An effective way for the automotive industry to tackle the growing concern of increasing CO2 emissions is to reduce the vehicle's overall weight, without compromising its performance and passenger safety. With the increasing demand for steels with enhanced properties in the last decades, the development of advanced high-strength steels (AHSSs) has been focused on the design of complex microstructures, leading to exceptional combinations of strength and ductility. On the other hand, stainless steels also offer significant potential in automotive lightweight applications to complement carbon steel AHSS grades. Stainless steels can offer additional advantages, including better corrosion resistance (no need for galvanising) and increased strength and fatigue resistance. However, a significant barrier to their application is the alloying cost (nickel in austenitic stainless steels) or low ductility and formability (standard martensitic stainless steels)....@en

This work discusses the microstructure evolution observed in a quenching and partitioning (Q&P)-processed martensite/austenite stainless steel during the partitioning step at 400 °C for 300 s, where distinct microstructural bands rich in austenite due to elemental segregation, evolve into a uniform distribution of austenite grains. This phenomenon is characterised and investigated using a model for the carbon partitioning from martensite to austenite coupled with the movement of the martensite-austenite interface. The observed elimination of microstructural bands is found to be related to the topological distribution of austenite grains and the heterogeneity of the thermodynamic equilibrium regime at the various interfaces governing the partitioning process. Furthermore, the concurrence of banding elimination (local equilibrium) and phase growth towards the global equilibrium phase fractions is investigated in the simulations in terms of the role of Mn. It is found that the local equilibrium-negligible partitioning (LENP) conditions lead to the most realistic outcome.

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This work presents an investigation of the microstructure development during the application of the quenching and partitioning (Q&P) process to two stainless steels with different Mn content. The results are compared with calculations based on the constrained carbon equilibrium theory, paying special attention to the presence of reactions competing for the carbon available for partitioning and to the effect of alloying element segregation. Results show that chromium carbides must be considered when accounting for the carbon available for austenite stabilisation. Moreover, manganese/chromium segregation bands play an important role in the microstructure development, particularly in martensite formation, with important consequences in the microstructure development during the following processing steps.

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The present article investigates the influence of chemical composition and phase fractions on the corrosion behaviour of industrially produced quenching and partitioning (Q&P) martensitic stainless steels. Localised corrosion was analysed by scanning Kelvin probe force microscopy (SKPFM) and scanning electrochemical microscopy (SECM) in 3.5 wt.% NaCl solution. SKPFM revealed a Volta-potential difference of around 40 mV between inclusions and the matrix, which is larger than the Volta potential variations within the matrix. This difference in surface potential is a driving force for selective dissolution (corrosion initiation) at inclusions and inclusion/matrix interfaces. SECM detected early pitting initiation, particularly in alloys containing MnS and TiN inclusions. Results suggest that pitting initiation and propagation occur at those specific regions. This study emphasised that irrespective of chemical composition and phase fraction, localised corrosion initiation in Q&P-processed martensitic stainless steels is predominantly governed by the presence of inclusions.

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In present work, the formation, evolution, and distribution of the primary Fe-rich phase in an Al–Mg–Si–Cu–Zn–Fe–Mn alloy are coupling controlled by ultrasonic melt treatment (USMT) and thermomechanical processing (TMP). It is shown in the results that the size of grains and Fe-rich phase in the as-cast state can be greatly reduced by the applied optimum USMT at 680 °C. Additionally, the transformation rate of β-Fe-rich phase to α-Fe-rich phase can be also enhanced. After the coupling control of USMT and TMP, the number density and distribution uniformity of multiscale Fe-rich particles can be greatly increased or improved, which contributes to the fine-grained recrystallization microstructure and weakened texture. Finally, compared with the 6xxx series Al alloys (such as AA6016 and AA6111), the alloy sheet in the pre-aging state exhibits substantially improved bendability and strength (the plastic strain ratio and tensile strength are 0.67 and 304 MPa, respectively). The effect of USMT on the formation and transformation of primary Fe-rich phase and the mechanisms of improved bendability and strength are deeply discussed.

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Recent studies have demonstrated the viability of quenching and partitioning (Q&P) treatment for processing martensitic stainless steels showing an improved balance of high strength and sufficient ductility. However, to date, the fatigue behaviour of these materials has not been explored. This study examines the effect of their complex hierarchic microstructure on high cycle fatigue performance. Three steels with different alloying element contents underwent Q&P processing, resulting in multiphase microstructures rich in retained austenite. High cycle fatigue tests and analysis of fatigue fracture surfaces were performed using SEM and EBSD techniques. The results indicate satisfactory high cycle fatigue performance in Q&P treated martensitic stainless steels, surpassing traditional counterparts. Fatigue cracks predominantly form and propagate along martensite packet and block boundaries, while prior austenite grain boundaries and MnS inclusions have minimal influence on fatigue crack formation and growth. Microplastic deformation at the fatigue crack tip enhances local KAM values and triggers localized transformation of retained austenite grains. It is hypothesized that the developed Q&P treated martensitic stainless steels exhibit improved resistance to low cycle fatigue.

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Quenching and partitioning (Q&P) treatment has been proven effective in manufacturing advanced high strength steels with high content of retained austenite, showing the improved balance of high strength and sufficient ductility. This method has been very well elaborated for carbon steel processing over the last two decades. Though it can also be potentially applied for processing other steel families, this has been scarcely studied. This article focuses on the effect of chemistry and heat treatment parameters on the microstructure and properties of Q&P treated martensitic stainless steels. Three different martensitic stainless steels with different contents of alloying elements are subjected to Q&P processing with varying quenching temperature or partitioning temperature and partitioning time. The tensile behavior of the Q&P treated steels is studied. The effect of chemistry and Q&P treatment parameters on the microstructure and tensile properties is analyzed. The effect of plastic deformation on the microstructure of the Q&P treated steels is also investigated. It is demonstrated that the Q&P treated martensitic stainless steels can show a good combination of enhanced strength and sufficient tensile ductility. Their uniform elongation increases with the increasing volume fraction of retained austenite due to the transformation induced plasticity (TRIP) effect. The ability of the martensitic matrix to accumulate plastic deformation also plays an important role. The Q&P process - microstructure - property relationship is discussed.

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Scanning electrochemical microscopy (SECM) is employed to characterize the evolution of local electrochemical surface activity during lithium-based conversion layer formation on legacy aerospace aluminium alloy AA2024-T3. Initially, three types of studied intermetallic particles - S-, θ- and constituent phases - act as active cathodic areas. Subsequently, θ- and constituent phases show passivation preceding that of S-phase particles during the later conversion layer formation stages. The entire surface, including the matrix region, shows a higher reactivity at the beginning and then gradually shows decreasing reactivity. Hydrogen evolution-generated bubbles attach to the alloy surface and locally hinder the conversion layer formation, weakening the corrosion protection the conversion layer provides at those locations.

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The intergranular corrosion (IGC) resistance of age-hardening Al–Mg–Si–Cu alloys is closely related to the precipitation behavior adjacent to grain boundaries. In this study, we proposed to regulate the interaction of solute atoms and solute partitioning of Zn-containing Al–Mg–xSi–Cu alloys by introducing dislocations, which can synergistically decorate the intergranular and intragranular precipitation behavior. Consequently, the continuity of grain boundary precipitates and width of solute-depleted precipitate-free zones are inhibited accompanied with high number density or coarse precipitate in the matrix. As a result, the IGC resistance is greatly improved without strength and ductility loss, and the related mechanism has been proposed.

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A new non-isothermal pre-aging treatment was proposed and utilized in Al-Mg-Si-Cu-Zn alloys, together with natural aging and artificial aging. The influence of cooling rates on subsequent precipitation behaviors was investigated by experimental and thermodynamic simulations. The results show that by controlling the formation of clusters/GP zones through changing pre-aging cooling rates, i.e. PA-0.2, PA-0.3 and PA-0.4 (°C/min, from 80 °C to 40 °C), an excellent bake hardening increment and natural aging stability can be obtained. The highest bake hardening increment can reach 180 MPa for PA-0.4 sample, which is twice higher than those of Al-Mg-Si-(Cu) alloys. The microhardness remains almost unchanged within NA for 14 days at a lower level of approximately 85 HV_0.2. Thermodynamic simulations estimate the solvus temperatures and chemical composition for GP zones, revealing the strengthening and stabilising mechanisms behind: a) Mg-Zn- clusters formed during pre-aging can suppress Mg-Si- clusters formation in the natural aging process, b) non-isothermal hinders the precipitates growth, a faster cooling rate leads to smaller and softer Mg-Zn- clusters, and c) the formation of a heterogeneous microstructure contributes to the high bake-hardening response without changing the type of strengthening phase β″. Finally, the clustering and aging process was illustrated and explained.

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The effect of Sn micro-alloying on microstructure evolution, formability and precipitation behaviour of Al-Mg-Si-Cu-Zn alloys were systematically studied by experimental techniques and theoretical calculations. Results show that Sn addition can accelerate both the precipitation and re-dissolution of the Fe-rich phase during casting and homogenising treatments, which thereby determined the final microstructure. A significant retarding effect to natural ageing precipitation was observed with increasing Sn content in quenching samples, but this effect was weakened in pre-aged samples, as explained by DSC and simulations. The different number densities of the strengthening phase β″at the same artificial aging state are mainly attributed to the changed activation energy of the β″ phase affected by the formed Sn-containing Mg-Zn clusters and Mg-Si clusters. Trace Sn participating in the formation of GP zones, Sn-containing MgZn₂ phase and new precipitating sequences during ageing were proposed for the first time.

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Synergy of Ni micro-alloying and thermomechanical processing on the phase distribution, formability and bendability of Al–Mg–Si–Cu–Zn–Fe–Mn alloys was systematically studied in this paper. With the addition of micro-alloying Ni, the Ni-containing Fe-rich phase can be formed, which not only serves as nucleation sites of Mg–Si precipitates (such as, Q phase) during the casting process, but also improves the uniform distribution level of Fe-rich phases after homogenization. The formability and bendability of Ni-containing alloy can be both improved to a certain level due to the positive effect of Ni micro-alloying. In comparison, if increasing the cold rolling deformation between hot rolling and annealing, the distribution of multi-scale Fe-rich phases can be significantly improved based on the synergy of Ni micro-alloying and thermomechanical processing. And finally, this improvement further results in the great improvements in the microstructure, texture, formability (average r = 0.688, △r = −0.09) and bendability of the alloy together. Based on the microstructure evolution, the synergy mechanism of Ni micro-alloying and thermomechanical processing is put forward in this paper.

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The coupling control of quenching rate and pre-aging and its positive effect on the age-hardening response of Al–Mg–Si–Cu–Zn–Fe–Mn alloy was systematically investigated. The larger and more stable solute clusters can be formed in alloy with fast age-hardening response by using the lower quenching rate (5.3 °C/min) and an appropriate pre-aging, in which the deterioration of natural aging also can be obviously suppressed. Additionally, the highest bake hardening increment of the alloy can reach 145.2 MPa, which is much higher than those of traditional Al–Mg–Si–(Cu) alloys (such as, 6016 and 6111 alloys). Based on the detailed precipitation behavior characterization of alloys with different quenching rates and the same pre-aging, the quenching rate change can result in the significant differences in the size, number density of precipitates in the both paint baking and peak aging states, but the type of precipitates basically keeps the same, i.e., Mg–Si precipitates, and no Mg–Zn precipitates can be observed. Finally, the related mechanisms of coupling control of quenching rate and pre-aging were also discussed in this paper. The developed coupling control method shows great potential and could significantly increase applications of Al–Mg–Si–Cu–Zn–Fe–Mn alloys with a fast age-hardening response.

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The effects of vanadium content on the microstructural and mechanical properties of a newly developed hot-work die steel (MPS700V) at room and elevated temperatures have been investigated in this paper. It shows that the ultimate tensile strength (UTS) of MPS700V at room temperature and 700 °C are both strongly dependent on the V content. With the increase of V content from 0 to 1.2% V, The UTS at room temperature increases from 1127 MPa to 1442 MPa, and the UTS at 700 °C increases from 400 MPa to 550 MPa. The transmission electron microscope (TEM) shows that the addition of V is beneficial to increase the nucleation rate and volume fraction of MC nano-carbides during the tempering process, and the size of the MC nano-carbides decreases with the increase of V content. The addition of V also increases the thermal stability of MC nano-carbides at 700 °C, which can be related to the reducing the interface mismatch between MC and matrix. Furthermore, a transformation from Orowan bypass strengthening mechanism to shearing strengthening mechanism caused by the V addition is proposed based on the theoretical analysis and TEM observation.

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