Properties and influence of microstructure and crystal defects in Fe2_2VAl modified by laser surface remelting

Laser surface remelting can be used to manipulate the microstructure of cast material. Here, we present a detailed analysis of the microstructure of Fe$_2$VAl following laser surface remelting. Within the melt pool, elongated grains grow nearly epitaxially from the heat-affected zone. These grains are separated by low-angle grain boundaries with 1{\deg}-5{\deg} misorientations. Segregation of vanadium, carbon, and nitrogen at grain boundaries and dislocations is observed using atom probe tomography. The local electrical resistivity was measured by an in-situ four-point-probe technique. A smaller increase in electrical resistivity is observed at these low-angle grain boundaries compared to high-angle grain boundaries in a cast sample. This indicates that grain boundary engineering could potentially be used to manipulate thermoelectric properties

Search for heavy resonances decaying to two Higgs bosons in final states containing four b quarks

A search is presented for narrow heavy resonances X decaying into pairs of Higgs bosons (H) in proton-proton collisions collected by the CMS experiment at the LHC at root s = 8 TeV. The data correspond to an integrated luminosity of 19.7 fb(-1). The search considers HH resonances with masses between 1 and 3 TeV, having final states of two b quark pairs. Each Higgs boson is produced with large momentum, and the hadronization products of the pair of b quarks can usually be reconstructed as single large jets. The background from multijet and t (t) over bar events is significantly reduced by applying requirements related to the flavor of the jet, its mass, and its substructure. The signal would be identified as a peak on top of the dijet invariant mass spectrum of the remaining background events. No evidence is observed for such a signal. Upper limits obtained at 95 confidence level for the product of the production cross section and branching fraction sigma(gg -> X) B(X -> HH -> b (b) over barb (b) over bar) range from 10 to 1.5 fb for the mass of X from 1.15 to 2.0 TeV, significantly extending previous searches. For a warped extra dimension theory with amass scale Lambda(R) = 1 TeV, the data exclude radion scalar masses between 1.15 and 1.55 TeV

Measurement of the top quark mass using charged particles in pp collisions at root s=8 TeV

Peer reviewe

Search for supersymmetry in events with one lepton and multiple jets in proton-proton collisions at root s=13 TeV

Peer reviewe

Search for anomalous couplings in boosted WW/WZ -> l nu q(q)over-bar production in proton-proton collisions at root s=8TeV

Peer reviewe

Search for standard model production of four top quarks in proton–proton collisions at

A search for events containing four top quarks (t¯tt¯t) is reported from proton–proton collisions recorded by the CMS experiment at √s=13TeVand corresponding to an integrated luminosity of 2.6fb−1. The analysis considers the single-lepton (e or μ)+jets and the opposite-sign dilepton (μ+μ−, μ±e∓, or e+e−)+jets channels. It uses boosted decision trees to combine information on the global event and jet properties to distinguish between t¯tt¯tand t¯tproduction. The number of events observed after all selection requirements is consistent with expectations from background and standard model signal predictions, and an upper limit is set on the cross section for t¯tt¯tproduction in the standard model of 94fb at 95% confidence level (10.2×the prediction), with an expected limit of 118fb. This is combined with the results from the published CMS search in the same-sign dilepton channel, resulting in an improved limit of 69fb at 95% confidence level (7.4×the prediction), with an expected limit of 71fb. These are the strongest constraints on the rate of t¯tt¯tproduction to date.We congratulate our colleagues in the CERN accelerator depart-ments for the excellent performance of the LHC and thank the technical and administrative staffs at CERN and at other CMS in-stitutes for their contributions to the success of the CMS effort. In addition, we gratefully acknowledge the computing centers and personnel of the Worldwide LHC Computing Grid for delivering so effectively the computing infrastructure essential to our analyses. Finally, we acknowledge the enduring support for the construc-tion and operation of the LHC and the CMS detector provided by the following funding agencies: BMWFW and FWF (Austria); FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS, MOST, and NSFC (China); COLCIEN-CIAS (Colombia); MSES and CSF (Croatia); RPF (Cyprus); SENESCYT (Ecuador); MoER, ERC IUT, and ERDF (Estonia); Academy of Fin-land, MEC, and HIP (Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hun-gary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); MSIP and NRF (Republic of Korea); LAS (Lithuania); MOE and UM (Malaysia); BUAP, CINVESTAV, CONACYT, LNS, SEP, and UASLP-FAI (Mexico); MBIE (New Zealand); PAEC (Pakistan); MSHE and NSC (Poland); FCT (Portugal); JINR (Dubna); MON, RosAtom, RAS, RFBR and RAEP(Russia); MESTD (Serbia); SEIDI, CPAN, PCTI and FEDER (Spain); Swiss Funding Agencies (Switzerland); MST (Taipei); ThEP-Center, IPST, STAR, and NSTDA (Thailand); TUBITAK and TAEK (Turkey); NASU and SFFR (Ukraine); STFC (United Kingdom); DOE and NSF (USA). Individuals have received support from the Marie-Curie pro-gram and the European Research Council and EPLANET (Euro-pean Union); the Leventis Foundation; the A. P. Sloan Founda-tion; the Alexander von Humboldt Foundation; the Belgian Fed-eral Science Policy Office; the Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture (FRIA-Belgium); the Agentschap voor Innovatie door Wetenschap en Technolo-gie (IWT-Belgium); the Ministry of Education, Youth and Sports (MEYS) of the Czech Republic; the Council of Scientific and In-dustrial Research, India; the HOMING PLUS program of the Foun-dation for Polish Science, cofinanced from European Union, Euro-pean Regional Development Fund, the Mobility Plus program of the Ministry of Science and Higher Education, the National Science Center (Poland), contracts Harmonia 2014/14/M/ST2/00428, Opus 2014/13/B/ST2/02543, 2014/15/B/ST2/03998, and 2015/19/B/ST2/02861, Sonata-bis 2012/07/E/ST2/01406; the National Priorities Re-search Program by Qatar National Research Fund; the Programa Clarín-COFUND del Principado de Asturias; the Thalis and Aris-teia programs cofinanced by EU-ESF and the Greek NSRF; the Rachadapisek Sompot Fund for Postdoctoral Fellowship, Chula-longkorn University and the Chulalongkorn Academic into Its 2nd Century Project Advancement Project (Thailand); and the Welch Foundation, contract C-1845

Suppression of Excited Υ States Relative to the Ground State in Pb-Pb Collisions at √sNN=5.02 TeV

The relative yields of Υ mesons produced in pp and Pb-Pb collisions at √sNN=5.02 TeV and reconstructed via the dimuon decay channel are measured using data collected by the CMS experiment. Double ratios are formed by comparing the yields of the excited states, Υ(2S) and Υ(3S), to the ground state, Υ(1S), in both Pb-Pb and pp collisions at the same center-of-mass energy. The double ratios, [Υ(nS)/Υ(1S)]Pb−Pb/[Υ(nS)/Υ(1S)]pp, are measured to be 0.308±0.055(stat)±0.019(syst) for the Υ(2S) and less than 0.26 at 95% confidence level for the Υ(3S). No significant Υ(3S) signal is found in the Pb-Pb data. The double ratios are studied as a function of collision centrality, as well as Υ transverse momentum and rapidity. No significant dependencies are observed

Development of high-strength aluminium alloys for laser powder bed fusion

Laser Powder Bed Fusion (L-PBF) is an additive manufacturing technique in which three-dimensional components are produced from computer-aided design models by melting metallic powder in a layer-by-layer manner. Since L-PBF enables the production of geometrically-complex components that cannot be fabricated by conventional manufacturing methods, it offers the opportunity to produce metallic components with improved functionality and reduced weight. Due to their low mass density, aluminium alloys are well-suited for the production of weight-optimised structural parts. However, the use of aluminium alloys in L-PBF is associated with difficulties. On the one hand, conventional high-strength aluminium alloys cannot be used in L-PBF since solidification cracks form during the process, resulting in premature material failure. On the other hand, most of the aluminium alloys available for L-PBF do not exhibit attractive strength-ductility combinations. Therefore, this thesis aims at developing aluminium alloys that simultaneously exhibit high strength and good processability by L-PBF, and further do not rely on expensive alloying additions. For this purpose, three different alloy design strategies are developed and implemented. First, the feasibility of using L-PBF to process the commercial, high-strength aluminium alloys 2065 and 7034 is investigated in laser surface melting experiments. In both alloys, solidification cracks are detected, indicating their poor processability. The formation of cracks is rationalised in terms of grain refinement and a quantitative criterion for solidification cracking, relating the crack formation in both alloys to their chemical compositions and microstructural features. Using laser surface alloying, the chemical composition of alloy 2065 is modified with the goal of suppressing the crack formation and thus enabling processing. While the addition of copper has a negligible effect on the cracking tendency, a titanium-modified 2065 alloy is shown to have a high resistance to crack formation during laser remelting, suggesting its use in L-PBF. Secondly, a novel Al-Ti-Si alloy is developed that employs the L12-Al3X-forming element titanium as the main precipitation-hardening element. Initially, a range of Al-Ti-based compositions is fabricated from elemental powder mixtures and subsequently investigated, including binary Al-Ti and ternary Al-Ti-X alloys with additions of nickel and silicon. All alloys are shown to exhibit crack-free and highly supersaturated microstructures after L-PBF production. However, massive solid-state precipitation of the desired L12-Al3Ti phase, accompanied by an increase in hardness, exclusively occurs in a silicon-containing Al-Ti-Si alloy. This is due to the influence of silicon on the precipitation process, which is elucidated on the basis of thermodynamic and kinetic effects. Thirdly, a high-strength aluminium alloy based on the eutectic Al-Ni system is developed using a mechanism-based alloy design approach. With the goal of creating a microstructure in which several strengthening mechanisms contribute to the total yield strength, theoretical models are used to identify alloying additions capable of forming homogeneous and supersaturated solid solutions during L-PBF. By this method, an Al-Ni-Zr-Cr alloy is developed. An in-depth microstructure characterisation reveals that Zr and Cr are primarily in solid solution in the as-produced material. During heat treatment, however, a high precipitate number density of up to 10^24 L12-Al3Zr particles per m^3 is produced, while Cr largely remains in solid solution, providing solid solution strengthening. Tensile properties and temperature stability of the Al-Ni-Zr-Cr alloy are compared to other aluminium alloys and rationalised in terms of the underlying strengthening mechanisms. This thesis introduces novel aluminium alloy compositions for L-PBF, presents methods for efficient alloy screening, and discusses metallurgical mechanisms, the understanding of which can advance the alloy design for L-PBF