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  • 1. Chaim, Rachman
    et al.
    Reshef, Ram
    Liu, Guanghua
    Shen, Zhijian
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Low-temperature spark plasma sintering of NiO nanoparticles2011Inngår i: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 528, nr 6, s. 2936-2940Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    NiO nanoparticles of 20 nm in diameter were spark plasma sintered between 400 °C and 600 °C for 5 and 10 min durations. Application of 100 MPa pressure from room temperature resulted in densities between 75% and 92%. The final grain size was between 26 nm and 68 nm. Lower densities were recorded when 100 MPa was applied at the SPS temperature. Two shrinkage rate maxima of 3.4 × 10−3 s−1 and 2 × 10−3 s−1 were observed around 390 ± 10 °C and at the SPS temperature. The two shrinkage rate maxima were related to densification by particle sliding followed by diffusional grain boundary sliding during the heating. The strong effects of the surface and interfacial processes which are active during the SPS were highlighted.

  • 2. Chen, Nan
    et al.
    Ma, Guoqiang
    Zhu, Wanquan
    Godfrey, Andrew
    Shen, Zhijian
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Wu, Guilin
    Huang, Xiaoxu
    Enhancement of an additive-manufactured austenitic stainless steel by post-manufacture heat-treatment2019Inngår i: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 759, s. 65-69Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effect of post-manufacture heat-treatment on the mechanical strength of an additively-manufactured austenitic stainless steel has been investigated. Microstructural investigations revealed that the as-manufactured material exhibited a multi-scale structure, composed of grains, cells, dislocations and nano-sized particles. Annealing at 400 degrees C resulted in a 10% increase in yield strength, associated with the additional precipitation of a population of nano-sized silicates. Annealing at higher temperatures resulted in a decrease in strength, attributed primarily to the thermal instability of the cell structure in the as-manufactured material. The results demonstrate that by careful control of annealing conditions the structure and mechanical properties of additively-manufactured austenitic stainless steel can be optimized by post-manufacture heat-treatment.

  • 3.
    Saeidi, Kamran
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Gao, X.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Zhong, Yuan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Shen, Zhijian James
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Hardened austenite steel with columnar sub-grain structure formed by laser melting2015Inngår i: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 625, s. 221-229Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Laser melting (LM), with a focused Nd: YAG laser beam, was used to form solid bodies from a 316L austenite stainless steel powder. The microstructure, phase content and texture of the LM stainless steel were characterized and compared with conventional 316L stainless steel. The crack-free LM samples achieved a relative density of 98.6 +/- 0.1%. The XRD pattern revealed a single phase Austenite with preferential crystallite growth along the (100) plane and an orientation degree of 0.84 on the building surface. A fine columnar sub-grain structure of size 0.5 mu m was observed inside each individual large grain of single-crystal nature and with grain sizes in the range of 10-100 mu m. Molybdenum was found to be enriched at the sub-grain boundaries accompanied with high dislocation concentrations. It was proposed that such a sub-grain structure is formed by the compositional fluctuation due to the slow kinetics of homogeneous alloying of large Mo atoms during rapid solidification. The local enrichment of misplaced Mo in the Austenite lattice induced a network of dislocation tangling, which would retard or even block the migration of newly formed dislocations under indentation force, turning otherwise a soft Austenite to hardened steel. In addition, local formation of spherical nano-inclusions of an amorphous chromium-containing silicate was observed. The origin and the implications of the formation of such oxide nano-inclusions were discussed.

  • 4.
    Saeidi, Kamran
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Kevetkova, Lenka
    Lofaj, Frantisk
    Shen, Zhijian
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Novel ferritic stainless steel formed by laser melting from duplex stainless steel powder with advanced mechanical properties and high ductility2016Inngår i: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 665, s. 59-65Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Stainless steel bodies with relative density of 99.5% (with the theoretical density being 7.8gr/cm3) were manufactured by laser melting (LM) of duplex 2507SAF steel powder. The crystalline phases of starting powder were fully ferrite with only a small trace of austenite. The chemical composition was unchanged during laser melting. A unique mosaic-type structure with mosaics of 100-150 μm size was formed after LM. Recrystallized grains with 1-5 μm was formed in between the mosaic boundaries. A great number of entangled dislocation loops resembling a loops with 100-200 nm size were also formed inside each of these mosaics and also within recrystallized micron size grains at the mosaic boundary zones. Nitrogen enriched areas and nitride phase were detected in the inner microstructure of the laser melted samples. The measured tensile strength, yield strength and microhardness were 1214 MPa, 1321 MPa and 450 HV, respectively, which is superior to that of conventional ferritic, austenitic and duplex stainless steels. The Enhanced mechanical properties are due to a number of nano- and microstructure factors such as the nano-sized dislocation loops restricting dislocation movements, different crystalline grain orientation of grains within the mosaics and boundary inclusions and precipitates that inhibit slip/slide effects. Despite of high strength and hardness, the laser melted ferritic steel was very ductile according to stress-strain curves and fracture analysis.

  • 5. Yang, Y.
    et al.
    Liu, Y. J.
    Chen, J.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Wang, H. L.
    Zhang, Z. Q.
    Lu, Y. J.
    Wu, S. Q.
    Lin, J. X.
    Crystallographic features of alpha variants and beta phase for Ti-6Al-4V alloy fabricated by selective laser melting2017Inngår i: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 707, s. 548-558Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The present study investigated the crystallographic features of a variants and beta phase for Ti-6Al-4V alloy fabricated by selective laser melting. The <100>(beta) fiber texture parallel to the building direction was ascertained on the basis of a reconstruction method realized by the manipulation of stereographic projection. The SLMed alloy has no alpha/alpha' variants selection but contains a special crystallographic area exhibiting random orientation which cannot be reconstructed as a parental columnar beta grain with the present introduced method due to its nature as the overlapped area between adjacent melt pools resulting from the heterogeneous nucleation in front of the liquid-solid interface. With increasing the heat treated temperature, alpha variants selection occurs. Especially at higher temperature of 905 degrees C, the intergranular beta phase following a reversed crystallographic path as parental beta phase->alpha variants->intergranular beta phase would be precipitated, therefore the intergranular beta phase keeps the same orientation with the parental beta phase. Once the alloy was heat treated at 975 degrees C close to T-beta, the microstructure is characterized by primary alpha variants selection and a large amount of secondary a widmanstatten structure with a homogeneous orientation which accounts for the lowest tensile strength. The decomposition of twelve alpha variants proved the BOR <110>(beta)//(0001)(alpha), <111>(beta)//<2-1-10>(alpha). The misorientation between two variants sharing a common parental <100>(beta) pole with a similar color consisting of all Euler angles was identified to be [0001] (alpha)/10.53 degrees.

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