كليدواژه :
آلومينيوم 2024 , عمليات حرارتي T8 , رفتار كششي , سازوكارهاي شكست
چكيده فارسي :
در اين پژوهش، تأثير عمليات حرارتي پيرسازي مصنوعي به همراه نورد سرد
m(T8) بر رفتار كششي آلياژ آلومينيوم 2024 در حضور و عدم حضور شيار مورد بررسي قرار گرفته است. براي تعيين سازوكارهاي شكست، با استفاده از SEM سطوح شكست، نمونه‌هاي كششي تحت زمان‌هاي متفاوت پيرسازي مطالعه شد. نتايج رفتار كششي نشان داد كه با ايجاد شيار در نمونه‌ها، تنش تسليم و نهايي افزايش پيدا كرده و ماده نسبت به شيار مستحكم مي‌شود؛ اگرچه به دليل كاهش چشمگير كرنش شكست در اثر حضور شيار، چقرمگي نمونه‌هاي شياردار به شدت كاهش مي‌يابد. اين كاهش در اوج پيري بيشترين ميزان را دارد. همچنين به نظر مي‌رسد تغيير شكل زمينه، ذرات ترك‌خورده و جدايش ذره از زمينه، سازوكارهاي شكست در تمامي شرايط پيرسازي و در كل نمونه‌ها هستند، اما حضور شيار به شدت كسر ذرات ترك خورده را افزايش داده و همراه با كاهش حجم زمينه، سبب افت چشمگير در كرنش شكست و چقرمگي مي‌شود.
چكيده لاتين :
The effect of artificial ageing on tensile properties and fracture behavior of 2024 Al alloy in the presence and absence of notch was investigated. The tensile samples having two V notches, as well as, common tensile specimens were prepared according to ASTM E8. T8 heat treatment consisting of solutionizing, cold rolling, and aging was selected to establish precipitation hardening in Al2024 alloy. Considering T8 condition, following solutionizing of Al sheet at 5000C,
samples were cold rolled with 30% reduction. Finally artificial aging was applied at 2000C via different times. The results of Brinell test shows that the maximum value of hardness was achieved at 1 hour, which indicated the peak aging condition. Moreover, soaking samples in aging condition at 2000C
for 15 minutes and 4 hours were considered as under-aging and over-aging
states, respectively. Similar to hardness results, the superior amount of yield
strength and ultimate tensile strength of un-notched specimens were observed at
peak aging time, while in notched samples, the role of aging time was not prominent due to the presence of V shaped flaws. By applying notches on specimens, the yield and ultimate stress were found to increase with simultaneously decrease in elongation at fracture. It was found that due to
localizing plastic deformation around flaws, the presence of notch increases
the strength of 2024 Al alloy, while elastic behavior controls deformability
far from notches. The energy which was absorbed till fracture, was indicated as
toughness and it was found to be absolutely dominated by the elongation at
fracture and significantly decreased in notched specimens. The peak-ageing
condition of notched tensile specimens had the lowest toughness compare to
under-ageing and over-ageing conditions. Although three fracture mechanisms,
such as matrix deformation, particle cracking and matrix/particle debonding
observed in all samples, the distribution of these mechanisms were more
homogeneous in un-notched specimens. In the other words, one may claim that the presence of flaws enhanced the cracked particles, which accompanied with
lowering matrix deformability lessened the elongation and toughness.
