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Saturday, August 13, 2011
Powerful Science Search Engine -WORLDWIDESCIENCE.ORG
Those of my readers who read the previous post will notice that this finding arose from the research done to write the previous post entitled: "Superplasticity - superelasticity
Superelasticity-Superplasticity Update _Adhoc Introductory Research Notes
Its good to be back on the blog.
This post is motivated by a news item in the R&D section of my professional house journal ,IOM3's Materials World (MW Aug 2011) entitled "High-strength alloy takes the strain" It reports briefly on Tohoku Univ. (Jp) work on superelastic materials.
Superelastic Effect in Polycrystalline Ferrous Alloys
In superelastic alloys, large deformation can revert to a memorized shape after removing the stress. However, the stress increases with increasing temperature, which limits the practical use over a wide temperature range. Polycrystalline Fe-Mn-Al-Ni shape memory alloys show a small temperature dependence of the superelastic stress because of a small transformation entropy change brought about by a magnetic contribution to the Gibbs energies. For one alloy composition, the superelastic stress varies by 0.53 megapascal/°C over a temperature range from –196 to 240°C.
T. Omori*, K. Ando, M. Okano, X. Xu, Y. Tanaka, I. Ohnuma, R. Kainuma, K. Ishida
Science 1 July 2011: 68-71.
Expanding the Repertoire of Shape Memory Alloys
The exceptional properties of many materials often come at the expense of limited performance in other areas. For example, conventional metals and their alloys are strong—they are good at resisting stress (i.e., an applied load)—but they tolerate only a very small amount of strain (i.e., deformation) before they are
irreversibly deformed. Rubber can easily return to its original shape, even after large deformations, but is much weaker than conventional metals. However, some metal alloys exhibit “shape memory”; they are strong but can recover from being deformed when heated. This process seems counterintuitive, but these alloys take advantage of solid-to-solid “diffusionless” phase transitions: The atoms rearrange how they pack into crystals in an orderly fashion, and this process changes the material's macroscopic shape. Few other materials possess this combination of strength and flexibility (see the figure), and clever engineering has exploited these properties—for example, in implanted medical devices such as stents. On page 1488 of this issue, Tanaka et al. (1) report on a superelastic alloy that almost doubles the useful range of deformation that can be induced in such alloys.
irreversibly deformed. Rubber can easily return to its original shape, even after large deformations, but is much weaker than conventional metals. However, some metal alloys exhibit “shape memory”; they are strong but can recover from being deformed when heated. This process seems counterintuitive, but these alloys take advantage of solid-to-solid “diffusionless” phase transitions: The atoms rearrange how they pack into crystals in an orderly fashion, and this process changes the material's macroscopic shape. Few other materials possess this combination of strength and flexibility (see the figure), and clever engineering has exploited these properties—for example, in implanted medical devices such as stents. On page 1488 of this issue, Tanaka et al. (1) report on a superelastic alloy that almost doubles the useful range of deformation that can be induced in such alloys.
Ji Ma and Ibrahim Karaman, Science 19 March 2010: 1468-1469.
Ferrous Polycrystalline Shape-Memory Alloy Showing Huge Superelasticity
Shape-memory alloys, such as Ni-Ti and Cu-Zn-Al, show a large reversible strain of more than several percent due to superelasticity. In particular, the Ni-Ti–based alloy, which exhibits some ductility and excellent superelastic strain, is the only superelastic material available for practical applications at present. We herein describe a ferrous polycrystalline, high-strength, shape-memory alloy exhibiting a superelastic strain of more than 13%, with a tensile strength above 1 gigapascal, which is almost twice the maximum superelastic strain obtained in the Ni-Ti alloys. Furthermore, this ferrous alloy has a very large damping capacity and exhibits a large reversible change in magnetization during loading and unloading. This ferrous shape-memory alloy has great potential as a high-damping and sensor material.
Y. Tanaka1, Y. Himuro1, R. Kainuma2,*, Y. Sutou1, T. Omori1 and K. Ishida1,
Science 19 March 2010: Vol. 327 no. 5972 pp. 1488-1490
DOI: 10.1126/science.1183169
MATERIALS SCIENCE
OTHERS WHO REPORTED THESE FINDINGS
- Fellow blogger : http://gblogger-metallurgy.blogspot.com/2010/03/ferrous-polycrystalline-shape-memory.html
- My professional house journal, IOM3’s Materials World.
FUNDAMENTAL APPROACHES
REVIEWS OF TOPICAL PROBLEMS PACS numbers: 62.20.Fe, 75.30.Kz, 75.80.+q, 81.30.Kf
Shape memory ferromagnets
A N Vasil'ev, V D Buchel'nikov, T Takagi, V V Khovailo, E I Estrin
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KEY WORDS: alloy,superplastic,forming
Monday, June 6, 2011
This-Above-All: Geoffrey West On the Scale and Unity of Life from Cells to Cities
This-Above-All: Geoffrey West On the Scale and Unity of Life from Cells to Cities
Just as dimensional analysis [pdf] is a fundamental tool of engineering in getting the right units and checking phenomena so too is scaling fundamental to coming to grips with complex systems.
Thursday, June 2, 2011
Solar-Thermal Flat Panels Generate Electricity
Two technologies have dominated efforts to harness the power of the sun's energy. Photovoltaics convert sunlight into electric current, while solar-thermal power generation uses sunlight to heat water and produce thermal energy. Photovoltaic cells have been deployed widely as flat panels, while solar-thermal power generation employs sunlight-absorbing surfaces feasible in residential and large-scale industrial settings.
Because of limited material properties, solar-thermal devices have so far failed to economically generate enough electric power. The team has introduced two innovations — a better light-absorbing surface through enhanced nanostructured thermoelectric materials, which was then placed within an energy-trapping, vacuum-sealed flat panel. Combined, both measures added enhanced electricity-generating capacity to solar-thermal power technology, as reported by Zhifeng Ren, a Boston College professor of physics and a co-author of the study. The team is comprised of Gang Chen, MIT's Soderberg professor of power engineering; Boston College and MIT graduate students; and researchers at GMZ Energy, a Massachusetts clean energy research company founded by Ren and Chen.
Ren claims that their team has developed a flat panel that is a hybrid capable of generating hot water and electricity in the same system," he said. "The ability to generate electricity by improving existing technology at minimal cost makes this type of power generation self-sustaining from a cost standpoint."
Their technology involves using nanotechnology engineering methods. The researchers combined high-performance thermoelectric materials and spectrally selective solar absorbers in a vacuum-sealed chamber to boost conversion efficiency.
The findings open up a promising new approach that has the potential to achieve cost-effective conversion of solar energy into electricity, an advance that should affect the rapidly expanding residential and industrial clean energy markets, claims Ren who further comments, "Existing solar-thermal technologies do a good job generating hot water. For the new product, this will produce both hot water and electricity," he said. "Because of the new ability to generate valuable electricity, the system promises to give users a quicker payback on their investment. This new technology can shorten the payback time by one-third."
Solar-Thermal Flat Panels Generate Electricity
Because of limited material properties, solar-thermal devices have so far failed to economically generate enough electric power. The team has introduced two innovations — a better light-absorbing surface through enhanced nanostructured thermoelectric materials, which was then placed within an energy-trapping, vacuum-sealed flat panel. Combined, both measures added enhanced electricity-generating capacity to solar-thermal power technology, as reported by Zhifeng Ren, a Boston College professor of physics and a co-author of the study. The team is comprised of Gang Chen, MIT's Soderberg professor of power engineering; Boston College and MIT graduate students; and researchers at GMZ Energy, a Massachusetts clean energy research company founded by Ren and Chen.
Ren claims that their team has developed a flat panel that is a hybrid capable of generating hot water and electricity in the same system," he said. "The ability to generate electricity by improving existing technology at minimal cost makes this type of power generation self-sustaining from a cost standpoint."
Their technology involves using nanotechnology engineering methods. The researchers combined high-performance thermoelectric materials and spectrally selective solar absorbers in a vacuum-sealed chamber to boost conversion efficiency.
The findings open up a promising new approach that has the potential to achieve cost-effective conversion of solar energy into electricity, an advance that should affect the rapidly expanding residential and industrial clean energy markets, claims Ren who further comments, "Existing solar-thermal technologies do a good job generating hot water. For the new product, this will produce both hot water and electricity," he said. "Because of the new ability to generate valuable electricity, the system promises to give users a quicker payback on their investment. This new technology can shorten the payback time by one-third."
Solar-Thermal Flat Panels Generate Electricity
Iceland’s Jet Engine Stopping Volcanic Ash
Iceland’s Jet Engine Stopping Volcanic Ash
As one involved in superalloy R&D, Quality Assurance and Manufacture this tweet from Materialinsight is worth recording for future aero-engine conception, QA andmanu specs.
Thanks Materialsinsight for the info
As one involved in superalloy R&D, Quality Assurance and Manufacture this tweet from Materialinsight is worth recording for future aero-engine conception, QA andmanu specs.
Thanks Materialsinsight for the info
Sunday, May 8, 2011
Korosi Celah (Cresive Corrosion)
(http://www.cdcorrosion.com/mode_corrosion/corrosion_crevice.htm)
Tahap – tahap terjadinya korosi celah:
- Terjadi reaksi korosi merata
- Pada daerah celah tempat jebakan air, terjadi penipisan kadar oksigen sehingga pembentukan OH- terhambat. Akibatnya terjadi kekurangan ion negatif.
- Ion negatif dari luar celah, misal ion Cl- berdifusi masuk ke dalam celah untuk menyeimbangkan muatan.
- Ion M+ terhidrolisis sehingga menyebabkan penurunan pH di dalam celah
- Penurunan pH menyebabkan reaksi korosi semakin parah
- Korosi celah ini bersifat autokatalitik artinya begitu reaksi awal terjadi, sel – sel tidak lagi bergantung pada keadaan luar
Pengendalian korosi celah dapat dilakukan dengan cara:
(1) memilih material yang tahan korosi.
(2) Menurunkan agresifitas larutan dengan menurunkan kandungan klorida, keasaman dan atau temperaturnya, menghambat aliran proses pembentukan deposit, dan mengeliminasi terakumulasinya hidrolisa produk korosi.
(3) Memberi unsur penghambat di larutan (inhibitors). Penerapan cara ini harus diperhitungkan dengan baik, karena apabila kandungan inhibitor yang terdapat dilarutan tidak cukup, maka pada beberapa bagian peralatan dapat terjadi kerusakan berupa lubang kecil yang dalam.
(4) Menggunakan protekasi katodik untuk peralatan yang digunakan di lingkungan laut, tetapi cara ini tidak selalu menjadi pilihan yang memungkinkan untuk aliran proses kimia yang agresif.
(5) Melakukan perencanaan dengan menghindari adanya celah-celah. Peralatan harus direncanakan lengkap dengan saluran pembuangan dan menghindarkan daerah yang menyebabkan tertahannya atau mengendapnya larutan. Sambungan las temu (butt-joint) pada struktur akan lebih baik diaplikasikan dibanding sambungan paku keling atau sambungan ulir.
(6) Membersihkan permukaan logam apabila memungkinkan, akan menurunkan terjadinya korosi sumuran dan korosi celah. Menghilangkan partikel padat yang dilakukan untuk meminimalkan pembentukan deposit.
Friday, May 6, 2011
Korosi Batas Butir
Intergranular corrosion (IGC) atau intergranular attack (IGA)atau korosi batas butir adalah serangan korosi pada daerah sepanjang batas butir atau daerah sekitarnya tanpa serangan yang cukup besar terhadap butirnya sendiri. Seperti kita ketahui, logam merupakan susunan butiran-butiran kristal seperti butiran pasir yang menyusun batu pasir. Butiran-butiran tersebut saling terikat yang kemudian membentuk mikrostuktur. Adanya serangan korosi batas butir menyebabkan butiran menjadi lemah terutama di batas butir sehingga logam kehilangan kekuatan dan daktilitasnya.
Sebagian besar paduan logam rentan terserang korosi batas butir ketika dihadapkan pada lingkungan agresif. Hal ini disebabkan batas butir merupakan tempat pengendapan (precipitation) dan pemisahan (segregation), dimana membuat mereka secara fisik dan kimia berbeda dengan butirnya. Presipitasi dan segregasi terjadi oleh adanya migrasi impuriti atau unsur pemadu (alloying element) menuju batas butir. Apabila kadar unsur tersebut cukup besar, maka akan terbentuk fasa yang berbeda dengan yang ada di bulk. Misalnya fasa intermetalik Mg5Al8 dan MgZn2 pada paduan aluminum dan Fe4N pada paduan besi.
Pada paduan nikel dan austenitic stainless steel, kromium sengaja ditambahkan untuk memberikan sifat ketahanan korosi. Sekitar minimal 12% kromium dibutuhkan untuk membentuk lapisan pasif yang tidak nampak pada permukaan stainless steel. Lapisan ini berfungsi untuk melindungi logam dari lingkungan korosif. Apabila stainless steel mengalami pemanasan pada 550-850 °C (misalnya selama produksi, fabrikasi, perlakuan panas, dan pengelasan), maka kromium karbida (terutama Cr23C6) akan tumbuh dan mengendap pada batas butir saat terjadi pendinginan. Sebagai konsekuensinya, wilayah yang berdekatan dengan batas butir akan kekurangan kromium. Daerah yang kekurangan kromium itu menjadi lebih rentan terserang korosi dalam lingkungan agresif dibandingkan daerah yang jauh dari batas butir.
Pengendapan atas beberapa karbida sering disebut sebagai “sensitasi”. Sensitisasi merupakan penyebab terjadinya serangan korosi batas butir. Sensitisasi terjadi saat pendinginan perlahan dari suhu 550-850 °C.
Sensitisasi pada stainless steel dapat dicegah dengan cara:
-pemanasan di atas 1000 °C kemudian dilakukan pendinginan secara cepat di dalam air. Akibatnya kromium karbida akan larut ke dalam butiran dan tidak sempat terjadi presipitasi. Metode ini dikenal dengan solution treatment.
-menambahkan titanium, niobium, dan tantalum. Ketika unsur tersebut akan membentuk titanium karbida, niobium karbida dan tantalum karbida yang lebih stabil daripada kromium karbida. Baja yang mengandung unsur-unsur penstabil ini disebut stabilized steel.
- menurunkan kadar karbon di bawah 0,02%.
Uji Ketahanan Terhadap Korosi Batas Butir
Terdapat beberapa metode untuk pengujian korosi batas butir. Secara umum digunakan larutan oksidator asam dengan pH, potensial dan temperatur tergantung pada metode yang digunakan. pemilihan metode tergantung pada steel grade dan komposisi batas butir yang akan diuji.
Beberapa uji ASTM untuk stainless steel dirangkumkan pada tabel berikut:
korosi batas butir pada stainless steel tersensitisasi
Sebagian besar paduan logam rentan terserang korosi batas butir ketika dihadapkan pada lingkungan agresif. Hal ini disebabkan batas butir merupakan tempat pengendapan (precipitation) dan pemisahan (segregation), dimana membuat mereka secara fisik dan kimia berbeda dengan butirnya. Presipitasi dan segregasi terjadi oleh adanya migrasi impuriti atau unsur pemadu (alloying element) menuju batas butir. Apabila kadar unsur tersebut cukup besar, maka akan terbentuk fasa yang berbeda dengan yang ada di bulk. Misalnya fasa intermetalik Mg5Al8 dan MgZn2 pada paduan aluminum dan Fe4N pada paduan besi.
Pada paduan nikel dan austenitic stainless steel, kromium sengaja ditambahkan untuk memberikan sifat ketahanan korosi. Sekitar minimal 12% kromium dibutuhkan untuk membentuk lapisan pasif yang tidak nampak pada permukaan stainless steel. Lapisan ini berfungsi untuk melindungi logam dari lingkungan korosif. Apabila stainless steel mengalami pemanasan pada 550-850 °C (misalnya selama produksi, fabrikasi, perlakuan panas, dan pengelasan), maka kromium karbida (terutama Cr23C6) akan tumbuh dan mengendap pada batas butir saat terjadi pendinginan. Sebagai konsekuensinya, wilayah yang berdekatan dengan batas butir akan kekurangan kromium. Daerah yang kekurangan kromium itu menjadi lebih rentan terserang korosi dalam lingkungan agresif dibandingkan daerah yang jauh dari batas butir.
Pengendapan atas beberapa karbida sering disebut sebagai “sensitasi”. Sensitisasi merupakan penyebab terjadinya serangan korosi batas butir. Sensitisasi terjadi saat pendinginan perlahan dari suhu 550-850 °C.
Sensitisasi pada stainless steel dapat dicegah dengan cara:
-pemanasan di atas 1000 °C kemudian dilakukan pendinginan secara cepat di dalam air. Akibatnya kromium karbida akan larut ke dalam butiran dan tidak sempat terjadi presipitasi. Metode ini dikenal dengan solution treatment.
-menambahkan titanium, niobium, dan tantalum. Ketika unsur tersebut akan membentuk titanium karbida, niobium karbida dan tantalum karbida yang lebih stabil daripada kromium karbida. Baja yang mengandung unsur-unsur penstabil ini disebut stabilized steel.
- menurunkan kadar karbon di bawah 0,02%.
Uji Ketahanan Terhadap Korosi Batas Butir
Terdapat beberapa metode untuk pengujian korosi batas butir. Secara umum digunakan larutan oksidator asam dengan pH, potensial dan temperatur tergantung pada metode yang digunakan. pemilihan metode tergantung pada steel grade dan komposisi batas butir yang akan diuji.
Beberapa uji ASTM untuk stainless steel dirangkumkan pada tabel berikut:
Uji | temperatur | waktu | aplikasi | evaluasi |
A Uji screening etsa asam oksalat | Ambien | 1,5 menit | hanya untuk sensitisasi kromium karbida | mikroskopik: klaisifikasi struktur etsa |
B Ferric Sulphate 50% Sulphuric Acid | mendidih | 120 jam | kromium karbida | Laju korosi/ kehilangan berat |
C. 65% Nitric Acid | Mendidih | 240 jam | Kromium karbida dan fasa sigma | Laju korosi/ kehilangan berat |
D. 10% Nitric Acid 3% Hydrofluoric acid | 70 °C | 4 jam | Chronium carbide in 316, 316 L, 317 and 317 L | Corrosion ratio compared to solution annealed specimen |
E 6% Copper Sulphate 16% Sulphuric Acid Metallic Copper | Mendidih | 24 jam | Kromium karbida | Examination for fissures after bending |
F. Copper Sulphate 50% Sulphuric Acid Metallic Copper | Mendidih | 120 jam | Kromium karbida pada cast 316 and 316 L | Laju korosi/ kehilangan berat |
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