compensates this as yield strength remains unchanged or even increases during tempering.

Abstract A novel Cu-bearing Quench-Partitioned and Tempered (Q&P-T) steel was designed to achieve an excellent strength–ductility balance. Tempering after Q&P treatment increased the tensile and yield strength from 1263 and 706 MPa to 1393 and 1003 MPa, and simultaneously improved the uniform and total elongation from 13.2 and 21.1% to 17.0 and 24.0%, respectively. Dilatometric, XRD and APT analyses as well as SEM and TEM observations revealed that the multiphase structure consisting of fresh martensite, tempered martensite and ultrafine filmy retained austenite was formed via Q&P-T process. Copious Cu precipitates with a diameter of 0.66 nm–5.89 nm, which formed during tempering at 500 °C for 30 min can account for the remarkable increase in strength. Cu particles invisible under TEM may not harm the ductility due to their coherency with the matrix. Whilst the retained austenite fraction decreased slightly by tempering, it is likely that carbon partitioning further proceeded between the austenite and the martensite matrix, and increased the stability of ultrafine filmy retained austenite, which led to the notable enhancement of ductility.

Abstract Cu-rich precipitates formation is associated with the precipitation hardening of Fe-Cu based steels and the embrittlement of reactor pressure vessel steels under neutron irradiation. The accurate modeling of the time evolution of Cu-rich precipitates is therefore of fundamental importance for the design of Fe-Cu based steels and the prediction of the irradiation induced shift of the ductile to brittle transition temperature of reactor pressure vessels. This work applies cluster dynamics with mobile Cu monomers and clusters to model Cu precipitation in dilute Fe-Cu alloys at several temperatures. Optimized model parameters can be used to simulate the mean radius, number density, volume fraction, and matrix composition evolution during isothermal annealing with reasonable accuracy. The possible reduction of the mobility of Cu-rich clusters due to additional alloying elements in Fe-Cu based steels is discussed.

We present a systematic first-principles study on the segregation behavior of alloying element X (X=Al, Mn, Ni, and Si) at coherent body-centered cubic Cu precipitate/Fe matrix interfaces with three different orientations. The calculations predict a modest thermodynamic driving force favoring Fe/Cu interfacial segregation for Al, Mn and Ni and strong energetic preference of Si to partition to Fe matrix, consistent with experimental observation. The results give direct evidence that the interfacial segregation decreases the interfacial energy and explains the promotion of nucleation and retardation of coarsening of Cu precipitates by Al and Ni addition.

Abstract We report on the alloy design strategies, precipitation mechanism and mechanical properties of an ultra-high strength steel hardened by co-precipitation of nanoscale NiAl and Cu particles. The steel, developed through a computational-aided alloy design approach, exhibits a tensile strength of ∼1.9 GPa, an elongation of ∼10% and a reduction in area of ∼40%. Atom probe tomography (APT) reveals an interesting type of co-precipitation mechanism of NiAl and Cu nanoparticles, in which the NiAl particles first come out of the supersaturated solid solution and the rejection of Cu solutes leads to the heterogeneous precipitation of Cu particles adjacent to the NiAl particles. The observed precipitation sequence of “supersaturated solid solution → NiAl → NiAl + Cu” is substantially different from the one previously reported in Cu-strengthened steels, which involves the process of “supersaturated solid solution → Cu → Cu + NiAl”. The modulation of the precipitation sequence is attributed not only to the relatively high Ni/Cu and Al/Cu ratios but also the synergistic combination of Ni, Al, Mn and Cu additions in the steel. In addition, APT also reveals the precipitation of a small amount of nanoscale Fe 3 (Mo, W) 3 C- and NbC-type carbides. The combination of the strengthening effects from the nanoscale NiAl particles, Cu particles and carbides contributes significantly to the overall ultra-high strength of the steel.