Microstructures in a resistance spot welded high strength dual phase steel

The microstructure of a high strength dual phase steel resistance spot welded with tempering-pulse technology is characterized in this paper. In the fusion zone, there is a needle-like microstructure identified as acicular or side plate ferrite that has a cube-on-cube orientation relationship with respect to the surrounding martensite. In contrast to the microstructures produced by the lower cooling rate arc or laser welding techniques, the nucleation of this fine intragranular ferrite takes place independent of inclusions. Further, a leaf-like microstructure within the martensitic matrix is found to contain primitive orthorhombic Cr₃C₂ and face-centered cubic CrC chromium carbides, rather than Cr₂₃C₆ or Cr₇C₃ as is commonly observed in steel alloys. The formation histories of both the ferrite phase and the chromium carbides are analyzed.     

Microstructure and Mechanical Properties of Al25 − xCr25 + 0.5xFe25Ni25 + 0.5x (x = 19, 17, 15 at%) Multi‐Component Alloys

A series of Al_{25 ? x}Cr_{25 + 0.5x}Fe₂₅Ni_{25 + 0.5x} (x = 19, 17, 15 at%) multi‐component alloys are prepared by arc‐melting and rapid solidification of copper molds. The technique of thermal‐mechanical processing is further applied to the master alloys to improve their mechanical properties. These alloys consist of face‐centered cubic (FCC) and body‐centered cubic (BCC) structure. The volume fraction of the BCC phase increases as Al content increase and Cr and Ni contents decrease, accompanied with a microstructural evolution from dendritic structure to lamella‐like structure. Due to the increase of volume fraction of BCC phase, the master alloys exhibit an increased strength and a declined ductility as Al content increases. The rapid solidified alloys have more BCC phase compared with the master alloys, which enhances the strength and decreases the ductility. After homogenization, hot‐rolling, and annealing at 1000 °C, the Al₈Cr_33.5Fe₂₅Ni_33.5 alloy displays excellent combination of strength (yield strength is ～635 MPa and fracture strength is ～1155 MPa) and ductility (tension strain is ～11%).

     

Microstructures and mechanical properties of (AlCoCrFeMn)100 − xCux high-entropy alloys

A series of (AlCoCrFeMn)_100???xCu_x high-entropy alloys (0, 4, 8, 12, 16 at.-%) were prepared by vacuum arc furnace melting and their phase composition, microstructure and mechanical properties were systematically investigated. The results show that Cu can induce phase transformation from the orthorhombic phase to the Laves phase in (AlCoCrFeMn)_100???xCu_x high-entropy alloys and the volume fractions of Laves phase increases from 0 to 70% with increasing Cu content. The compression fracture strain increases from 5 to 16% and the compression fracture strength also increases from 1380 to 2140?MPa with Cu content increases. The increased volume fraction of the Laves phase is the main factor for the ductility increases.     

Improvement of room temperature fracture toughness in Cr2Nb-based alloys by heat treatment

The microstructure and room temperature fracture toughness of binary Cr/Cr₂Nb alloys annealed at 1653 K for 30 h were investigated at both the hypo- and hypereutectic compositions. The experimental results indicate that the high temperature heat treatment has a beneficial effect on the room temperature fracture toughness of the Cr₂Nb/Cr alloys. After the heat treatment, the room temperature fracture toughness of the hypo- and hypereutectic alloys are increased by about 212 and 203%, which are 15 and 8 times higher than that of as-cast Cr₂Nb Laves phase (1·2 MPa m^1/2). The fractographic analysis indicates that in the annealed condition, the strengthen of lamellar eutectic cohesive strength can provide significant toughening of the matrix by crack deflection, crack blunting and crack bridging mechanisms.     

Influence of silicon,aluminium, phosphorus and boron on hot ductility of TRansformation Induced Plasticity assisted steels

Abstract

The hot ductility of steels having high aluminium or phosphorus contents, which are currently being considered as possible replacements for the conventional high silicon TRansformation Induced Plasticity (TRIP) steel, has been examined. Tensile specimens were cast in situ and tested in the temperature range 750 - 1000 ° C at a strain rate of 3 × 10^-3 s^-1. The ductility trough for the conventional high silicon TRIP steel was controlled by the austenite - ferrite transformation, intergranular failure occurring when a thin band of the softer ferrite phase formed around the austenite grains. Void formation at the sulphides situated in the soft ferrite at the boundaries then occurred, and the strain concentrated locally there. The thin bands of ferrite were deformation induced and, as such, formed at temperatures above Ar ₃ and could form at as high a temperature as Ae ₃. Adding ferrite formers such as silicon, phosphorus and aluminium increased the Ae ₃ temperature and thus widened the trough. The high aluminium (2%) TRIP steel exhibited good ductility throughout the temperature range examined, since large amounts of ferrite were always present, preventing strain concentration, and the AlN particles were too coarse to influence the hot ductility. In contrast, the 1%Al containing steel gave poor ductility below 850 ° C, the band of strain induced ferrite being extremely thin. The ductility trough in the titanium containing high phosphorus steel was poor, owing to fine precipitation of TiN. Adding boron to the steel and reducing the manganese content from 1.4 to 1% resulted in better ductility. Generally, the TRIP type steels had superior ductility to the conventional niobium containing high strength low alloy steel.     

Thermal aging of 16Cr – 5Ni – 1Mo stainless steel Part 1 – Microstructural analysis

Abstract

Specimens of 16Cr - 5Ni - 1Mo stainless steel were solution treated at 1050 ° C for 1 h followed by heating in the temperature range 400 - 750 ° C for different holding times (1 - 16 h). After heat treatment, optical microscopy, scanning (SEM) and transmission (TEM) electron microscopy, and X-ray diffraction examinations were conducted. The microstructure of all aged specimens was found to consist of martensite with variable fractions of δ ferrite and reversed austenite. Very fine precipitates of Mo carbides were revealed in the specimens aged at 475 ° C. The specimens aged at 625 ° C showed a decrease in the dislocation density and a high volume fraction of austenite and precipitation of Fe₂Mo Laves phase was detected by X-ray analysis. Above 625 ° C, Cr₂₃C₆ and TiC became the predominate carbides heterogeneously precipitated in the martensitic matrix. Partial transformation of reversed austenite to unaged martensite was observed at temperatures above 625 ° C.     

Effects of Nd and B contents on the thermal stability of nanocomposite (Nd,Zr)2Fe14B/α-Fe magnets

The effects of Nd and B contents on the microstructure and thermal stability of nanocomposite (Nd,Zr)₂Fe₁₄B/α-Fe magnets have been investigated. It is shown that for Nd_xFe_93−xZr₁B₆ (x = 9–11) alloys, the volume fraction of Nd₂Fe₁₄B increases with increasing Nd content, and the sample with x = 10 exhibits the optimal microstructure and thermal stability. Though the room-temperature _iH_c of Nd₁₁Fe₈₂Zr₁B₆ sample is the highest, it decreases more rapidly than that of Nd₁₀Fe₈₃Zr₁B₆ as temperature increases, indicating the deterioration of the temperature coefficient β. For Nd₁₀Fe_89−yZr₁B_y (y = 5–8) alloys, the remanence and the temperature coefficient α deteriorate with increasing B content. The coercivity and the temperature coefficient β first improve with increasing B content, reaching the optimal values at y = 7, then deteriorate with further increasing B. Coarse grains and the Fe₃B phase are observed in the Nd₁₀Fe₈₁Zr₁B₈ alloy.     

Formation of ultrafine grained ferrite during thermomechanical treatment in microalloyed steel

Abstract

In the present work, the formation of ultrafine grained ferrite has been studied by applying suitable thermomechanical treatment. A high amount of deformation (～80%) at varying strain rates (0·01–10 s^?1) was applied in the temperature range of Ar₃ to Ac₃ followed by water quenching. This treatment resulted in a two-phase ferrite–martensite microstructure as compared to fully martensite structure after quenching without deformation. The formation of ultrafine ferrite (?3 μm) during deformation was favourable at a lower temperature and a slower strain rate. A maximum ～50% ferrite formed during deformation at 780°C with a strain rate of 0·01 s^?1. Experimental rolling with a high strain (～1·3) with finish rolling temperature just above Ar₃ (～750°C) resulted in fine ferrite–pearlite of ?3 μm, and the properties showed a high value of strength as compared to steels rolled in a conventional way. Dual phase microstructure (ferrite and martensite) was produced after partial austenisation to 780°C followed by quenching in water, and this resulted in an excellent combination of properties (high ultimate tensile strength, low yield strength/ultimate tensile strength, high elongation and high n values).     

Fabrication and mechanical characterization of a series of plastic Zr-based bulk metallic glass matrix composites

Coupling the high yielding strength with enhanced plasticity under compression at ambient temperature, a series of Zr-based bulk metallic glass matrix composites are designed based on a pseudo ternary phase diagram. The largest compressive fracture plastic strain of 17.0% with the yielding strength of 1070 MPa is available for Zr_60.0Ti_14.7Nb_5.3Cu_5.6Ni_4.4Be_10.0 bulk metallic glass matrix composite. The relationship between the cooling rate and the microstructure, the microstructure and the mechanical properties, and the fractographs of the composites is carefully identified.     

Tensile properties of partially austenitised and austempered ductile irons with dual matrix structures

Abstract

In the present study, an unalloyed ductile iron containing Fe–3·50C–2·63Si–0·318Mn–0.047Mg (wt-%) were intercritically austenitised (partially austenitised) in two phase region α+γ at various temperatures of 795, 805, 815 and 830°C for 20 min and then quenched into salt bath held at austempering temperature of 365°C for various times to obtain different ausferrite volume fractions (AFVFs). Results showed that dual matrix structure containing proeutectoid ferrite, new ferrite (also called epitaxial ferrite) and ausferrite (bainitic ferrite+high carbon austenite, which is retained or stabilised austenite) has been developed. Within each of the austempered series in α+γ temperature range, new ferrite volume fraction increased with increasing intercritical austenitising temperature (ICAT). Although, transforming percentage of new ferrite from parent austenite present at ICAT increased with decreasing ICAT. Some specimens were also conventionally austempered from 900°C for comparison. The new ferrite was absent in these samples. The volume fraction of proeutectoid ferrite, new ferrite and ausferrite can be controlled to determine the strength and ductility. Austempered specimens in α+γ temperature range exhibited much greater ductility than conventionally austempered ones. The tensile strength increased while ductility decreased with increasing AFVF. On the other hand, the ductility increased with increasing proeutectoid ferrite and new ferrite volume fractions at the expense of strength. The specimen with ～47·2%AFVF exhibited the best combination of high strength and ductility. The strength and ductility of this material is much higher than that of ferritic grades. Its strength is at the same level as while ductility almost more than four times higher than that of pearlitic grades. Meanwhile, the specimen with ～ 75%AFVF exhibited the best combination of high strength and ductility compared with those of pearlitic grades. The strength of this material is much higher and its ductility is almost more than two times higher than that of pearlitic grades yet slightly lower than that of ferritic grades. This material also meets the requirements for the strength of quenched and tempered grades and its ductility is higher than that of this grade.     

High temperature strength at 1773 K and room temperature fracture toughness of Nbss/Nb5Si3 in situ composites alloyed with Mo

High temperature compressive strength at 1773 K and room temperature fracture toughness have been studied in terms of microstructure, phase stability and solid solution hardening in Nb-Si-Mo in situ composites consisting of niobium solid solution and Nb₅Si₃. Molybdenum addition stabilizes the -Nb₅Si₃ phase and makes unstable Nb₃Si phase in the in situ composite. It is found that molybdenum has a strong effect to increase the yield stress of the present in situ composite at 1773 K due to solid solution hardening. Yield strength depends not only on chemical composition and volume fraction but also the Nb₅Si₃ phase itself. Room temperature fracture toughness is very sensitive to microstructure and the content of ternary alloying element, but not to the volume fraction of constituent phases within the composition ranges investigated. It is suggested that plastic deformation of Nb solid solution and interface decohesion is responsible for high fracture toughness in this alloy system. Details are discussed in relation to microstructural features and Molybdenum alloying.     

Effects of Cr,Mn, Si,Cu and Zr on microstructure and mechanical properties of high carbon Fe–16Al alloy

Abstract

Effects of alloying elements Cr, Mn, Si, Cu and Zr on the microstructure and mechanical properties of Fe₃Al (Fe–16Al) based alloy containing ～0·5 wt-%C have been investigated. Six alloys were prepared by a combination of air induction melting with flux cover and electroslag refining (ESR). ESR ingots were hot forged and hot rolled at 1373 K and were further characterised with respect to microstructure and mechanical properties. The base alloy and the alloys containing Cr, Mn, Si and Cu exhibit a two phase microstructure of Fe₃AlC_0·5 precipitates in Fe₃Al matrix whereas the alloy containing Zr exhibits a three phase microstructure, the additional phase being Zr rich carbide precipitates. Cr and Mn have high solubility in Fe₃AlC_0·5 precipitates as compared to Fe₃Al matrix whereas Cu and Si have very high solubility in Fe₃Al matrix compared to Fe₃AlC_0·5 precipitate and Zr has very low solubility in both Fe₃Al matrix and Fe₃AlC_0·5 precipitate. No significant improvement in room and high temperature (at 873 K) strengths was observed by addition of these alloying elements. Furthermore, it was observed that addition of these alloying elements has resulted in poor room and high temperature ductility. Addition of Cr, Mn, Si and Cu has resulted in marginal improvement in creep life, whereas Zr improved the creep life significantly from 22·3 to 117 h.     

Deformation behavior of a Co-Cr-Fe-Ni-Mo medium-entropy alloy at extremely low temperatures

We report the mechanical and microstructural characteristics of a medium-entropy alloy, Co_17.5Cr_12.5Fe₅₅Ni₁₀Mo₅ (atomic percent, at%) at cryogenic temperatures, down to a record low temperature of 0.5 K. The alloy exhibits excellent strength and ductility combined with a high strain-hardening rate in the entire temperature range investigated. Its property profile, including the yield strength, ultimate tensile strength, strain hardening capability, and absorbed mechanical energy, is better than those of most alloys and HEAs used in cryogenics. Within the interval of extremely low temperatures considered (0.5–4.2 K), the alloy exhibits several unusual features, including anomalies of the temperature dependence of the yield strength and tensile ductility, discontinuous plastic deformation (DPF), and a change in the propensity for the deformation-induced martensitic transformation. While the occurrence of these effects in the same temperature interval may be fortuitous, we hypothesize that they are interrelated and provide a tentative explanation of the observed phenomena on this basis.     

High temperature magnetic properties of mechanically alloyed Fe–Zr powder

We report the preparation and characterization of amorphous/non-equilibrium solid solution Fe_100 − xZr_x (x = 20–35) alloys by mechanical alloying process. The microstructure and magnetic properties of milled powders have been studied as a function of Zr substitution. The effective magnetic moment of as-milled powders decreases as concentration of Zr is increased. Thermomagnetization measurements confirmed that the Fe₈₀Zr₂₀ sample exhibits two clear magnetic phase transitions due to the co-existence of an amorphous phase and a Fe rich non-equilibrium solid solution. All the other samples exhibiting an amorphous structure showed a single magnetic phase transition with Curie temperature of ~ 570 °C,which did not vary much with different composition. The Curie temperature of the mechanically alloyed powders is noticeably higher than those of melt-spun amorphous ribbons.     

Microstructure and mechanical properties of a novel refractory AlNbTiZr high-entropy alloy

The microstructure and mechanical properties of a novel refractory AlNbTiZr high-entropy alloy (HEA) with a low density of ～5.85?g?cm^?3 were investigated after arc melting and homogenisation at 1473?K for 5?h. The as-cast HEA exhibits a single-phase ordered body-centred cubic (B2) structure. A hexagonal Zr₅Al₃-type second phase is introduced into the HEA through homogenisation treatment, resulting in increase of the yield strength, ultimate compressive strength and fracture strain by 70?MPa, 308?MPa and 9.2%, respectively. These results indicate that the introduction of the hexagonal Zr₅Al₃-type second phase into the B2 matrix can simultaneously improve the HEA strength and ductility, showing a strength–ductility combination superior to those of most reported refractory HEAs.     

Influence of Titanium on Hydrogen-Induced Transformations in the Laves Phases Based on Zirconium

By the methods of differential thermal, and X-ray phase analyses, we study the influence of hydrogen (hydrogenation, disproportionation, desorption, and recombination) on the phase transformations in Zr_{1 − x} Ti_xCr₂ (x = 0.1 and 0.2) alloys under a pressure of hydrogen of 5 MPa at temperatures varying from the room temperature to 1238°K. For 3–5 MPa and 1163–1223°K, we observe the decomposition of the Laves phase with C14-type structure accompanied by the formation of Zr_{1 − x} Ti_xCr₂H_y and ZrH_x hydrides and Cr. Under a pressure of 5 MPa, on holding at 1223°K for more than 4 h, the compounds completely disproportionate into zirconium and titanium hydrides and chromium. On holding in a vacuum at 1193– 1238°K, the compounds recombine into the phases with C14- or C15-type structure or their mixture.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 6, pp. 67–72, November–December, 2004.     

Transformation of the quasicrystalline phase Al-Cr-Fe induced by rapid solidification

A quasicrystalline phase, Q, with icosahedral symmetry was detected by X-ray diffraction and transmission electron microscopy in Al-3Cr-xFe (x=0 1 or 3 at %) alloys elaborated by hot extrusion of rapidly solidified powders. Chemical microanalysis showed the average composition of this phase to be 75 ± 0.5% Al, 12 ± 1%Cr, 12 ± 1% Fe. Annealing treatments led to its transformation into the equilibrium phases Al₁₃Cr₂ and Al₁₃Fe₄, directly at high temperature, or through a metastable and unknown phase, X, at intermediate temperature. This transformation was followed by X-ray diffraction, calorimetry and in situ electron microscopy. The convergent-beam technique was used for characterization of the X phase.     

Effects of Fe/Ni Ratio on Microstructure and Properties of FeNiCrAlNb High-Entropy Alloys

Herein, the effects of Fe/Ni ratio on the microstructure, mechanical properties, and corrosion resistance in a 3.5 wt% NaCl solution of Fe_xNi_65−xCr₂₀Al₁₀Nb₅ are investigated systematically. It is found that the phases shifted from the FCC-dominated to the BCC-dominated with the molar ratio of the Fe/Ni increased. The strength of Fe_xNi_65−xCr₂₀Al₁₀Nb₅ increases with the molar ratio of Fe/Ni further increased, while the plasticity decreases. The yield strength reaches 1,653 MPa at x = 45. The alloy exhibits the best corrosion resistance when x = 35 which is attributed to the dominant FCC phases in the dendritic region.     

Self-propagating high temperature synthesis and magnetic properties of Ni0.35Zn0.65Fe2O4 powders

Ni-Zn ferrite powders were synthesized by self-propagating high temperature synthesis (SHS) method. X-ray diffraction, TEM and vibrating sample magnetometry (VSM) were used to characterize the phase composition, microstructure and magnetic properties of the combustion products. The effect of the combustion temperature (T _c), the major parameter of the SHS process, on particle size, phase composition and magnetic properties of the products was also studied. The results showed that particle size grew with the increasing combustion temperature. The maximum saturation magnetization,M _s, increased with combustion temperature indicating the growth of grain size and high degree of ferritization, while residual magnetization,M _r, and coercive force,H _c, decreased. Compared with other methods, Ni_0.35Zn_0.65Fe₂O₄ ferrite powders with improved magnetic properties can be obtained by SHS at 1000°C.     

Superior mechanical properties of a selective-laser-melted AlZnMgCuScZr alloy enabled by a tunable hierarchical microstructure and dual-nanoprecipitation

Achieving high mechanical strength and ductility in age-hardenable Al7000 series (Al–Zn–Mg) alloys fabricated by selective laser melting (SLM) remains challenging. Here, we show that crack-free AlZnMgCuScZr alloys with an unprecedented strength–ductility synergy can be fabricated via SLM and heat treatment. The as-built samples had an architectured microstructure consisting of a multimodal grain structure and a hierarchical phase morphology. It consisted of primary Al₃(Sc_x,Zr_1−x) particles which act as inoculants for ultrafine grains, preventing crack formation. The metastable Mg-, Zn-, and Cu-rich icosahedral quasicrystals (I-phase) ubiquitously dispersed inside the grains and aligned as a filigree skeleton along the grain boundaries. The heat treated SLM-produced AlZnMgCuScZr alloy exhibited tunable mechanical behaviors through trade-off among the hierarchical features, including the dual-nanoprecipitation, viz, η′ phase, and secondary (Al,Zn)₃(Sc₉Zr), and grain coarsening. Less coarsening of grains and (Al,Zn)₃(Sc₉Zr) particles, due to a reduced solution treatment temperature and time, could overwhelm the more complete dissolution of I-phase (triggering more η′ phase), resulting in higher yield strength. Optimal combination of the hierarchical features yields the highest yield strength (∼647 MPa) among all reported SLM-produced Al alloys to date with appreciable ductility (∼11.6%). The successful fabrication of high-strength Al7000 series alloys with an adjustable hierarchical microstructure paves the way for designing and fine-tuning SLM-produced aluminum engineering components exposed to high mechanical loads.