Microalloying Additions Research Articles

Analysis of the Static Recrystallization Behavior of Nb-Ti Microalloyed Steels Including Low Strain Levels

Semi-empirical models for predicting the austenite static recrystallization behavior are widely used in designing thermomechanical treatments to improve final mechanical properties. However, a problem with these models is that their utility can be limited to the range of deformation conditions and chemical compositions they were developed for. This work focuses on the study of the applicability of current recrystallization models to the range of low strain conditions and/or high Nb microalloying additions (≈0.1%). To do so, the recrystallization behavior of two low carbon Nb-Ti microalloyed steels (0.04 and 0.11% Nb and ≈0.01% Ti) has been investigated by torsion tests. Experimental results for recrystallization time and recrystallized grain size have been compared to previously developed equations. It has been observed that at low strains (ε = 0.1) the predictions fail. A dependence of the n Avrami exponent both on temperature and applied strain was also found.

Some Metallurgical Issues Concerning Austenite Conditioning in Nb-Ti and Nb-Mo Microalloyed Steels Processed by Near-Net-Shape Casting and Direct Rolling Technologies

As thin slab direct rolling technologies are moving to the production of higher quality steel grades, chemical compositions based on Nb-Ti and Nb-Mo become a good option. However, with the use of multiple microalloying additions, the as-cast austenite conditioning becomes more complex. This paper analyzes some of the microstructural features that should be taken into account during the as-cast austenite conditioning in Nb-Ti and Nb-Mo microalloyed steel grades. In the case of Nb-Ti grades, it has been observed that the process parameters during solidification and post-solidification steps affect the austenite evolution during hot rolling. This is due to the differences in the size and volume fraction of TiN particles that can be formed. Fine TiN precipitates have been shown to be able to delay recrystallization kinetics. Moreover, the solute drag effect of Ti cannot be ignored in the case of hyperstoichiometric Ti/N ratios. It is observed that Nb-Ti grades tend to have lower non-recrystallization temperatures compared to Nb grades, which means that pancaking of the austenite is more difficult for these steels. The opposite is observed for the Nb-Mo grades, although in both cases the behavior is affected by the nominal content of Nb.

Improving precipitation hardening behavior of Mg[sbnd]Zn based alloys with Ce[sbnd]Ca microalloying additions

The precipitation hardening behavior of newly developed MgZnCaCe alloys, with modified texture and improved ductility, is studied to delineate the microstructural characteristics that lead to effective hardening upon ageing treatments. Advanced electron microscopy and atom probe techniques are used to analyze the structural characteristics in relevance to the hardening potential. It has been found that the formation of a new basal precipitate phase, which evolves from a single atomic layer GP zone, and is finely distributed in both under-aged and peak-aged microstructures, has a significant impact in the improvement of the hardening response compared with the base MgZn alloys. It has also been found that the β′1 rod precipitates, commonly formed during ageing treatments of MgZn alloys, have their size and distribution significantly refined in the CaCe containing alloys. The role of alloy chemistry in the formation of the fine basal plate GP zones and the refinement in β′1 precipitation and their relationships to the hardening behavior are discussed. It is proposed that Ca microalloying governs the formation of the GP zones and the enhancement of hardening, particularly in the under-aged conditions, but that this is aided by a beneficial effect from Ce.

Effects of Cerium and Zirconium Microalloying Addition on the Microstructures and Tensile Properties of Novel Al-Cu-Li Alloys

Effects of Ce and Zr additions (combinative addition and single addition) on the microstructures and tensile properties of novel Al-5.8wt%Cu-1.3wt%Li alloys with an elevated Cu/Li ratio were investigated comparatively by a microscopy method and tensile test. The microstructural observation shows that the intermetallic dispersoid in the case of combinative addition of Ce and Zr is more significantly refined from coarse polygonal shape to fine irregular particles compared to that in the case of single addition of Ce or Zr. Due to the refinement and the modification of intermetallic dispersoid, the corresponding fracture mode changes from brittle intergranular fracture to ductile transgranular fracture during tension. In addition, microstructural analysis reveals that the addition of Ce promotes the precipitation of the T 1 phase which is the predominant strengthening phase in Al-Cu-Li alloy. In comparison to Ce containing Al-Cu-Li alloy, Ce+Zr-containing Al-Cu-Li alloy raises the degree of supersaturation of Cu in matrix due to less Cu atoms being trapped in finer AlCuCe dispersoid after solution treatment. Thus it is found that the ultimate tensile strength (UTS) and yield strength (YS) for the later alloy are increased by 19.6% and 16.1%, respectively. And the elongation (El) is similar, owing to decreasing of the phase size, changing of precipitation type to T 1 phase only and further grain refinement.

Controlling the corrosion and cathodic activation of magnesium via microalloying additions of Ge

The evolution of corrosion morphology and kinetics for magnesium (Mg) have been demonstrated to be influenced by cathodic activation, which implies that the rate of the cathodic partial reaction is enhanced as a result of anodic dissolution. This phenomenon was recently demonstrated to be moderated by the use of arsenic (As) alloying as a poison for the cathodic reaction, leading to significantly improved corrosion resistance. The pursuit of alternatives to toxic As is important as a means to imparting a technologically safe and effective corrosion control method for Mg (and its alloys). In this work, Mg was microalloyed with germanium (Ge), with the aim of improving corrosion resistance by retarding cathodic activation. Based on a combined analysis herein, we report that Ge is potent in supressing the cathodic hydrogen evolution reaction (reduction of water) upon Mg, improving corrosion resistance. With the addition of Ge, cathodic activation of Mg subject to cyclic polarisation was also hindered, with beneficial implications for future Mg electrodes.

In vitro evaluation of biodegradable magnesium alloys containing micro-alloying additions of strontium, with and without zinc

Event Abstract Back to Event In vitro evaluation of biodegradable magnesium alloys containing micro-alloying additions of strontium, with and without zinc Junlan Wang1*, Shayanti Mukherjee2*, David Nisbet2*, Nick Birbilis1* and Xiaobo Chen1* 1 Monash University, Materials Science and Engineering, Australia 2 The Australian National University, Research School of Engineering, Australia Abstract: The in vitro degradation of magnesium (Mg) alloys containing low levels of strontium (Sr, 0.05, 0.1 and 0.2 wt.%) with and without additions of zinc (Zn, 0.5 and 1.0 wt.%), was studied for potential use in orthopaedic fracture treatments. Sr-containing Mg-alloys were selected as a promising strategy to utilise the biological role of Sr through alloying, to induce accelerated bone tissue growth. The influence of controlled alloying upon degradation rate was studied via electrochemical measurements and immersion tests in minimum essential medium (MEM). Immersion testing revealed a comparable degradation rate of the alloys tested herein, indicating no detrimental effect of Sr on degradation. Cytotoxicity experiments on primary mouse osteoblasts indicated good biocompatibility and enhanced proliferation of osteoblasts for all the tested Mg alloys. Potentiodynamic polarisation testing further confirmed that low-level additions of Sr imparted a minor influence on cathodic kinetics, with a slight inhibition of anodic kinetics. In contrast, the addition of Zn as a ternary element moderated both anodic and cathodic kinetics of Mg-Sr alloys. Xiaojian Xia Keywords: Tissue Engineering, in vitro, material design, Biodegradable metal Conference: 10th World Biomaterials Congress, Montréal, Canada, 17 May - 22 May, 2016. Presentation Type: Poster Topic: Metallic biomaterials and alloys Citation: Wang J, Mukherjee S, Nisbet D, Birbilis N and Chen X (2016). In vitro evaluation of biodegradable magnesium alloys containing micro-alloying additions of strontium, with and without zinc. Front. Bioeng. Biotechnol. Conference Abstract: 10th World Biomaterials Congress. doi: 10.3389/conf.FBIOE.2016.01.01355 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 27 Mar 2016; Published Online: 30 Mar 2016. * Correspondence: Dr. Junlan Wang, Monash University, Materials Science and Engineering, Melbourne, Australia, junlan.wang@monash.edu Dr. Shayanti Mukherjee, The Australian National University, Research School of Engineering, Canberra, Australia, shayanti.mukherjee@gmail.com Dr. David Nisbet, The Australian National University, Research School of Engineering, Canberra, Australia, david.nisbet@anu.edu.au Dr. Nick Birbilis, Monash University, Materials Science and Engineering, Melbourne, Australia, nick.birbilis@monash.edu Dr. Xiaobo Chen, Monash University, Materials Science and Engineering, Melbourne, Australia, xiaobo.chen@monash.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Junlan Wang Shayanti Mukherjee David Nisbet Nick Birbilis Xiaobo Chen Google Junlan Wang Shayanti Mukherjee David Nisbet Nick Birbilis Xiaobo Chen Google Scholar Junlan Wang Shayanti Mukherjee David Nisbet Nick Birbilis Xiaobo Chen PubMed Junlan Wang Shayanti Mukherjee David Nisbet Nick Birbilis Xiaobo Chen Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Effect of strain-rate on the deformation response of D03-ordered Fe3Al

The mechanical response of centrifugally cast Fe3Al with the composition Fe–27Al (at.%) containing micro-alloying additions of Nb, Zr, C, and B was investigated over a wide range of strain rates between 10−4 and 103 s−1 at room temperature. Tests were carried out in compression using a (i) screw-driven load frame, (ii) drop impact tester, and (iii) split-Hopkinson pressure bar at quasi-static, intermediate and dynamic strain rates respectively. Post deformation analysis was carried out by DSC, SEM/EBSD, TEM and micropillar deformation. In all instances, the stress-strain curves show initial hardening (∼first 5% plastic strain) followed by a plateau in stress. A loss in work-hardening occurs at the highest strain rates examined (>103 s−1) and is likely associated with shear localization; in addition, (21¯1) [111¯]-type twinning was observed at these high strain rates at room temperature. This observation is in line with previous theoretical calculations of the antiphase boundary (APB) energy. The consequence of a continuously increasing yield stress with strain rate and a loss in work hardening at the highest strain rates together yields a maximum in flow stress at the intermediate strain rate.

The effect of Sc and Yb microalloying additions and aged‐hardening heat treatment on corrosion behavior of Al–Mg alloys

Sc‐containing Al alloys are potentially applied in saline environment. However, the corrosion behavior and its mechanisms of Sc–containing Al alloys have not been paid much attention. In this research, the corrosion behavior of Al–4Mg, Al–4Mg–0.3Sc, and Al–4Mg–0.24Sc–0.06Yb alloys was investigated in 3.5 wt% NaCl solution by immersion and potentiodynamic polarization analysis. Scanning electron microscope (SEM), transmission electron microscopy (TEM) and high‐resolution TEM were used to characterize intermetallic phases and localized corrosion. The results showed that the Fe–containing intermetallic particles are cathodic with respect to the matrix and can lead to localized corrosion, whereas the high content of Mg makes the Fe‐containing intermetallic particle less noble to the matrix. Yb‐containing intermetallic particles did not show any dissolution of the matrix at the periphery of these particles. The results indicated that active dissolution and localized corrosion were the dominant corrosion mechanisms for Al–4Mg alloy. However the polarization curves of Al–4Mg–0.3Sc and Al‐4Mg–0.24Sc–0.06Yb alloys showed spontaneous passivation followed by pitting corrosion. Results suggested that age‐hardening heat treatment improved the stability of the passive film on Al–4Mg–0.3Sc and Al–4Mg–0.24Sc–0.06Yb alloys.

Influence of a microalloying addition of Ag on the precipitation kinetics of an Al–Cu–Mg alloy with high Mg:Cu ratio

Effects of microalloying with Ag on the precipitation process in an Al–Cu–Mg alloy with high Mg:Cu ratio have been investigated during artificial ageing of the ternary and quaternary compositions Al–1.5wt.%Cu–4wt.%Mg(–0.5wt.%Ag) at 175°C, combining transmission electron microscopy and positron lifetime spectroscopy. In addition, coincidence Doppler broadening of the positron–electron annihilation radiation was used to provide information about the chemical environment surrounding vacancy-like defects. The results obtained for the silver-free alloy indicate that, immediately after quenching from the solution treatment temperature, vacancy v-Cu–Mg clusters are formed in the supersaturated solid solution. During the early stages of ageing these clusters become richer in Cu and Mg and laths of the S phase (Al2CuMg) nucleate preferentially on dislocation lines. Continued ageing led to gradual precipitation of an equi-axed phase in the matrix between the S phase laths which is believed to be the cubic Z phase thought previously to form only if silver is present. For the silver-containing Al–Cu–Mg alloy, v-Cu–Mg–Ag aggregates also form immediately after quenching and the solute transport mechanisms are the same. However, precipitation of the S phase is suppressed during ageing and the silver addition promotes accelerated formation of a finer dispersion of smaller equi-axed Z phase precipitates. For both alloys, no evidence was found that the Z phase is preceded by any pre-precipitate or GP zones. Combination of coincidence Doppler broadening and microanalysis results revealed the Z phase to have a Mg:Cu ratio of 2 which indicates that it differs from the well known T phase (Al6CuMg4) which the equilibrium phase diagram indicates should form in Al–Cu–Mg alloys with high Mg:Cu ratios. Additional ageing experiments at 240°C showed that neither the Z phase or T phase are formed in the ternary alloy, but only precipitates of the S phase, while in the quaternary alloy the S phase is strongly suppressed and mainly Z phase precipitates are formed.

In vitro evaluation of biodegradable magnesium alloys containing micro-alloying additions of strontium, with and without zinc.

The in vitro degradation of magnesium (Mg) alloys containing low levels of strontium (Sr, 0.05, 0.1 and 0.2 wt%), with and without addition of zinc (Zn, 0.5 and 1.0 wt%), was studied for potential use in orthopaedics for fracture treatment. Alloying Mg with Sr was selected as a promising strategy to utilise the biological effect of Sr in inducing accelerated bone tissue growth. The influence of controlled alloying upon degradation rate was studied via electrochemical measurements and immersion tests in minimum essential medium (MEM). Immersion testing revealed a comparable degradation rate of the alloys tested herein, indicating no detrimental effect of Sr on degradation. Cytotoxicity experiments on primary mouse osteoblasts indicated good biocompatibility and enhanced proliferation of osteoblasts for all the tested Mg alloys. Potentiodynamic polarisation testing further confirmed that addition of low-levels of Sr had a minor influence on cathodic kinetics, with a slight inhibition of anodic kinetics. In contrast, the addition of Zn as a ternary element moderated both anodic and cathodic kinetics of Mg-Sr alloys.

First-principles-aided design of a new Ni-base superalloy: Influence of transition metal alloying elements on grain boundary and bulk cohesion

A new approach to the design of Ni-base polycrystalline superalloys is proposed. In this approach, we assume that the creep–rupture characteristics of a superalloy are mostly determined by the strength of interatomic bonding at grain boundaries (GBs) and in the bulk of γ matrix. The ideal work of separation, Wsep, of a GB is used as a fundamental thermodynamic quantity that controls the mechanical strength of an interface, whereas the partial cohesive energy, χ, of an alloy component serves to characterize its contribution into the strength of the bulk. Using the Σ5 (210)[100] symmetric tilt GB as a representative high-angle GB in Ni, we calculate Wsep,χ, and GB segregation energies, Eseg, for the complete set of 4d and 5d transition metal impurities, to which we add B (a typical microalloying addition), S and Bi (notoriously known as harmful impurities in Ni-base superalloys). The purpose of the analysis is to identify the elements that demonstrate a high tendency to segregate to GBs, have positive (preferably high) partial cohesive energies in the bulk, and have positive impact on Wsep of GBs. We refer to these elements as low-alloying additions. Our study reveals Zr, Hf, Nb, Ta and B as the most promising low-alloying additions. Our next step is to introduce the elements found in the first step into a new powder metallurgy (P/M) Ni-base superalloy. The results of the subsequent testing confirm that the newly created P/M superalloy indeed demonstrates superior mechanical properties at high temperatures compared to the existing Russian P/M alloy EP741NP.

Improving microstructure and ductility in the Mg–Zn alloy system by combinational Ce–Ca microalloying

A strategy is proposed to enhance the microstructure and mechanical properties of Mg–Zn alloys by combining microalloying additions of the rare earth element Ce and the non-rare earth element Ca. The double additions of Ce–Ca are found to significantly increase tensile elongation compared to binary Mg–Zn, or single additions of either Ce or Ca. Microstructure analysis reveals that the Ce–Ca additions increase ductility by modifying texture and refining grain size. Texture modification is attributed to solute effects from the microalloying elements, particularly Ca, while grain refinement is additionally influenced by a fine dispersion of Mg6Ca2Zn3 precipitates that form during rolling and pin grain boundaries. The microalloying element additions also lead to large secondary phase particles in the alloys, which can limit ductility enhancement by promoting early fracture. By scaling Zn content in the Mg–Zn–Ce–Ca alloys, the Mg6Ca2Zn3 phase fraction and Zn solute content can be controlled for optimum ductility or strengthening potential.

Thermal Cycling Reliability of Alternative Low-Silver Tin-Based Solders

Sn-3.0Ag-0.5Cu (SAC305) alloy is the most widely used solder in electronic assemblies. However, issues associated with cost and drop/shock durability have resulted in a search for alternative lead-free solder alloys. One approach to improve the drop/shock reliability has been to reduce the silver content in Sn-Ag-Cu alloys. Another approach is doping Sn-Ag-Cu solder with additional elements. In 2008, the International Electronics Manufacturing Initiative (iNEMI) started the “Characterization of Pb-Free Alloy Alternatives” project to provide a comprehensive study of 15 tin-based solder interconnect compositions benchmarked against the eutectic tin-lead solder. For this study, temperature cycle durability was the primary focus and solders were selected to study the effect of varying silver content, microalloy additions, and aging. This paper reports the findings from one of the test conditions conducted under the iNEMI project. The cycles to failure for a temperature cycling test condition from −15°C to 125°C, with dwell times of 60 min at both extremes, are presented. The test assembly consisted of 16 of the 192 I/O BGAs and 16 of the 84 I/O BGAs soldered onto an LG451HR laminate. Test results revealed that the reduction of silver resulted in a reduction in cycles to failure. In all cases, the 15 tin-based solders were more durable than the eutectic SnPb solder. Aging at 125°C for 10 d did not affect the cycles to failure in SAC105 solder; however, the cycles to failure decreased with aging in SAC305 solder. In addition, aging resulted in a wider distribution of cycles to failure in 192 I/O BGAs. Failure analysis was carried out on all solder materials to identify the failure site and failure mode.

Magnetocaloric effect in Fe-Zr-B-M (M = Ni, Co, Al, and Ti) amorphous alloys

The amorphous soft magnetic Fe-Zr-B-M (M = Ni, Co, Al, and Ti) ribbons have been successfully fabricated and their magnetocaloric effect was investigated through the measurements of magnetization in this work. The magnetocaloric effect, both the maximum entropy change and the refrigerant capacity, of the precursor material Fe88Zr8B4 amorphous alloy is enhanced with microalloying addition. The Curie temperature increases with the addition of Co but decreases with the addition of Al and Ti. The field dependence of the entropy change was studied. All specimens show the same power law to the field, which is in agreement with the reported amorphous alloys. The specimen containing 1 at. %Co, whose Curie temperature is 295 K and entropy change reaches 1.48 J kg−1 K−1 at an applied field of 1.5 T, is suitable for a magnetic refrigerant at room temperature. On the other hand, the specimen containing 1 at. %Ti with Curie temperature of 270 K and refrigerant capacity of 183.5 J kg−1 is a proper work material for a magnetic refrigerant at subroom temperature.

Influence of Nb Additions on Microstructural Evolution of a V-Microalloyed High-Carbon Wire Steel During Patenting

This study investigated the feasibility of microalloying strategies for improving the strength of high-carbon wire products subjected to industrial patenting heat treatments for two eutectoid steels: a 0.8C-0.5Mn-0.2Cr-0.08 V alloy (wt.%) and the same composition with an additional 100 ppm Nb. A Gleeble 3500 thermomechanical simulator (Dynamic Systems Inc., Poestenkill, NY, USA) was used to perform heat treatments consisting of a 30 s austenitization at 1093 °C, 950 °C, or 880 °C followed by a 15 s isothermal transformation step at 650 °C, 625 °C, 600 °C, or 575 °C. Vickers hardness, field-emission scanning electron microscopy, and pearlite interlamellar spacing measurements were conducted to assess the effects of the heat treatments. Niobium microalloying additions were found to provide no hardness increase, but they extended the pearlitic regime to lower isothermal transformation temperatures.

The Effects of Heating Rate and Cold Work on the Development of Dual-Phase Steel Microstructures

The effects of heating rate and prior cold work on the development of dual-phase steel microstructures in three low carbon steels were evaluated with samples processed on a Gleeble 3500 thermomechanical processing simulator. The nominally 0.2 wt pct carbon steels included a plain carbon steel and modified alloys incorporating higher manganese contents, boron additions, and microalloy additions. Each alloy was prepared with two different cold rolled reductions. Heating rates from 1 to 1000 oC/s were selected to span the rates typically experienced in conventional furnace heat treating up to rates for induction heating. Critical transformation temperatures were obtained from dilatometric curves. Dual-Phase microstructures after heat treatment with different heating rates were compared. Transformation temperatures decreased with an increase in cold work and increased with an increase in heating rate. The steels with higher manganese and carbon additions exhibited lower Ac3 values across all heating rates and the steels with higher silicon higher Ac1 temperatures across all heating rates. Ac1 increased less than Ac3 with increasing heating rate. The increase in transformation temperatures between 100 and 1000 °C/s was smaller than values exhibited over other increments in heating rate, and decreased in one steel; contributing factors were identified for this behavior.

Influence of microalloying additions on Al-Mg alloy. Part 2: Phase analysis and sensitisation behaviour

A range of alloys based on the Al-4Mg-0·4Mn system were produced with selected quaternary microalloying additions. In Part 1 of this study, the electrochemical and corrosion response was studied. To characterise the sensitisation behaviour of these alloys, where sensitisation is the major mode of degradation of 5xxx alloys, heat treatment at 150°C was carried out and followed by the Nitric Acid Mass Loss Test (NAMLT) according to ASTM G67-04. Herein the alloying elements studied include silicon, zinc, titanium, zirconium and strontium. The results indicate that strontium (Sr), silicon (Si) and titanium (Ti) have a significant influence in reducing intergranular corrosion (IGC) susceptibility.

Multipass hot deformation behaviour of high Al and Al–Nb steels

The effect of Nb (up to 0.07wt%) and high Al content (up to 2wt%) on the multipass deformation behaviour of steels with 0.2% C and 2% Mn was studied with the aid of hot torsion simulations. From the tests, the critical Non-Recrystallisation (Tnr), Recrystallisation Limit and Stop Temperatures (RLT and RST) and the ferrite phase transformation start temperature (Ar3) were determined. It was observed that an increase in Al content from 1% to 2% or a microalloying addition of 0.03% Nb to 1% Al steel both led to a significant increase in the recrystallisation critical temperatures, which is greater than 100°C in the case of the Tnr. However, the value of the Tnr was not affected when 0.03% or 0.07% Nb was added to the 2% Al steel. Specimens quenched after several deformation passes were examined by optical and TEM means in order to study the interaction between static recrystallisation, strain-induced precipitation and γ→α phase transformation, and determine the mechanisms leading to strain accumulation in the steels investigated. The results suggest that for the 1% Al steels, the Al and Nb solute drag effect is the main mechanism leading to the increase in the critical recrystallisation temperatures, while for the 2% Al steels the occurrence of γ→α phase transformation at temperatures close to the Tnr is the main mechanism involved in softening retardation, with a limited contribution of Nb. However, γ→α phase transformation taking place at temperatures close to the Tnr resulted in a loss of hot ductility, which can limit the industrial applicability of the 2% Al steels.

Microstructure and property evaluation of high-pressure die-cast Mg–La–rare earth (Nd, Y or Gd) alloys

Microstructure, castability and tensile properties were investigated in high-pressure die-cast Mg–La–Nd, Mg–La–Y and Mg–La–Gd alloy series, with a constant La concentration at approximately 2.5wt.% and the concentrations of Nd, Y or Gd were varied. All three alloy series had a dendritic microstructure with a Mg–La-rich eutectic with increasing Nd, Y or Gd content and containing a Mg12RE intermetallic phase. The morphology of the eutectic at ternary alloying additions of equal to or less than 1.0wt.% was lamellar but became increasingly divorced at higher ternary concentrations. This was however more obvious in Mg–La–Y and Mg–La–Gd than Mg–La–Nd alloys. The hot tearing susceptibility in all three alloy series increased markedly with even micro-alloying additions of Nd, Y or Gd, and began to decrease again in alloys with more than 0.5wt.% Y or 1.0wt.% Gd, but did not decrease significantly for Mg–La–Nd. A model using the temperature–fraction solid curves as input parameters was used to estimate hot tearing susceptibility for Mg–La–Nd alloys. Tensile testing at room temperature showed that Mg–La–Nd alloy series had higher 0.2% proof stress and lower elongation to failure than either the Mg–La–Y or the Mg–La–Gd alloy series for Nd concentrations greater than 1wt.% due to a greater effectiveness of grain boundary reinforcement.

Formation of Nano/ultrafine Grain Structure in a Ti-modified 201L Stainless Steel through Martensite Thermomechanical Treatment

The formation of nano/ultrafine grain structure in a Ti-modified 201L austenitic stainless steel using the martensite thermomechanical treatment was investigated. Effects of Ti micro-alloy addition on the formation of strain-induced martensite (SIM) during cold rolling and austenite reversion during subsequent annealing were studied. The results showed that the nano-grained Ti-modified 201L steel with the average grain size of 45 nm possessed the yield strength of 1000 MPa, ultimate tensile strength of 1330 MPa and total elongation of 42%. Such improvement in mechanical properties of the thermomechanically processed steel is believed to be due to the formation of SIM supplemented with precipitation of Ti carbides.